r.j. gayler - bg tuning manual

The BG Tuning Manual

Page 1

CHAPTER ONE

POWER!

Horsepower.... What exactly is it?

Wherever people gather to talk motoring, motorcycling or motor sport of any kind it is only a matter

of time before the word power comes into the conversation.

But very few people really understand what horsepower is.

The dictionary definition is....

A unit to measure the rate of doing work...

Not a lot of help for someone trying to make crucial

decisions on how to tune an engine!

Yet every rider and driver knows the feel of power...

The surge of acceleration as the throttle is opened, the

clutch dropped, and suddenly the wheels are spinning -

thats the feel of high power.

Even as you wind back the twist grip of your 50cc

moped, its power that gets you moving, even though not

quite so sexy.

In fact, nothing moves without power - clockwork toys,

elastic band propelled pellets, human beings on bicycles,

Harrier jump jets... all need power, large or small, to

move at all.

A simple falling stone uses motive power from the

kinetic energy imparted when someone or something lifts

it against the pull of gravity.

So power can also be defined as the result of converting

work into movement.

And the internal combustion engine happens to be one of

the most convenient ways of producing movement.

All you need to do is pour the fuel in at one end, pull the

right knobs and levers... and instant power comes out the

other end.

The engine thus converts fuel into movement.


Page 2

But what exactly, is fuel?

Fuel is neatly packaged heat...

Anything that burns when a match is applied is potentially capable of producing power, in this case

petrol/gasoline or methanol.

So the engine is a heat converter and of course this is why, when this subject is studied in college, it

is entitled heat engines.

Two stroke or four stroke, one cylinder or sixteen, the conversion process is always carried out in

the same way - by drawing in a mixture of fuel and air, compressing it, igniting it and using the

resultant combustion to drive a piston or turbine down or round, and rotate a crank.

When the engine converts fuel into power, the process is rather inefficient and only about a quarter

of the potential energy in the fuel is released as power at the flywheel. The rest is wasted as heat

going down the exhaust and into the air or water.

This ratio of actual to potential power is called the THERMAL EFFICIENCY, of the engine.

The machine we use to measure engine performance is a dynamometer and the way in which it

works is closely tied to the explanation of power.

MEASURING POWER

The term Horsepower, was evolved during the 19th century to describe the capability of engines to

carry out a measure of work related to the conversion of energy into motion by the horse.

At that time it was a realistic way of comparing mechanical power to horse power.

The unit of engine horsepower, which used to be called B.H.P, the abbreviation of Brake Horse

Power, is now being replaced with the Watt, hitherto used to quantify the power of electric

motors and other appliances.

One B.H.P is the equivalent of 746 Watts.

Both of them are a measurement that describes the power that is actually measured at the flywheel.

A dynamometer is not actually capable of showing power as a direct reading, but measures torque


Page 3

and R.P.M, from which power is calculated.

Engine Data Acquisition Systems are electronic measuring system that captures information from

an engine or chassis dynamometer, then automatically calculates the power.

Torque is the amount of work that an engine is actually doing at any given moment, that is, the

turning force being exerted at the crankshaft, and is measured in Foot Pounds, or Newton

Metres, depending on whether you are using the British Imperial or the International Metric

System.

Imagine an engine sited at the top of a deep well turning a drum,

which is four feet in diameter, i.e. 2 feet radius.

A rope attached to the drum is hanging down the well with a weight

of 100 lbs. on the end.

As the engine turns the drum it will lift the weight. The drum is

four foot in diameter and the rope is being pulled in at two foot

from the centre of rotation; therefore the work being done or torque

is measured as 2ft x 100lbs = 200 foot pounds

The speed at which the drum is rotating is measured as Revolutions

Per Minute (R.P.M).

B.H.P is calculated as follows

TORQUE X R.P.M

B.H.P = -------------------

CONSTANT

The constant depends on the units of torque, which are being

measured. As we are using ft.lbs it will be 5250, so if we say that

the engine is turning at 1000 R.P.M then:

200 x 1000

B.H.P = ------------------- = 38

5250

Because we cannot calculate B.H.P without knowing the R.P.M, it means that B.H.P is a measure of

the speed at which work is done as previously mentioned, a unit to measure the rate of doing work.


Page 4

To understand the way in which the

dynamometer works, imagine anchoring a

spring balance to the ground, with a rope

attached to the top eye and wrapped around a

drum with a slipknot is tightened as the drum

is rotating, the rope will be tensioned and the

balance will extend to indicate this tension as a

weight. As the knot is further tightened,

friction between rope and drum will slow the

drum and its driving engine until, at 1000

R.P.M, the spring balance reads 100 lbs.

The weight being lifted is 100 lbs and the

speed of the drum or engine will then be used

in the formula to calculate the horsepower.

If the speed of the engine/drum were 1000 R.P.M the B.H.P being exerted would be 38. If the speed

were 1500 R.P.M this would mean the engine was lifting the weight faster and exerting more power

to do it. The calculation would then be:

200 x 1500

-------------- = 57 B.H.P

5250

THE DYNAMOMETER

So a piece of rope, a spring balance, a rev counter and an engine fitted with a flywheel or drum to

take the rope is all you need to make a dynamometer

Well, yes but! if the throttle is wide open and nothing is moving where is all the power going?

The answer is, that its turning back into heat again.

Where?

Youve guessed it! Between the drum and the rope - as friction.

So although the idea of a cheap dyno sounds good, in fact the power being used, turning into

friction heat, would set fire to the whole lot.

Unless, of course, we cool it by pouring water over it, and thats just what the modern dynamometer

does.

It uses a device like the torque - converter of an

automatic transmission to do the job of the rope

and drum and is running in a continuous flow

of cooled water to absorb the heat.

The engine turns the inner part of the torque -

converter and the water drag thus created tries

to turn the outer casing, which is coupled to a

big accurate weighing machine reading torque.


Page 5

Now that we understand the meaning of power and how to measure it just how important is it?

Does an increase in power automatically mean higher speeds?

Not necessarily power is only of value when applied with suitable engineering skill. Smooth,

controllable power will often return better results than ultimate B.H.P peaking over a narrow rev

band. This is often the secret behind the out-performance of many smaller racing machines over

their more powerful contemporaries.

WHAT CONTROLS POWER?

HOW CAN IT BE IMPROVED?

An engine very much like a human being. It takes in air and fuel(food).

It burns the fuel (digestion). It converts the energy released into power (muscles).

Discharge of exhaust (bowels). Oil pump and circulation (heart).

Cooling system (pores). Pistons and cylinders (lungs).

The camshaft (brain) coordinates of the whole sequence of operations in the same way as the brain.

The efficiency of these individual functions effects engine performance in the same way they would

affect a human.

Poor or contaminated fuel will have low energy content.

Bad ignition or combustion chamber shapes will reduce the ability to digest the fuel fully, resulting

in an unpleasant and dirty exhaust.

Clogged or inadequate oil filters or a worn oil pump will result in component failure.

A dirty cooling system with clogged radiator pores will result in overheating.

A poorly designed camshaft will result in erratic breathing.

They all work together to produce the final flywheel muscle power, however good or bad.

To understand the process well start with the breathing cycle.

If we think of the engine as an air pump then theoretically it should draw in and exhaust its own

volume of air each time it cycles - that is, once every revolution if its a two stroke and once every

two revolutions if its a four stroke. In fact, ordinary production engines dont achieve this and only

manage to shift about 80% of their volume.

This ratio of possible air pumped to actual air pumped is called Volumetric Efficiency and this is

what we have to improve to get more power. The difference in appearance between two engines of

similar type, one of which is in standard road trim and one in full race trim can be seen on the left.


Page 6

Fig.11 is a standard street Honda 750 four and Fig.12 is a

full race version of the same model. The noticeable

external differences in the engine preparations are to

increase the VOLUMETRIC EFFICIENCY.

Volumetric Efficiency = Breathing Ability = Power

A chain of parts controls the breathing cycle, each one of

which depends on the others to work at its best.

The process starts right back at the air cleaner which

varies from being a large box containing a large paper

filtration and silencing element, necessary for silent

operation and engine protection under a variety of dusty

and sandy conditions, through to the light and minimal

filters of rally cars and speedway bikes, to the completely

open bell mouths of full circuit drag machines.

The next link in the chain is carburetion. The process of mixing the fuel and air and feeding them to

the engine in balanced doses, that is about fifteen times as much air as fuel.

Fifteen to one - air fuel/ratio - another important controller in a final power output!

Although we call it carburetion, here it can also include fuel- injection, just another method of

delivering fuel to the engine.

So, via inlet manifolds or stubs we move to the next link and here it is where the two stroke and

four stroke engines divide.

11

12


Page 7

FOUR STROKE TWO STROKE

AIR BOX OR CLEANER

RAM - PIPE

CARBURETTOR

FUEL - INJECTION

INLET MANIFOLD

CYLINDER HEAD CRANK CASE

PORTING PISTON VALVE

INLET VALVE PISTON SUPERCHARGER

CAMSHAFT TRANSFER TIMING

COMBUSTION

EXHAUST VALVE EXHAUST PORT TIMING

SIMPLE PARALLEL COMPLEX MULTI-

PIPE EXHAUST SYSTEM TAPER SYSTEM ONLY

COUPLED WITH MULTI COUPLED WITH 3

CYLINDER ENGINES CYLINDERS

POSSIBILY A EXPANSION CHAMBER,

SIMPLE REVERSE CONE

MEGAPHONE RESTRICTIVE TAIL PIPE

150+ BHP / LITRE 200 BHP / LITRE


Page 8

The four-stroke chain of power

The mixture

enters the cylinder

head and is

induced through

the inlet valve into

the combustion

chamber. The way

in which it enters

the chamber is

controlled by the

port shape and finish inlet valve timing, which

in turn is controlled by the camshaft.

The camshaft is probably the most important single component in the four stroke engine, as far as

power production is concerned, and it is certainly the most complicated piece to design and produce

(P.17).

Compression takes place followed by ignition and combustion - the point at which four- and two-

stroke reunite in a common process.

Exhaust in a four-stroke engine is again first controlled by the camshaft operating the exhaust valve,

and then by the design of the port and the exhaust system, which in turn, has a considerable effect

on exhaust efficiency (P.18).

The two-stroke chain of power.

There are several ways in which the two-stroke engine will

work but we will consider the modern loop-scavenge design,

which is the most commonly used production version.

(Fig.19)

As the piston rises, a depression is created in the crankcase

and the mixture is drawn in at the point where the piston

skirt starts to uncover the inlet point. As the piston comes

down, the inlet port is closed and the charge is compressed

and driven up the transfer port into the combustion chamber.

Because the mixture is being forced into the chamber under

pressure, this is really a form of supercharging and is one of

the reasons that this type of engine can produce so much

power relative to its size.

19


Page 9

As the piston rises again, compression takes place, followed by ignition and combustion, driving the

piston down which opens the exhaust port and drives a new charge into the combustion chamber at

the same time. Because these two happen together, the design of the transfer port and the exhaust

system must be just right in order to clear the foul gases and ensure a full charge of new mixture

without wasting any down the exhaust port.

Although the two designs appear to be very different, the overall function of both is the same - fuel

into power, the level of which is governed by volumetric efficiency.

The significant part of the power train chart is the point where the two engines coincide at

combustion. The key to high power output is fast, controlled, near total burning of the compressed

mixture. This key is common to all types of internal combustion engine and is dependant on all the

parts around it.

Next well start to look at those parts in detail.


Page 10

CHAPTER 2. BREATHING - THE INDUCTION SYSTEM.

AIR BOXES AND CLEANERS.

The need to use an air box at all must be judged by the environment surrounding the engine and in

which the vehicle is going to be used.

For example, while it is unlikely that a motorcycle needs any additional cold air fed into the carbs,

the engine compartment of a car is often very hot and poorly ventilated, so the argument for an air

box here is to get cold air from the front of the car, under the bonnet to the carbs.

On the other hand, if the bike or car is going to be used in excessively dusty or sandy conditions,

then an air box and filter are essential.

General rule of thumb is that vehicles used for pure circuit work or fast roadwork, including

dragsters, in Europe, can do without any air box at all, except during the occasional long hot

summer when the dust level is high.

This all supposes that you are prepared to put up with the induction roar of unsilenced intakes.

It goes without saying that autocross, rally, dirt and grass track vehicles all need cleaners. The

important thing is to apply the air box in the right way.

In order to avoid upstream restrictions, it should be sited at the

point at which the induction pulse can expand in the same way as

if there were no box there at all, i.e. the bell mouths should be

allowed to protrude into the box with adequate clearances and the

measurement "A" should be at least as great as the carburettor

bore size.

Tubular couplings as seen here will effectively increase the

tuned ram length to an RPM level that will probably be too

low.

RAM LENGTH is effectively the tuned length from the

inlet valve head or piston port face, to the end of the intake

trumpet.


Page 11

At the moment that the valve or port first starts to open, a pressure wave starts to travel back and

forth through the inlet tract, changing from plus pressure to minus pressure, or effective suction.

If the wave front is caught at the right moment, it can be used to help ram mixture into the engine,

thus inducing a form of mild supercharging. You can calculate approximate correct ram length

using the formula below, where L = length.

For a four-stroke:

228 x T 5791 x T

L in ins = --------------- in mms = -------------------

N N

Where T = Total intake valve duration in degrees

N = The estimated RPM at which max. power is required, minus five hundred

And for a two-stroke:

1150 x T 29210 x T

L in ins = --------------- in mms = --------------------

N N

Where T = Inlet port opening in degrees

N = The estimated RPM at which max. power will be achieved.

The two-stroke calculation will result in a length that is too great to be practical and may be divided

by 3 or 5 to fit installation requirements.

Ram pipes or trumpets should have fully round ends.

Contrary to general belief, much of the intake air is drawn in around

the edges of the mouth and, if there are sharp edges in this area, flow

will be interrupted and turbulence will cause restriction in the bell

mouth.

This is also the reason why bell mouths should be allowed to protrude

into the air box rather than finish flush with the wall.

The position of the air entry to the box is not critical provided it is not

within 50mms or so, of the bell mouths.

CARBURETTORS.

In order to increase power the process of improving volumetric efficiency is invariably tied up with

an increase in the operating RPM. However, if we increase RPM, then we will inevitably be

increasing the air speed in the inlet tract, which includes the carburettor.

The size of carburettor that is normally suitable for the standard engine is rarely large enough for

any appreciable increase in the state of tune other than stage 1.

Stage 1 modifications can vary from one engine to another but generally comprise a mild increase

in cam profile on a four-stroke or lengthening of inlet and exhaust timings by about 5 degrees on a

two- stroke, raising compression by about one ratio, smoothing out porting and possibly fitting a

high performance exhaust system.

Even at this level the engine can easily be strangled by its standard carburetion. So any further

increase means that we are inevitably faced with the need to up rate the carburetion.


Page 12

To evaluate the European options available, we will first divide them into two types depending on

their principles of operation:

Fixed choke, butterfly air control.

Weber and Dellorto

Other fixed choke carburettors are available but are impractical due to lack of adjustable features.

Variable choke, butterfly air control, constant depression (CD) S.U., Stromberg C.D., Mikuni.

Variable choke, manual slide control of air and fuel. Amal, Bing, Keihin, Mikuni, Dellorto.

Suitability and convenience of application:

1. Weber D.C.O.E. series side draught

I.D.A. and D.C.N series downdraught

Size range from 38mm to 48mm.

Without doubt the best high performance carburettor available in the world for application in cars.

Wide range chokes and jets readily available for applying

to any engine.

The sealed throttle spindles make them particularly

suitable for upstream supercharging.

Usable for motorcycles but rather big and bulky for

average application, also needs special fuel delivery

requirements, i.e. pump delivery or modified needle

valves.

2. Dellorto D.H.L.A. series sidedraught

F.Z.D. series sidedraught

Size range from 20mm to 48mm

D.H.L.A. series similar to Weber D.C.O.E. but not quite as efficient in terms of air flow.

Not quite the same range of variables available.

Other series are readily adaptable to motorcycle applications.

3. S.U. - Size range from one and a quarter to two inch .

The most widely used performance carburettor up to the late 70s.

Very popular for road conversions. Very adaptable with wide range of tuning needles available.

Tend to suffer fuel surge on high cornering G-forces.

4. STROMBERG C.D. - Size and range from one and a quarter to one and three quarters of an

inch.

Similar in operation to S.U. Not such a wide range of needles, application not quite so simple.

5. MIKUNI & KEIHIN - Both make C.D. carburettors for use on road bikes and also make manual

slide control versions for use on racing bikes.

They are available as kits for some production models but generally, they are difficult to apply to

other models due to the lack of non-standard jets and needles.

Properly set up, they make efficient and reliable racing carburetion.


Page 13

6. AMAL - size range 25mm - 42mm

The most popular and successful motorcycle carburettor over many years.

Very adaptable to almost any engine and very forgiving to slight errors in tuning.

All carburettors, whatever the make, suffer, to a degree, from two major faults...

Fuel frothing due to vibration and float chamber fuel surge due to cornering G-forces.

In many cases these can be minimised by careful attention to flexible mountings, float levels,

needle valves and springs where applicable.

CARBURETTOR SELECTION

Refer to the Staging table in the camshaft section of the manual for Stage descriptions.

Stage 1. There is no real point in modifying even slightly if the standard engine is not of the GT

type, which is really the manufacturer's first mild production tuning step from his basic engine.

Carburetion at this stage will generally be two small C.D.'s or a single progressive twin-choke

which will only require needle or jet changes for Stage 1.

Stage 2. Increase in volumetric efficiency at this point will call for an increase in size of C.D.

carbs. e.g. 1300cc engine running on 2 x 1.25 inch S.U.s will need to move up to 1.5 inch units. Or

will need a large choke version of twin-choke DD carb.). Also will need a change of air cleaners to

free-flow type. At this stage single choke injection systems can be considered as a possible

alternative.

Stage 3. Further volumetric efficiency improvements now create a "grey area" in which well-

engineered twin carbs, or single choke injection, will still do the required job, but the move toward

one choke per cylinder must be seriously considered. Cylinder sizes up to 400 cc will require a

40mm carburettor with 30 to 34 mm chokes fitted e.g. Ford 2000cc four cylinder will need 2 x

48DCOE Weber or DHLA Dellorto.

Stage 4. One choke per cylinder is now essential to fully justify other engine modifications.

Available choices are Weber DCOE, Dellorto DHLA, Amal, Keihin or Mikuni smooth bores.

Carburettor sizes will need to be as follows for various cylinder sizes:

250cc 40mm with 32-34mm choke or injection bore.

400cc 45mm with 36-40mm choke or injection bore.

500cc 48-52 with 40-46mm choke or injection bore.

Stage 5/6 As above except that we now move into the area where individual engine build

specifications will dictate precise intake breathing requirements, and it is no longer possible to

predict general sizes.

Only dynamometer testing of the engine build combinations will produce the most effective results

from carburetion or injection variants.

At best, initial carburetion choice can only be a compromise generally suited to the user's overall

requirements and will eventually have to be adjusted accordingly.

Genuine designers and suppliers of good high performance equipment will be able to advise you on

jet settings for your individual needs.

Beware of buying cheap unrelated tuning parts; nobody will be prepared to advise you on the

tuning details needed to complete a satisfactory job.


Page 14

FUEL INJECTION, THE ALTERNATIVE

The term "fuel-injection" should really only describe the process of injection directly into the

combustion chamber, in the same way that a diesel engine functions.

The last petrol engine of any note to do this, was the Mercedes 300SL and generally, the design

was discarded due to impracticality and high cost. That engine also used desmodromic valve gear,

but more of that later.

However, direct injection has recently been introduced in a small range of modern cars.

Generally, present day injection systems squirt fuel into the inlet manifold or cylinder head close to

the inlet valve. They are really just another form of carburetion, classified under the expression

"pressurised fuel metering systems", and of course, are now all controlled by microprocessor

engine management systems.

All injection systems work on the same basic principle. A series of signals from the engine are

monitored and used to produce a squirt of fuel of the correct size at the correct time. The important

signals that need to be measured are:

1. Engine speed - sensed electronically by a transducer on the crankshaft.

2. Throttle position - measured electronically and indicating the driver's power call-off from the

engine at any time, e.g. the sudden change from one eighth open to full open indicates driver

requirement for sudden acceleration and therefore mixture richness is fed in accordingly.

3. Engine air consumption or volumetric efficiency. A big thorn in the side of injection equipment

designers. Measured electronically now, but on earlier systems, measured mechanically by use of

manifold pressure sensing devices or by a 'floating' air bell in the induction tract. This is very

sensitive but can be subject to problems of dirt deposits causing 'sticky' operation. It is also

difficult to modify when the engine is up rated because it is critical in design and in itself offers an

obstruction to clean airflow.

Various other controls are introduced depending on the sophistication of the system or the

individual requirements of the manufacturer. Such devices as atmospheric pressure, temperature

and exhaust gas analysis sensors, feed signals to a microprocessor, which in turn, digitally instructs

the metering system to correct the fuel flow accordingly.

But, regardless of the level of the additional sophistication, all systems have to work around the

three basic control parameters listed above.

Most injection systems are purpose-designed for the vehicle to which they are fitted and are

difficult to re-adapt for other engines.

Systems used on formula 1 and 2 cars are available in component form to be used on any engine,

but the responsibility for adapting and fitting, include determination of the computer control

software to suit fuel requirements lies with the customer, a formidable task for anyone who does

not have expensive test equipment.

Bosch were far and away the world leaders in supply of electronic fuel injection systems or

technology licensing, but an increasing number of other manufacturers are now offering

alternatives, many based on the Bosch technology.

Although their then "state of the art" systems, fitted to high performance market leaders like

B.M.W. and Mercedes were based on the fully electronic "L" system, other then current models

like the 4/4 Ford Sierra, still used the out of date mechanical "K" system, though at the expense of

poor fuel consumption and high emission levels and maintenance costs.


Page 15

Carburettor manufacturers Weber, started to offer retro-fit fuel injection system, programmably

adjustable with a PC desktop computer, and incorporating an electronic ignition system controlled

from the same processor.

A low-cost alternative system designed to be fitted at manufacturing stage, was developed in the

Piper workshops over a period of a few years, and was made available as an electronic multi-point

carburetion system for high volume production vehicles.

The table below lists the variations of driving conditions and the resulting demands put on any fuel

metering system, whether carburetion or injection.

Driving Throttle Engine Volumetric Fuel

Condition Position Speed Efficiency Requirement

Cold start Closed to Low Poor Rich

15%

Hot start 15% Low Poor Weak

Cruise at 70

motorway flat 20% Med/high Poor Weak

or downhill

Flat out flat

road 100% High Good Med.

Accelerate on

steep hill 50 - 100% Low Poor Med.

Dragster start

from lights 100% Low Poor Rich

Cruise at 70

slight uphill 60% Med Med Med.

Shut throttle

suddenly from Closed High Poor Near zero

150 mph

The difficulty of meeting all these possible combinations makes the carburettor look good value for

money and it is.

However the requirements of greater economy, together with reduced exhaust emission levels, were

necessitating the change to a much greater level of sophistication in carburettor design and were

inevitably leading us into the era of electronic carburetion.

In racing, this had been the case since the early seventies.

Further discussion of electronic fuel systems may be found in the section on electronic engine

management.


Page 16

INLET PORTS AND VALVES

The transition of the manifold from the

carburettor or injection air control body

to the port and valve should be as a

gradually reducing cross-sectional area

from the bell mouth at the atmospheric

end of the tract, right through to the

intake valve head (Fig. 43), in order to

ensure that efficient maximum gas

velocity is achieved.

This is around 400ft/sec. (122

metres/sec.) and should be achieved as

close to the valve as possible.

In the case of the four-stroke, this will be just upstream of the valve guide boss where the port must

start opening out to reduce velocities in the throat and around the valve head.

For the two-stroke, maximum velocity can occur at the piston skirt face.

Having calculated the correct minimum throat diameter, the intake tract should progressively

converge to that size from the bell mouth, at a rate of approximately 4mms in 100, (4%) or an

included angle of 2-3 degs.

This taper in the intake tract is to compensate for the gas drag that occurs in any flow system, and

that would otherwise tend to restrict the effective cross-sectional area.

Minimum intake throat diameter, related to cylinder capacity and RPM.

Sq Mms/Litre/Cylinder

1 sq. in. = 645 sq. mms

So if you have a 500cc

cylinder capacity and the

engine is tuned to produce

max. power at

7000 rpm, then the

minimum port

section at

the throat should be

1800/2 = 900 sq. mms.

= 1.39 sq. ins.

1.4 sq. ins. area = 1.33 ins. dia.

43


Page 17

When we refer to the valve, we mean of course the poppet valve in the four-stroke engine as

opposed to the sleeve valve, long ago discarded in four-stroke engines, but still retained by the two-

stroke in the form of the piston skirt opening and closing the cylinder ports.

It is a great pity that the efficient flow characteristics of the poppet valve are denied the two-stroke,

due to the difficulty of driving them at crankshaft speed.

Nevertheless, the facts are

unavoidable: a well

designed inlet port and

valve (Fig. 45) achieves a

discharge coefficient

approaching 100%, whilst

the sleeve valve of the

two-stroke (Fig. 46) only

reaches about 80% at its

most efficient.

The discharge coefficient is the ratio of the amount of gas that will pass, compared to its size.

There are many publications illustrating the detailed cylinder head modifications that should be

carried out on specific models of engine, so I do not intend to waste time and space by re - covering

old ground here.

However, there are some vital signs and rules that should be observed, regardless of the hardware

that is being worked on.

Gas flow within the intake system, particularly around

the valve head and stem, is highly complex and almost

impossible to visually depict other than in an animated

diagram.

Every irregularity in the flow path causes vortex

shedding to occur (Fig. 47), which effectively reduces

the efficiency of the system.

Vortex shedding is the effect of tiny swirls of gas being

generated at every sudden change of direction or size.

When re-working an intake system, the simplest

approach is to imagine yourself as the slug of intake gas,

moving along the tract.

Each obstruction or change of section that would upset

or irritate your path will be equally disturbing to the

inlet charge flow.

All inlet tract joints should be accurately matched, i.e. carb to manifold and manifold to port,

including gaskets.

Every stepped joint will cause turbulence in the stream and will sap valuable energy from the

ingoing charge.

45

46

47


Page 18

All corners should be radiussed to remove

potential flow interruption, but not at the cost

of removing too much metal and losing the

correct shape for example, in (Fig. 48)

careless removal of metal at "A" will result

in a port flow that is so bad it would have

been better left as standard.

The black area shows the correct

modification.

Inlet valve guide bosses can be smoothed

and streamlined but the valve stem is

unalterably round and will eventually control

the turbulent pattern in the throat.

In some ports there is a case for removing the inlet guide and boss right back to the port wall,

providing there is sufficient guide length left to fully support the valve and hold it square on the

seat.

Areas adjacent to the inlet valve seat should be radiussed to create a smooth blend from port wall to

valve seat and from seat to combustion chamber roof. Careful attention in this area alone can raise

the flow coefficient by over 10%.

Similarly, the shape of the valve head is of great importance and, although true individual valve

head shape can only be determined by knowing the characteristic of the port, as a general rule of

thumb, "flat" approach ports, as used in Mini A-series engines work best with "penny on a stick"

valve heads, while downdraught ports give best flow with a "tulip" or spherically backed valve

head.

Highly developed racing engines invariably have steeply down-draughted intake ports to assist the

inlet flow across the valve head.

Although the general design for these engines is the pent-roofed, four valve layout, the Yamaha

FZ750 used 5 valves per cylinder and had even experimented with seven valves, in the interests of

improving the intake flow process.

Two-stroke inlet valves.

Two-stroke piston skirts can be re-worked to ensure that they fully clear the top of the port and, if it

has any downdraughting, should be chamfered to match.

Although the sharp edges can be broken to assist flow at small openings, radiussing should not be

carried out because this will make the port timing unstable.

Inlet valve timing.

Four-stroke.

Average production engine timing duration is from 240 to 260 degrees, with opening points

varying from 5 degrees to 25 degrees before T.D.C. and closing from 40 degrees to 55 degrees after

B.D.C.

Valve lifts range from 8.0mm for a 250cc cylinder to 10.0mm for a 500cc cylinder. These engines

will be giving about 60 Bhp/L and produce maximum power between 5200 and 5800 rpm.

A

48


Page 19

If we increase the level of tune in "stages", then these characteristics will alter as indicated in the

chart below.

Stage Application Inlet V Timing Valve lift (mms) Potential

BTDC ATDC 250cyl 500cyl Bhp/L

Std. Std. Saloon 20 50 7.0 10.0 50

St 1 Improved std. 30 60 9.0 10.5 62

St 2 Rally M/cross

Grasstrack 40 70 9.5 11.0 80

St 3 Race 1 50 80 10.0 11.5 95

Race 2 58 88 10.0 11.5 105

SPL Super/ch Drag 62 92 10.0 12.0 150

SPL Turbo/ch Race 45 85 10.0 12.5 150

Specially developed racing four-strokes running high boost pressures, and ultra-high revving two-

strokes will develop in excess of 200 Bhp/L.

TWO - STROKE PORT TIMING

Because the piston uncovers the ports equally on its up and down strokes, it has symmetrical

timing unless disc or other forms of rotary valve timing control are added.

These methods of control invariably increase the cost and complexity of the engine, which is a pity

because the two-stroke does respond to asymmetric changes in inlet timing.

Whilst, in general, slight changes in inlet opening do not give power improvements, the same

changes in inlet closing certainly do. Therefore, while it is necessary to lengthen inlet timing to

increase power and rpm, the low speed power losses are enormous, hence the need for a lot of gear

ratios.

This effect can be offset slightly by the use of reed valves in a piston ported engine, but, whilst they

increase low and mid-range power, they lose a certain amount of top end power due to the flow

restriction and the fact that, even though the reed is very light, it still has inertia which has to be

overcome by the inlet stream. This in turn means that the small amount of energy required to move

the reeds must be taken from the inlet stream which therefore loses some of it's high speed ram

effect.

Previous reference has been made to the fact that the loop/scavenge or conventional crankcase

induction two-stroke is really a supercharged engine because it sucks the gas into the crankcase and

then blows it into the combustion chamber.

Why, therefore, doesn't it produce as much power as an externally supercharged engine?

The reason is that the petrol/air mixture is fouled, either by its own intrinsic oil content or by being

exposed to injected hot oil being sprayed around the crankcase.

It is also pre-heated by thermal transfer, from the surrounding hot components, further heated by

compression through the transfer ports before being subjected to combustion.

All this means that the combustion process is not as efficient as a four-stroke but, because it

happens twice as often, it can still produce good power.

Approximate changes in staged tuning of the inlet are shown in this table.

Application Inlet duration RPM Potential

Open Close at BHP/L

BTDC ATDC max.

power

Std Street 70 70 7000 80-90


Page 20

Stage 1 78 78 8500 100-120

Stage 2 90 90 12000 150-250

The choice between piston-valved and disc-valved

two-stroke racing engines is still argued out and

successful machinery is divided into both categories.

The successful Yamaha's (Fig. 51) retained piston

valves as did most of the motocross bikes (Fig. 52),

but tended to combine reed valves as well.

On the other hand the RG 500 Suzuki and KH250

Kawasaki (Fig. 53), got their results using rotary disc

valves.

51

52

53


Page 21

CHAPTER 3.

COMBUSTION - THE POWER STROKE.

Four-stroke and two-stroke characteristics re-unite during the process of combustion.

In both cases, a fast efficient burn is THE major key to high power output, and the factors that

control it are common to both.

As the piston rises, the mixture is compressed and consequently undergoes a rise in temperature.

As the cylinder pressure rises to about 13.6 bar (13.6 x Standard Atmospheric Pressure), which will

occur somewhere between 20 and 40 degs. before T.D.C., the plug fires and the combustion

process starts.

Once again, it must be stressed.....

IT IS A PROCESS OF BURNING! - IT IS NOT AN EXPLOSION!

An explosion is the result of uncontrollable detonation, which does occur in the internal

combustion engine occasionally and, if allowed to continue unchecked, produces disastrous results.

As combustion continues, the cylinder pressure rises to between 54 to 68 bar (Fig. 54), to create the

driving force on the piston crown.

This maximum pressure varies with engine design, but a good thumb rule is that it will be about

one hundred times the compression ratio.

54


Page 22

The efficiency with which combustion takes place is controlled by a number of important factors

that readily respond to adjustment or modification and are equally applicable in both two- and four-

stroke engines.

They are as follows :

Swirl - Control of movement of the inlet mixture as it enters the combustion chamber, as a result of

the correct shaping or reshaping of the inlet port and valve, or the transfer port.

This results in a turbulent pattern within the inlet charge, which in turn creates a pre-mixing effect

on the richer and weaker portions of the charge and assists even burning.

Fig. 55 shows an example of perfect swirl control imprinted in the light carbon on the piston crown

of a well-modified engine.

Fig 55a shows a graphical example of the same effect, as it would be created by the correct shaping

of a two-stroke port layout.

In engines with two valves per cylinder, swirl will take place around a vertical axis, forming

layered pancakes or doughnuts of mixing gas.

In engines with four valves, the swirl is created around a horizontal axis forming wave-like rolls of

mixing gas.

In either case, the object is to induce a gas movement that holds the fuel vapour in suspension with

the air and packs it evenly into the cylinder space, ready for compression.

55 55a

56


Page 23

Combustion chamber shape and finish - A neat, minimum combustion chamber area, ensures that

flame-spread is rapid and progressive, permitting the ignition process to be delayed as long as

possible, i.e. a minimum of spark advance. (See section on ignition control)

This is one area in which the two-stroke engine really scores, because

its' lack of valve head intrusion in the combustion chamber means

that the chamber can be of near perfect, small part spherical design,

with the plug in the centre. (Fig. 57)

In all cases combustion chamber surface finish should be smooth and

highly polished to reflect heat back into the chamber and to retard the

build-up of carbon related deposits that can cause detonation or run-

on.

Having selected the desired compression ratio, chamber volumes should be balanced to within half

a c.c. between all cylinders.

Squish - The final compressive shock received by the charge, immediately before the start of

combustion.

Created by close proximity of certain areas of the piston

crown to the cylinder face at T.D.C. (Fig. 58), squish is

used to drive the compressing charge into the most suitable

area for start of combustion.

The squish gap should be as small as is practically

possible, and can be as little as 0.5 mm or less, depending

on crank and con-rod stretch at high R.P.M.

Remember that the "inertia weight"... that is the effective

weight of the piston and connecting rod small end at high

engine RPM, is enough to stretch the upper limit of piston

crown travel by more than 0.3mm at high speed.

Good squish

control allows

mixture that

otherwise may

be trapped in the

"dead areas" of

the combustion

chamber, to be

driven towards

the combustion

chamber centre,

and closer to the

ignition point.

57

58

59


Page 24

Compression ratio - The ratio of the swept volume of the cylinder to the compressed volume of the

cylinder.

Calculated as follows:

Swept volume + Clearance volume

Compression ratio = ------------------------------------------

Clearance volume

Compression ratios vary from 8 to 1, used for standard engines, up to 12 to 1 for petrol burning

race engines and 15 to 1 for alcohol burning race engines.

Compression ratios for two-stroke engines are often calculated using trapped volume above the

exhaust port, instead of true swept volume. This means that C.R. varies with exhaust timing and,

calculated on this basis, the results are much lower and vary from 7 to 1 up to 9.5 to 1.

Measuring Compression Ratio.

This apparently simple task is often carried out incorrectly, to the considerable detriment of the

maximum power output.

If the engine is a twin cam 2 or 4 valve layout, the combustion chamber will usually finish up as a

part spherical or pent-roof shape. (Fig. 59)

The piston may have a slight rise on the crown and may also have valve clearance pockets, and the

flat area above the top ring land, will probably not reach the top of the bore at TDC.

Cylinder head gaskets will vary in thickness and valve heads will protrude into the chamber.

This complexity of layout often means that the simplest way to check the compression ratio, is to

set one cylinder to TDC, smear a thin seal of grease around the top ring land, then build the

cylinder head into place in the normal manner and torque it down.

Then, if the spark plug is not upright, tilt the engine until the plug axis is vertical and, using a

pipette or measuring cylinder, fill the chamber with water until it is half way up the plug thread.

Remember, when using water as the measuring liquid, to allow for the meniscus curve caused by

the surface tension.

That is always measure to the top or bottom of the curve. It doesn't matter which as long as you

are consistent.

This should have then fully filled the combustion

chamber and allowed for the plug body volume, but just

to be sure, use a piece of thin wire to agitate the liquid

through the plug hole, to help shift or burst any air

bubbles that may still be trapped. (Fig. 60)

This method should give you a fairly accurate measure

of the true clearance volume, and of course, the swept

volume is the bore area times the stroke.

If you decide to measure the head and block volumes

separately, then make sure you allow for the volume of

the compressed head gasket as well.

This can typically represent one complete ratio on a

"square" bore/stroke ratio 400 c.c. cylinder.

60


Page 25

FUELS.

The type of fuel in use obviously heavily influences the combustion process.

As previously mentioned, power is derived from the conversion of heat, and fuel is merely neatly

packaged heat.

The amount of heat (and consequently power energy) in a particular type of fuel, is quantified as its'

"calorific value".

Just as consumed calories from food, release body heat, so consumed calories from fuel release

combustion heat.

One calorie is the amount of heat required to raise one gram of water one degree centigrade.

Straight petrol (gasoline), regardless of octane level, has a calorific value of around 44

KiloJoules/Kg or 19000 British Thermal Units/Lb.

However, in order for a fuel to give out its' maximum energy potential, it must also be resistant to

knock.

Knock or detonation is the uncontrolled process of combustion that occurs due to the presence of

trapped gas in hotspots.

So although fuel additives don't actually increase a fuels' calorific value, they do enable it to deliver

more of its' potential by holding off the onset of knock, thus allowing suitable amounts of spark

advance to be used for maximum power.

In effect therefore, the fuel additive is giving an energy or heat conversion boost.

As well as boosting effective heat content by reducing spark advance knock, fuel additives can also

enhance the power output of a particular fuel by allowing the use of higher compression ratios.

Table of fuel properties :

Effective Octane

Energy Value Rating

2 - star pump petrol 80 92

4 - star pump petrol 100 97

Unleaded petrol 95 94

Avgas aviation petrol 110 104

4 - star with fuel booster 108 102

Methanol 118 110

4 - star/Nitromethane 120 100

4 - star/Nitrous Oxide 120 97


Page 26

CHAPTER 4.

THE EXHAUST SYSTEM.

Four stroke engines.

The four-stroke exhaust system consists of considerably more than the visible external manifold

and pipe, and starts at the exhaust valve.

The valve itself needs to be much heavier in design at the back of the head than the inlet valve, in

order to cope with the hot and scouring exhaust blast.

Do not be tempted to remove any of the protruding valve guide.

The valve is continuously struggling to shed its heat, and relies on contact with the guide and seat

for this process.

For the same reason, the seat width must always be greater than for inlet valves and should be

between 2.0 and 2.5 mm, depending on cylinder and valve size.

Modifications to the exhaust port should consist of increasing the bowl area around the stem and

guide, and then a smooth blend to the chosen pipe size.

As far as shape is concerned, a short straight section blended into an updraughted port gives the

best flow/diameter ratio. (Fig. 61)

Just as with the inlet port, radiussing sharp corners will also improve flow.

Effect of the exhaust cam.

When fitting a full-race cam, the valve lift at T.D.C. is considerably increased.

A 400cc cylinder, in standard trim, will have about 1.0mm of lift at overlap T.D.C.

When a full-race cam is fitted, this will increase to around 5.0mm. This happens to both inlet and

exhaust valves, so valve to piston clearance and valve to valve clearance is reduced considerably.

One way to alleviate this problem is to pocket the exhaust valve head. (Fig. 62).

Unlike the inlet, the flow capability of the exhaust actually improves with partial masking.

61

62


Page 27

This phenomenon presents us with the convenient option of recessing the exhaust valve to improve

valve-to-valve clearance.

However this procedure also results in the exhaust valve stem being too long for the valve spring

fitted length, or too close to the cam lobe to allow suitable clearance adjustment.

This problem is dealt with further in Chapter 8.

Exhaust pipe length and size.

As the exhaust valve opens, a positive or pressure wave front is created which travels down the

exhaust pipe at the speed of sound.

As this pressure wave reaches the end

of the pipe, it expands and a negative

or suction pulse travels back up the

pipe towards the engine. As the

negative wave front in turn reaches the

cylinder, it reverses again and moves

back towards the end of the pipe.

This fluctuating pressure pulse effect

can be used to great advantage in

tuning the engine.

If the system is designed in such a way

that the negative or suction pulses return to the cylinder at overlap T.D.C., then they will assist in

clearing the combustion chamber of exhaust gases.

In turn, this will cause a depression at the inlet valve, which will help draw in the inlet charge.

Coupling the pipes of multi-cylinder engines will also mean that the pulse effects from one cylinder

can be used to assist the breathing of another.

The following formula can be used to calculate the ideal length for a given application:

129540 x E.T.

L = ----------------

R.P.M. x 6

Where:

L = Primary pipe length in mms measured from the exhaust valve head.

E.T. = Exhaust valve duration in degrees from point of valve opening before B.D.C plus the full

180 degree stroke up to T.D.C.

R.P.M. = The estimated revs, at which max. power will be achieved minus five hundred.

Example:

Exhaust timing = 80 B.B.D.C. to 50 A.T.D.C. Estimated maximum power R.P.M. = 7200

E.T. = 80 + 180 = 260

R.P.M. will be 7200 - 500 = 6700

Therefore :

129540 x 260

Primary pipe length = --------------- = 837 mms. or 32 ins.

6700 x 6

Having calculated the primary pipe length, we must now calculate the diameter as follows :


Page 28

Divide "L" by 10 to bring it to cms. Call this "L2". (83.7)

Take the cylinder capacity in ccs and double it. (Say 400 x 2 = 800)

Divide by "L2" as previously calculated. (800 / 83.7 = 9.56)

Divide by 3.4 (9.56 / 3.4 = 2.8)

Find the square root (2.8 = 1.67) Multiply by two and add 0.3 ((1.67 x 2) + 0.3 = 3.64)

Multiply by 10 to bring it back to mms. (10 x 3.64 = 36.4) 36.4mms = 1.43ins

This will give the O.D. of the tube in which at first sight will appear rather small.

This is because it assumes the use of a perfectly smooth straight pipe, which is impractical to use,

so the following allowances must be made.

To allow for the viscous drag created in the bends used in an "average" primary pipe and also to

allow for the slight pipe flattening that takes place at the bends, increase the internal cross-

sectional area by 10-15%, depending on how tortuous the system is.

This will probably finish up as a pipe size that is non-standard, so go for the nearest available stock

diameter above this figure.

Remember that "L" is from the exhaust valve head, so the exhaust port length will have to be

deducted to get the actual manufacturing length.

This will then give the joining point of the primary pipes.

From this point, the secondary or tailpipe length can be "L" or any multiple of "L" and its diameter

can be calculated using the method above, but by starting off with four times the cylinder capacity

for a four cylinder engine, or three times for a "six".

For maximum power development, "fours" should always finish up in a single tailpipe (Fig 64),

while "sixes" should finish up with twin pipes, one of which couples cylinder numbers 1,2 and 3,

the other coupling numbers 4, 5 and 6.

For street use, fours can also be designed with a secondary pipe set (Fig. 65) which, although not

giving quite the same maximum power, gives a much broader spread of power.

The secondary pipes need to equal or be a multiple of L, with the next stock diameter up on the

primary.

64 65


Page 29

Production V8s with 90 degree cranks should ideally have a crossover system (Fig. 66), but this is

usually impractical, in which case they can be treated as two "fours", but will need a balance pipe

linking the tailpipes at a tuned length, calculated as above.

Racing V8s with 180 degree cranks can be correctly treated as two "fours" with no balance pipe

necessary (Fig. 67).

If a silencer is used, expansion will take place at this point, so the start of the chamber should occur

at a "tuned" tailpipe length, with additional tailpipe added to clear exhaust gas as necessary.

Dragsters, using only wide-open throttle, requiring

little or no progression, will typically use open

stubs to give the highest, shortest band of power.

(Fig 68)

The following chart is only intended as a guide to typical characteristics and requirements.

Individual engine applications will vary with engine characteristics and slight changes will be

needed to achieve maximum performance.

Exhaust valve timings for turbocharged and supercharged engines are dealt with in the relevant

section, later in the manual.

Exhaust timing selection chart

Exhaust Exhaust Av.max.

Application opens closes power E.T.

B.B.D.C. A.T.D.C. R.P.M.

Standard Engine 50 20 5500 230

Stage 1 Street 65 30 6200 245

66 67

68


Page 30

Stage 2 Rally, M/Cross 75-80 40-50 6800 260

Stage 3 Advanced Rally

and Motor Cross 82 54 7400 262

Stage 4 Full Circuit Race 88 56 8000 268

Dragster 90+ 60+ 8000+ 270+

Two-stroke engines.

The pressure pulse reversal as described earlier also takes place in the two-stroke exhaust system

and can be used to much greater effect if correctly manipulated.

This is due to the fact that the whole breathing process is dependant on a transfer of pressure from

one area to another and is not positively valve controlled as it is in the four stroke.

The rules of good design are not nearly as easy to define, and best results are ultimately only

achieved by exhaustive dynamometer testing.

Even then the results obtained will not apply to another engine if there is any slight variation in

timing.

Multi-cylinder coupling is not mathematically feasible except with three cylinder configurations

and even then, the improvement comes only in the mid-range, often very useful to spread the torque

of highly tuned engines.

In order to obtain ultimate power from multi-cylinder two-stroke engines, it is necessary that they

be treated as a group of single cylinders.

This often creates great difficulty in

accommodating the mass of snake-like

hardware. (Fig. 70)

The separate geometric components of the exhaust system are laid out in Fig. 71 and function in the

following manner :

(A) Primary Pipe.

Often a parallel tube, particularly in cheaper production road machines, but ideally a tapered

primary pipe should be used to control the expansion rate of the high speed gas slug ejected

from the port and to convert its kinetic energy into pressure energy.

70


Page 31

(B) Primary Divergent Cone.

Controls initial expansion of the pressure pulse and is often combined with :

(C) Secondary Divergent Cone.

Which finally controls the pressure pulse expansion to

induce the negative pulse, which travels back to the port

to help scavenge the cylinder.

(D) Expansion Chamber.

Length acts as a time control before throttling of the gas

slug which starts at :

(E) Convergent Cone.

Which throttles down the slug to the :

(F) Tail Pipe.

The size of which controls the high back pressure reverse

"plug", which in turn pushes the overspill of intake charge

back into the cylinder before the piston shuts the door.

The whole sequence, using correctly designed

components, will result in a cylinder filling efficiency of

more than 100% at the "tuned" engine speed.

True design formulae for these systems are highly

complex and still not quite fully understood, but outlined

below is a simplified starting point for those who want to

have a go themselves :

41910 x TD

Length "G" in mms = ----------

R.P.M.

Where TD is transfer port duration in degrees.

R.P.M. is desired "on pipe" R.P.M.

41910 x ED

Length "H" in mms = ----------

R.P.M.

Where "ED" is exhaust port duration in degrees.

41910 x (TD + C)

Length "J" in mms = ---------------

R.P.M.

Where "C" will control the length of the parallel section

and lengthen time before reverse plug starts.

Find this empirically by starting at 0 and increasing in intervals of 5.

"De" will be around 2.25 x Piston diameter for high revs and 2.0 x Piston diameter for torque.

"Dt" will be around 0.45 x Piston diameter for high revs and 0.5 x Piston diameter for torque.

71


Page 32

Exhaust emissions.

Although not an important consideration in Europe when tuning for performance, exhaust

emission control is becoming a major factor of engine design consideration.

When the air/fuel mixture burns in the combustion chamber, a number of chemical changes

take place that result in the pungent and easily recognised exhaust fumes emitted from the tail

pipe.

These consist of harmless gases like oxygen and carbon dioxide, and the deadly gases like

nitrous oxides, carbon monoxide and hydrocarbons.

Nitrous oxides are usually the result of high combustion chamber temperatures, so these will

often rise when tuning takes place.

Hydrocarbons are formed when small, trapped pockets of gas are left partially unburnt or

mixtures are run too rich.

Well-tuned engines usually have low hydrocarbon outputs.

It is almost inevitable that, as Californian law already decrees, European exhaust emission

legislation will eventually mean that tuning kits may not be sold unless they conform to the

clean air laws.

Hopefully though, it will not apply to motor sport events.


Page 33

CHAPTER 5.

THE CAMSHAFT.

CAMSHAFT CHOICE.

The camshaft is undoubtedly the most important single component to be selected when tuning the

four-stroke engine.

The correct choice is often difficult to make and this chapter is principally concerned in setting out

the problems and facts, in order that the correct decision can be made for an individual application.

Recognition of the profile.

The cam lobe is made up of four essential elements (Fig.

79). The base circle or clearance circle, often called the

heel, together with the ramp - the flank - and the nose.

The BASE CIRCLE is the area of the cam in which little

or no contact takes place with the cam follower.

The centre of the base circle duration lies at

approximately 180 degrees from the nose centre line and

is the point at which valve clearances are normally set.

The RAMP is the area joining the base circle to flank, and

is designed to take up valve clearance in a controlled manner, immediately prior to the start of

valve lift.

The FLANK lifts the valve train with the spring in compression and accelerates to its maximum

speed.

The NOSE takes over at this point and controls the valve train deceleration until it momentarily

comes to rest at full lift, when the process reverses itself to the lower valve back to its seat, where

the ramp will re-open the clearance.

The whole procedure exerts an enormous strain on the components

involved, sometimes stacking-up contact stresses over the cam

nose as high as 1300 Meganewtons/sq.m. (200 lbs/sq.in.), calling

for a high degree of accuracy in design and manufacture, together

with the need for great care and attention when fitting.

The cam nose stress stack-up is the result of a number of

unavoidable design constrictions.

No matter how tight the engine manufacturing tolerances, there

will always be a small amount of mis-alignment or out-of-square

mating between components, sometimes causing hairline or

pressure point contact. (Fig. 80)

In order to minimise the effects of this possibility, cam lobes are often purposely machined with a

taper to mate with a spherically ground tappet face (Fig. 81), creating an intentional but calculable

high pressure footprint, slightly offset from the tappet centre-line to promote rotation of the tappet,

thus improving overall service life.

79

80


Page 34

This design feature means that individual cam lobes and

tappets bed together after an initial running period, and

should therefore never be allowed to re-mate with other

components during later re-assembly procedures.

Camshafts are made from either cast-iron or steel with the

latter usually recognisable by their smooth forged or turned

finish between the lobes as opposed to the rougher finish of

the former which are left as cast in this area.

In some cases they are also turned between the lobes to

reduce the core size, usually done when an increase in lift

dictates a smaller base circle diameter.

Cast iron shafts made of proferal or K iron, have the

lobes and gears heat treated by flame or induction hardening processes, whereas chill cast

components are hardened in the vital areas during the casting process.

Whatever production method is used, cast-iron cam lobes

finish up with a hardness pattern as in (Fig. 82), that is,

about a quarter of an inch depth of hardness over the nose,

tapering in depth down each flank.

This means that the base circle is usually relatively soft,

which is acceptable because there is little or no load at this

point. Because of this hardness pattern, these cams are

particularly suitable for regrinding; only requiring final

refinishing with a black, oil retaining, phosphate coating.

Steel cams, on the other hand, are case hardened, which means that they finish up with a thin

hardened layer, usually about 1.0mm thick, which is penetrated when the cam is reground.

This necessitates heat treatment or hard facing to regain acceptable hardness after a regrind.

Lobe hardness over the nose should be 50-53 Rockwell C on cast iron and 54-58 Rockwell C on

steel.

CAMSHAFT REGRINDING HOW DOES IT WORK?

A few years ago, by far the majority of European high performance camshafts were produced by

the process of re-profiling the standard cam.

Contrary to commonly held misconceptions, this procedure, if properly engineered, results in a

product that is equal, both in reliability and performance, to the same component made from raw

billet.

Although it would be ideal to make all camshafts from new billets to avoid the transportation

problems of exchange units, this is just not practical in Europe, due to the vast variety of makes and

models, often coupled with the non-availability of unmachined castings from the original

manufacturers and the obvious poor economics of producing special casting to meet the small

demands for one particular model.

This situation doesn't apply in the United States where a relatively small number of billets cover a

very wide range of vehicle.

81

82


Page 35

To many people the process of regrinding a camshaft is a black art, resulting in the inevitable

question :

How can you machine metal away from a cam, yet have it finish up with more duration and lift?

It works like this

In (Fig. 83), the outer contour represents the

original lobe shape and the cam lift is represented

as dimension A, the difference between the base

circle radius and the nose.

The duration of the lift is shown as 120 degrees at

the cam, which would be 240 degrees at the

crankshaft, because the camshaft rotates at half

crank speed.

The inner contour line shows the cam shape after it

has been reground and it can be clearly seen that

the lift has now been increased to the dimension

B, this time the difference between the new basic circle radius and the nose.

At the same time the duration has been increased to 160 degrees at the cam, that is 320 degrees at

the camshaft.

From this diagram it is also now possible to see why the cast iron cam lends itself so readily to

regrinding. The deep hardness pattern over the nose is still fully effective.

As contemporary high efficiency engines are being evolved, the process of regrinding will no

longer be acceptable for a number of reasons.

The increasing use of hydraulic tappets, fitted to automatically adjust running valve clearances to

compensate for changes in engine dimensions due to heating and cooling effects, means that

nominal base circle sizes may not be altered.

In this case, the replacement camshaft will need to be produced from a new billet or casting.

However, restrictions such as slide-in camshaft bearing diameters will often control the overall

height of the cam lobe and thus mean that the new billet cam will have lobes no larger than those of

a re-ground shaft.

This in turn means that the hydraulic followers will have to be

replaced with solid followers and a method of valve clearance

adjustment introduced.

In any circumstances, it is desirable to switch to solid lifter

operation wherever possible for efficient high performance engine

conversion.

For the same reasons, the use of various types of overhead cam

operating systems will have layout geometries that will not allow

significant changes in cam dimensions. (Fig. 84)

The high performance replacements for these will, not only need to

be manufactured from new billets, but will also need to use state of

the art profile design techniques to give the best results.

83

84


Page 36

It is a general, but mistaken impression that it is possible to look at a cam shape and say whether it

is for road or track.

This is clearly illustrated in (Fig. 85), which shows a

selection of racing cam shapes from a variety of

engines.

A is a standard Fiat, B is a racing XK140 Jaguar,

C is a racing Hillman Imp.

D, which most experts would define as a very mild

cam, was the profile used in the contemporary Golf

1.8 GTI racing engines.

E is a standard Triumph Bonneville, F is a

standard Ford 2.0 OHC, G is a racing Honda.

The shape of the lobe is entirely dependent on the

design of the components that work with it and will

vary hugely from engine to engine.

The technical requirements of successful high performance cam design require attention to a

number of simple but vital engine component functions.

The vast majority of production engines have valve train mechanisms that generate symmetrical

motion at the valve. (Figs. 86 & 87)

This is achieved by either directly moving the valve with conventional direct operating cam

technique (Fig. 86a), in which case a symmetrical cam profile imparts a symmetrical valve motion,

or by moving the valve via a rocker mechanism, which imparts symmetrical valve motion by

compensating for the constantly varying rocker ratio by using an asymmetrical cam profile.

However, modern computer design and analysis techniques allow for the development of cams that

impart asymmetrical motion to the valve, thus taking advantage of part of the generous safety

margins that are built into standard engine valve train dynamics.

86

85

87


Page 37

If we study the movement of a valve

during its' opening and closing cycle as

a graph (Fig. 88), then the actual valve

lift is shown as a single smooth curve

on the upper, or positive side of the zero

line.

The velocity is seen as two sharper

positive curves, representing the speed of

the valve as it lifts to full lift, pauses

momentarily, then speeds up once more

as it starts to close. (Fig. 89)

Finally, the acceleration is shown as

positive as the valve accelerates to

maximum velocity, then negative as it

decelerates to rest at full lift, followed by

a reverse cycle of acceleration and

deceleration as it closes.

The design of this acceleration pattern is the point at which all cam profile design starts.

Normally this pattern is symmetrical and is controlled by the maximum deceleration that occurs at

full lift, from which the valve spring dimensions are calculated, the speed at which the valve is

lowered back onto the valve seat in order to avoid valve bounce, and the amount of wind-up that

occurs in the valve train due to flexibility of the components.

This last factor is of course the reason why overhead direct acting valve trains can be made to

operate at far higher speeds than systems using rockers or push-rods, hence the evolution of the

twin OHC engine.

Using computerised design techniques and high precision manufacturing methods, the valve

dynamics can be modified by distorting the symmetry of the acceleration, velocity and lift curves,

to give a greater proportion of valve open, or breathing time, within any given valve duration.

The result of this strategy is that, for any given valve duration (i.e. time of valve leaving then

returning to its' seat), the actual open area through which gases can move is increased.

This means that a relatively short duration cam, giving good low-end performance, can also allow

enough breathing area for high speed volumetric efficiency and consequently increase power.

These poly-dynamic cam profiles represent a significant step towards the all-purpose high

performance camshafts, but it should be remembered that the high valve opening accelerations

impose stresses that are eroding the normal standard production safety limit margins.

88

89


Page 38

CAMSHAFT POSITION AND DRIVE

Referring again to Figs. 86&87, the camshaft and associated valve train component layouts

covering 95% of modern engine designs are shown, together with their applications, virtues and

disadvantages as listed below :

KEY USED BY VIRTUES DISADVANTAGES

(A)

Rover Gp. Single Camshaft Valve size

S.O.H.C. V.W. Audi Simple Drive Restriction due

Direct SAAB Honda Belt or Chain to in-line layout.

Operating many others. Bore size controls sum

of inlet and exhaust

seat diameters.

Ports often too

long.

(B) B.M.W.,Mazda Allows better Spark plug position

S.O.H.C. Colt,Moskvich, valve placing, is restricted due

with BL,Peugeot, still simple to rockers and shafts.

Rockers Porsche, Toyota drive.

Honda, Yamaha,

(C) Mostly Ease of Flexibility and

OHV servicing. weight of a long

industrial train of components.

Pushrod manufacturers.

(D) Datsun (Nissan), Ease of Flexible by normal

SOHC Fiat,Ford, Lada, servicing. OHC standards.

Loose Vauxhall. Choice of valve Excessive overall

follower position. height.

finger.

(E) AlfaRomeo, The only true Costly to manufacture.

Twin Cam.

Aston Martin, racing layout. Difficult to service,

Direct Ferrari, Honda, Allows freedom of but becoming easier

operating. Lancia, Lotus, valve and plug with hydraulically

Maserati, position. adjusting bucket

Jaguar, Toyota, followers.

Suzuki, Kawasaki

Many others.


Page 39

CAMSHAFT INSTALLATION.

As stated earlier, the camshaft is the most highly stressed component in the engine and therefore

requires particular care when being fitted.

The majority of cast-iron cams have a black phosphate coating.

The purpose of this is not, as many people think, a surface hardening process but it is for oil

retention during the early life of the cam. It carries out this function admirably but unfortunately

also retains any dirt that is brought into contact.

Even just handling a camshaft with grubby hands while fitting, can implant enough tiny particles of

grit to seriously shorten its life.

THE RULES OF SUCCESSFUL CAMSHAFT INSTALLATION.

Research indicates that the majority of cams that wear out, start to fail during the first few moments

of operation.

Many cams are irreparably damaged, even before the engine is started, because the basic rules of

camshaft break-in have not been followed.

The cause of premature cam and tappet failure is metal-to-metal contact between the tappet and

cam lobe. Should this contact occur due to lack of proper lubrication, or excessively high pressure

due to valve train interference shearing the oil film, then 'galling' will take place.

When this happens, metal is transferred from the tappet to the cam or vice versa in a process

comparable to welding. Microscopic high spots, which are present on all machined parts, become

overheated due to friction and pressure bond together, tearing sections loose from the tappet or

lobe. These pieces of metal remain attached to the mating part, creating further local overheating

during the following revolutions of the camshaft, leading to ultimate failure of the affected

components.

Listed below are the mistakes that lead to premature failure:

1.Inadequate lubrication during the initial rotation of the camshaft with full spring load applied.

2.Interference in the valve train due to improper installation and failure to check for interference.

Valve spring coil boxing, spring collar to guide contact, valve to valve contact and valve to piston

contact are the main problems.

3.Installation of used tappets with a new camshaft.

No matter how good tappets look, new tappets must be used with a new camshaft!!!

Beware of reclaimed tappets; they are usually ground flat whereas geometric relationships between

cam and follower, often involve the use of a tapered cam lobe working with a spherically radiussed

tappet foot.

Fig. 81 (above), shows an exaggerated view of this condition, which essentially comprises a cam

lobe taper of around 6-10 minutes! That's about 0.025mm over 13mm and a spherical radius on the

tappet foot of 1500-2500mm!

The centreline of the tappet is also offset from the lobe centreline by about 1.0mm.

4. Water, petrol or other contaminant in the oil that can lower film strength, or create abrasion.

5.Excessively long cranking on the starter. Oil will not reach cam lobes until engine is running.


Page 40

6.Low idle speeds during break-in. Cam lobes in pushrod engines usually depend on all thrown

from con-rods for lubrication. Oil delivery will not be sufficient at idle.

A word of warning !

Cam profiles, particularly high performance profiles, have sensitive areas of dimensional tolerance

at the point where the flank joins the ramp, and the contour over the nose.

Even at the point of original manufacture from the high precision master cam, small but acceptable

contour discrepancies occur.

However, any attempt to further copy the camshaft will degrade the profile contour, just as

videotape degrades with copying.

In order to avoid the problems that this causes in the valve train dynamics, together with the

subsequent reductions in performance and reliability, you should confirm that your camshaft

supplier is the original designer and production originator, rather than a machine shop specialising

in producing copies of other manufacturers' original designs.

Reputable camshaft manufacturers and suppliers will themselves take several other precautions by:

1. Supplying cam profile designs that are not overloaded or highly stressed.

2. Provide cams with the correct machined finish.

3. Phosphate or otherwise treat cams to assist oil retention.

4. Supply or recommend special oil for assembly.

The mechanic handling the installation bears the greatest responsibility for break-in of the

camshaft.

The following outlined steps will help ensure long and trouble-free life from the camshaft and

associated components :

1. Coat the cam lobes and cam face of the tappet with lubricant.

If a proprietary cam lube containing Zinc-Dio-Thio-Phosphate (ZDTP), like Piper Cam Lube, is

not available, then an E.P. 140 or 90 Hypoid rear axle oil is the next best alternative.

2. Check entire valve train for interference before attempting to start engine, and particularly

check that the cam eccentricity or wipe path across the follower, does not run off the edges.

High velocity cams will wipe across a much wider face path than standard cams.

3.Set pushrod engine valve clearances 0.003in to 0.005in smaller than specified for initial start-

up.

4.Before starting any engines, prime the oil by turning oil pump manually. Fill carburettor with

petrol, fill radiator and ensure correct ignition timing. Engine must start right away and not be

subjected to a long grind on the starter.

5.Do not idle engine during the first twenty minutes of operation.

Rpm should be kept at 2500 or above.

In pushrod engines oil throw-off from the crank may not be sufficient to lubricate the cam

followers. Also contact stresses at the nose of the cam are very high at low speed. Engines

may be run in the shop or on the road or strip. If adjustments need to be made during the

twenty minutes break- in period, shut the engine down. DO NOT LET IT IDLE!


Page 41

If the engine is dismantled for repair, maintenance or inspection, after any running at all, it is

important that the tappets be kept in order.

Each tappet will have mated to a cam lobe and swapping tappets may cause failure.

CAMSHAFT - SELECTING, CHECKING AND SETTING TIMING

r.j. gayler - bg tuning manual

Documents