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06-27-2006, 06:33 PM #1
BarryKCF Senior Member
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Info on Timing - long post!
Hi all
I am constantly seeing threads on here from people asking about timing questions. Theyseem to range from the most basic “how do I set my timing” to what should my timing beset at”, “I took my car to a mechanic and they re-set my timing and now the car has nopower”, “should my vacuum advance be on ported or full manifold source”, “do I need myvacuum advance”, and many others all related to timing.
Recently I’ve been attempting to answer a lot of these threads myself and came up withthe idea of this post that hopefully maybe the moderators could turn into a sticky so it’salways there to help others.
** DISCLAIMER:
Under NO circumstances whatsoever do I consider myself an authority or an expert ontiming. I leave that to people like Lars and JohnZ.
As a matter of fact, my experience working on cars started relatively recently but forgood reason. After buying my first Vette 2 ½ years ago I depended on taking it tomechanics for everything it needed. I quickly found out that each and every time I tookit to a mechanic it ran worst than before I gave it to them to work on. Most mechanicsanymore do not know how to work on vintage motors, especially older distributors suchas points style distributors, and most have NO understanding or concept of the vacuumadvance system.
I had to make a decision – learn how to do it myself or sell the car. Since I wasn’t willing
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I had to make a decision – learn how to do it myself or sell the car. Since I wasn’t willingto sell the car, I bought the tools I needed and proceeded to learn how to do it.
Believe me, this isn’t rocket science and if an idiot like myself can do this you can do ittoo!
I will attempt to explain as best as I can the steps you need to take to set the timing onyour own car so you do not need to depend on mechanics to do this relatively simpletask yourself.
••CREDITS:
I learned 95% of what I know about timing and the vacuum advance system from readinga number of tech articles. The last 5% was from asking questions here on the forumwhich was mainly to help clarify certain points that I couldn’t get through my thick skullbut was in these tech articles already.
All credit for the information in this post belongs to the authors of those articles, not me.Any mistakes that may be in this post belongs to me and me alone, not the sources Ipulled the information from. .
••RESOURCES:
The tech articles that I’ve pulled most information from are the following four articlesthat the authors generously allow me to host up on my website’s tech articles page. ISTRONGLY recommend that anyone that has questions regarding timing download andread these articles.The articles are:
Timing101article by John HinckleyMappingadvance by John HinckleyTiming for peak Performance by Lars GrimsrudVacuum Advance Specs by Lars Grimsrud
These tech articles can be found here on my website: http://www.lbfun.com/Corvette/Tech/vettetech.htmlClick on the Timing and Vacuum Advance section**************************************** ******
First, what is timing and why is it important for us to be concerned about it?
Timing is not a constant unchanging thing. As we drive a car it creates a continualchanging load on the motor which requires a different timing advance to allow the motorto work efficiently. At idle or steady cruising such as going a constant 50mph down thehighway the motor is working much differently and has different requirements than whenyou are accelerating, going up a hill, or running at WOT (wide open throttle).
These different and ever changing demands on the motor all require a change in thespark timing for the motor to run efficiently and well. This is the job of the distributor totake care of. As long as the distributor is working properly and is set up correctly it doesits job invisibly as we drive. If it isn’t set up properly than we can run into all kings ofissues from poor idle, weak throttle response, hotter operating temps, and poor fueleconomy to as bad as detonation that can destroy your motor.
*Before setting the timing it’s important to understand WHY timing changes underdifferent driving conditions and loads on the engine.
At idle or under light loads such as easy cruising the motor is working very easily and thefuel/air mixture is LEAN. A lean mixture (more air, less fuel) burns slowly.
As the load on the motor increase such as during acceleration the fuel /air mixturebecomes more RICH. A rich mixture (more fuel, less air) burns more quickly.
This difference in the burn rate of the varying mixtures is the ultimate key to timing soit’s important for you to remember this.
The objective of timing and the distributor is to fire off each sparkplug at precisely thecorrect time during the combustion stroke. Because a lean mixture burns slowly, it needsto be “lit” by the spark earlier but a rich mixture that burns quicker needs to be “lit” bythe spark later in the compression stroke.
When setting up the timing on your car there are basically four main areas we want toconcern ourselves with. INITIAL TIMING, TOTAL TIMING, CENTRIFUGAL ADVANCE CURVE,and VACUUM ADVANCE.
INITIAL TIMING: this is the timing set for when the car is at idle. At idle the car is at alean fuel/air mixture so the timing is set less advanced. This is also referred to as “basetiming”.
CENTRIFIGAL ADVANCE: This is controlled by weights and springs inside the distributorunderneath the rotor. This will advance spark timing based only on engine speed. As theengine speeds up the distributor will advance the timing. As the engine slows down thedistributor will retard the timing.
The operation of this is very simple. As engine speeds increase the centrifugal force ofthe distributor shaft turning causes the weights to “swing out”. The weights are retainedin place by springs. The weaker the spring is the easier the weights can swing out at a
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in place by springs. The weaker the spring is the easier the weights can swing out at aslower speed. The stiffer the springs are the higher the speed the engine must be toallow the weights to swing out and advance the timing.
TOTAL TIMING: the base or initial timing PLUS the full amount of the centrifugal advanceequals the total timing. Although all aspects of timing are important, total timing is thesingle most important aspect that you need to be concerned about. When you readposts that people say “set timing to 36º” it’s the total timing they are referring to.
VACUUM ADVANCE: The vacuum advance system is a vacuum canister that runs offvacuum from the motor that adds more advance to the spark timing. While thecentrifugal advance is solely based on engine speed, the vacuum advance is based onengine LOAD. A typical vacuum advance canister adds approx 15º of advance over andabove what the centrifugal advance adds into the timing. Although the two systems arecompletely different they work together to provide the proper amount of timing for the
motor at any given time.
How does the vacuum advance system work by engine load? At idle or steady cruisingthe motor is under very light load and produces high vacuum. That vacuum causes thevacuum canister to pull against the distributor’s breaker plate and add more advance tothe timing. Under acceleration the motor loses vacuum and at WOT throttle basically hasno vacuum at all so there is nothing to cause the vacuum canister to pull on the breakerplate to add advance. The vacuum advance system adds advance to the timing underlight loads and very little to no advance under heavier loads when it’s not needed.Remember, under heavier loads the motor has a rich fuel/air mixture burn burns morequickly so it needs to get “lit” later in the compression stroke, not sooner.
ADVANCE CURVE: This is going to be a VERY simplistic explanation compared to what thetrue “curve” really is on each distributor but I think it will suffice for our purposes here.For our use, what we need to be concerned with on the “curve” is WHEN the advancecomes in (how early) and when it’s “ALL IN”. “ALL IN” refers to when the centrifugaladvance advances all that it can and no matter how much higher the motor speedincreases there is no more advance available.
Typically, the stock set-ups had a relatively long curve so the advance wasn’t all in untilyou reached redline on the motor or close to it. This was set up this way from thefactory for a simple reason – it was safe, and the factory didn’t want cars coming backwith damage to the motor that needed service – especially while the cars were still underwarranty. Unfortunately this set-up on the curve also left a lot of performance on thetable.
One of the simplest ways to gain some performance in our cars is to re-set the curve sothat the timing is all in by around 2,500-3000rpm’s. This way we get full advance in therange of engine speeds that we normally drive in. We can adjust this to bring in theadvance sooner by installing “weaker” springs on the advance weights.
We want the centrifugal advance to come all in by around 2500-3000 rpm’s but at thesame time we don’t want it to come in too early either while the car is still only at idlespeed.
**************************************** ***********
SETTING TIMING:Finally, we are ready to set the timing on our car.
Tools required:
Timing light (I strongly recommend a “dial-back” timing light as it makes the job mucheasier)Dwell meter (not needed if you have a later HEI distributor)Vacuum gaugeBasic hand toolsService manual for your car that lists what the factory specs are for timing settings
(if you don’t have a dial-back timing light and are using only a regular standard timinglight than you need to index your balancer with timing degrees on it. See John’s and Larspapers I referenced above for the procedure on how to do this. For $69 it’s easier to runto Sears and get a nice Dial-back model timing light!)
**note: many, many people set their timing simply by setting the distributor at 36º totaltiming and letting everything else “fall where it may”. In my opinion this works fine IF youhave already gone through all the other steps and know EXACTLY what your distributor isdoing and how it’s set up and what the curve it. If you don’t know this than I suggesttaking the extra few minutes and go through the complete procedure from start to finish.When you are done you will not only know what your distributor is doing, but it will giveyou a more thorough understanding of how everything works.
Start your car and warm it up to operating temps. You can’t do this with the car coldand the choke still on.
1. SETTING DWELL:
If you have a points distributor the first thing you want to do is hook up the dwell meterand check and adjust dwell. Dwell is usually set between 28-32. I always set mine at 30.Dwell is set with the car at normal idle speed.
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Dwell is set with the car at normal idle speed.
You adjust and set the dwell with a 1/8” Allen wrench to adjust the screw behind thelittle pull-up window in the front of the distributor cap. Turning the screw one waydecreases the dwell and turning it the other way increases it.
It’s important to check the dwell at varying speeds though after you set it. If you findthat the dwell varies or fluctuates as you increase or decrease the engine speed, thatindicates that the distributor main-shaft has too much endplay and is out of normaltolerances. You will want to pull the distributor out and have it serviced to eliminate thisproblem. Dwell effects timing so if the dwell varies than the timing will also vary which willdefeat a lot of the effort you are going through to correctly set the timing on your car. Once the dwell is set and it does not vary you can proceed.
Timing must be set with the vacuum advance hose disconnected from the vacuumadvance canister on the distributor and plugged. Using a golf tee or a screw to plug itworks great.
Hook up your timing light, making sure to be careful that your wires don’t interfere withmoving parts such as the fanblades or lay on the hot exhaust manifolds.
(HINT: on C3’s where the battery is back behind the seats the power cord will not usuallyreach so it’s easy to get power for the light off the alternator connections.)
2. SETTING INITIAL (Base) TIMING.
If your distributor has not been serviced recently the springs on the weights for thecentrifugal advance may have weakens allowing the centrifugal advance to come in tooearly, even at idle speed. Too properly check and set initial timing we need to make surethat the centrifugal advance is not affecting initial timing so a good method to use is toremove the distributor cap and put a rubber band around the weights so they don’t moveand start adding in centrifugal advance. Install the rubber band for this and reinstall thedist. cap. Adjust your idle for the lowest idle speed you can.
Now check your initial timing at this idle speed. With a dial-back timing light it’s as easyas pointing it to the balancer and turning the dial on the light until the line on thebalancer aligns to the “0” mark on the index plate.
Depending on your year car, what motor you have, etc your initial timing will vary but forthe sake of this procedure I’ll use the timing of my ’65 327/365hp motor. My ’78 L82 hascompletely different timing specs.
In this case the initial timing is suppose to be at 12º. If your initial timing is different thanwhat the manual calls for loosen the hold-down bolt on the distributor and rotate thedistributor until you get the proper amount of initial timing.
Now, remove the distrib cap, remove the rubber band, reinstall the distrib cap, readjustthe idle speed to the correct idle speed your car calls for and recheck your initial timing.If it’s different than it was than your centrifugal advance is coming in too soon. If thereading is the same as it was when you set it you are good to go. If your centrifugaladvance is coming in too soon you will want to replace the springs to that the advancedoesn’t start until the motor is above idle rpm level. Write down or remember this initial timing number.
3. SETTING TOTAL TIMING & CENTRIFUGAL ADVANCE:
Please read John Hinckley’s tech article on Mapping Advance to get more details on thisstep, as I’m going to simplify things greatly although you will still get the general idea ofhow to do it.
What we want to do here is set the TOTAL timing and also check out how muchcentrifugal advance the distributor is giving us.
Set the dial on your timing light to 36º and watch the balancer and index plate timingmarks. Gradually increase engine rpms (by pulling on the accelerator rod or linkage off thecarb). You should be able to start to see the timing line on the balancer move towardsthe mark on the index plate. Keep increasing engine speed until the line stops moving.Once it’s stopped moving and increasing engine speed any more doesn’t make it moveany further the timing is “all in”. CAUTION – if the distributor still has the stock springs init it’s very possible that you need to rev the motor up to redline or even beyond to getthe timing all in. You do not want to do this for obvious reasons. I would not exceed5,000 rpm’s under any circumstances while you are working under the hood and probablyeven less than that. If the timing is not all in by than, plan on changing the springs tobring the timing in sooner. You want the centrifugal timing to be all in by around 2,500-3,000 rpm’s. If the line on the balancer aligns to the “0” mark on the index plate with the timing all inthan your timing is set at 36º (remember, we set the dial on the timing light to 36). If thelines don’t align than while keeping the revs up to a point that the advance stays all inturn the dial on the light until they do align up and read what that number is.
If the number is higher than 38 your timing is excessive and must be turned back. Timingthat is too high is a cause of detonation that can quickly destroy your motor. This meansalso that the distributor is giving you too much centrifugal advance. Usually it’s becausethe advance limiting bushings inside have fallen off or broken off and need to bereplaced. It may also mean that the advance “slot” on the cam plate has worn and isallowing a longer travel on the advance. In the case of my ’65, I found the centrifugaladvance was giving me 32º instead of the correct 24º which was extremely excessiveand reason was someone in the past deliberately made the advance slot longer (an oldhot-rod trick that was done when the fuels in the past allowed higher timing levels).
To correct this I had to have the slot welded up shorter to bring it back to the correct
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To correct this I had to have the slot welded up shorter to bring it back to the correctlength.
If your total timing exceeds 38º you must turn the distributor to retard the timing andreset it for 36º. This will change your initial timing but we can live with that as long asit’s not by too much. If it’s more than a few degrees I’d recommend having the distributor“recurved” so that you get the proper centrifugal advance set up so that BOTH yourtotal timing and your initial timing are at the correct levels. If you must make a “choice”for right now, set the total timing to 36º and let the initial “fall where it may”.
Now, you may ask, why I just said that if total timing doesn’t come in correctly with theinitial timing at specs why set it for total and let the initial timing fall as it may when atthe beginning of all this I recommended against that? It’s simple. Initial timing isimportant, but total timing is MORE important. At least by going thru all these steps youare seeing exactly what your distributor is going and how it’s set up. If the centrifugaladvance isn’t 100% correct to allow you the correct settings on both initial and totaltiming, at least set it for a correct total timing FOR NOW and you at least now know youwill want to work on your distributor to re-do the curve correctly as soon as you can. At this point what do we know? We know we set initial timing at 12º. We know we have36º on total timing. Take the 36º and subtract the initial timing of 12º and we now alsoknow that the distributor is giving us 24º of centrifugal advance. On my car the 12ºinitial, 24º centrifugal advance, and 36º total timing is dead on perfect.
Once your car is set with the total timing take it out for a test drive (yes, the vacuumadvance is still unhooked and plugged).
Check very carefully for any pinging or knocking. Especially good is to get the car in ahigher gear such as 4th gear at a low rpm such as 1200-1500 rpm and accelerate, thisplaces a heavy load on the motor and if it will knock or ping it will do it than. If you don’thear any detonation you are fine but if you do you must retard back the timing 2* at atime until it stops. Detonation must not be allowed at it will destroy your pistons.
4. VACUUM ADVANCE: Now that the total timing is set, unplug and reconnect the vacuum hose to the vacuum
can on the distributor. You should immediately see an increase in idle speed so now is thetime to readjust the idle speed back to it’s normal level, Depending on the motor,transmission, etc different vacuum cans give a different amount of advance but onaverage you will see about a 15º increase in timing advance once the can isreconnected. Check timing at idle now to confirm this. You should see a new timing levelthat equals: initial timing as set previously PLUS approx 15º advance from your vacuumadvance. Check what can you have (they all have a number stamped on them on thebar) against the specs listed in Lars paper Vac Adv Spec.pdf to see just how much thecan you have is giving you. (important, the specs list the vac, can advance amounts isDISTRIBUTOR degrees so you need to double that amount to get the correct number ofcrank degrees . If the can is listed at supplying 8º that means it’s really adding 16º ofadvance to the timing.
On my car the initial timing is 12º PLUS the vacuum can adds another 16º for a new initialtiming of 28º at idle with the vacuum can connected.
Next check total timing with the vacuum can connected. It should be 36º PLUS whateveramount your vac. can supplies. On my car it would be 36º + 16º from the can = 52º.
No matter what, you do NOT want to exceed 52º (total plus vacuum advance) or againyour timing will be too high and detonation becomes an issue.
Now that you see your vacuum advance is working take the car out for another testdrive. Again make sure you aren't hearing knocking or pinging plus feel carefully that thecar isn’t giving you a surging, chugging, or jerking feeling. If it is your vacuum advanceadding too much in over and above the centrifugal advance and needs to be lowered.
** Note: for more details on various vacuum cans, their specs, and requirements, readLars paper Vac Adv Specs.pf that I listed early on.
••• note on vacuum source: older cars such as my ’65 used a full manifold vacuumsource off the carb to run the vacuum advance. What this means is that at idle the carbsupplied full motor vacuum to the vacuum can. Later cars switched to what is called“ported” or “timed” vacuum. The difference is where the vacuum is picked up on thecarb. Full manifold vacuum is picked up BELOW the throttle blades on the carb so italways gets the full vacuum the motor produces. The ported vacuum is drawn off thecarb higher up ABOVE the throttle blades. This makes a large difference since at idle fullmanifold vacuum gives full vacuum but a ported vacuum port will give NO vacuum at idlebut than will work the same as a full manifold vacuum source after the throttle blades arecracked open when driving.
If your car is a later car and is set up to run off a ported vacuum source you aretypically better off switching it to a full manifold vacuum source port on the carb. If youcarb does not have a full manifold vacuum port it's possible to pick it up directly off theintake manifold. Many manifolds have a hole already drilled into them for various vacuumpurposes that are simply pluged and you can get a proper fitting to feed vacuum frommany of the Corvette parts suppliers such as Paragon, etc.
The ONLY reason they switched to a ported vacuum source was because of emissionsrequirements. One way they lowered hydrocarbon emissions was the A.I.R. system and tomake that work properly they needed a very retarded spark at idle. Since a Portedvacuum gives no advance at idle this worked perfect. The problem is that while a ported
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vacuum gives no advance at idle this worked perfect. The problem is that while a portedvacuum helps reduce emissions it’s lousy for performance!
Switching from a ported vacuum to a full manifold vacuum source will typically give youbetter idle characteristics, better throttle response, cooler operating temps, and betterfuel economy.
BASIC CARB TUNING:
The last thing we really want to do is make sure the carb is set up for the proper
mixture. First make sure your idle speed is set correctly.
Than hook up your vacuum gauge to a full manifold vacuum source and check it at idlespeed.
To set your fuel/air mixture you want to set it so that you achieve the highest steadyvacuum reading you can get.
I start by turning the mixture screws all the way in (do NOT turn them in tight you candamage the seats – just turn them in until they very gently hit so you know they arecompletely closed) than turn each one out 1 ½ turns.
Starting with one screw turn it until you get a higher reading and let the motor stabilizefor about 15-20 secs before doing the same on the other screw. Slowly go back andforth between both screws until you get the highest vacuum reading you can. When you are done, make sure both screws are pretty much set equally.
There, you are done!!
This may take a while the first time you do it but after doing it once or twice you shouldbe able to go thru the entire procedure in about 15 minutes or so.
The best part is that you now not only have your timing set correctly, you know exactlywhat your distributor is doing, you know it’s set-up and curved properly, and youprobably just saved $75-$150 that a mechanic would have charged you to do it and inMY experience you just did it better than most mechanics you would find.
Now that you have gone thru the entire procedure and you know your distributor is setup and curved properly, in the future it will also be much easier. Since you knoweverything is set up right, in the future all you will typically need to do to set your timingis make sure you are at 36º total timing with the vacuum can disconnected and plugged.Knowing how the distributor is set, and making sure the timing is at 36º you will alreadyknow where your initial timing is going to fall and that it’s in the correct range. That willtake you all of 2 or 3 minutes now and it’s easy to do.
I hope this has helped some of you with your timing questions and if you are someonethat depends on mechanics to do all of the work on your car I especially hope it helpsyou decide to start doing at least this simple aspect of work on the car yourself!
In some areas I simplified things to try to keep this post from being even longer than isalready became so again I strongly encourage you to read the four tech articles Ireferenced at the beginning to get a better understanding of everything.
Once again all the credit for this information goes to John Hinckley and LarsGrimsrud. Any mistakes are mine alone.
Barry
__________________
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Last edited by BarryK; 06-23-2008 at 08:37 PM.
06-27-2006, 08:23 PM #2
VT-tuna
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VT-tunaJunior Member
Garage is empty, add now My Corvette PhotosMember Since: May 2006
That was outstanding. I learned quite a bit and I'm one of those idiots that thoughtI already knew all that.Thanks.
06-27-2006, 09:26 PM #3
MuddywatersCF Senior Member
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My Corvette PhotosMember Since: Jan 2005Location: Salinas Ca.
Great thread... Needless to say, I downloaded all papers on your site!! By the
way, very nice site.
06-27-2006, 09:51 PM #4
desiCF Senior Member
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My Corvette PhotosMember Since: Sep 2002Location: Engineers do itbetter.
This couldn't have come at a better time. I find the information very helpful.
Thanks
06-28-2006, 09:55 AM #5
sperkinsCF Senior Member
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My Corvette PhotosMember Since: Oct 2005Location: Macon, GA
Nice sticky Barry. Looks like George came through for you. I'm sure this will help alot of folks.
06-28-2006, 10:38 AM #6
SIXFOOTERCF Senior Member
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Nice job Barry
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06-28-2006, 01:28 PM #7
pws69CF Senior Member
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Nice post, BUT, I disagree with this statement:
The ported vacuum is drawn off the carb higher up ABOVE the throttle blades.
It still comes from below the throttle plates, but it IS (as your dissertation states)switched.
Vacuum ABOVE the throttle plates is HIGH at WOT - period.
06-28-2006, 02:58 PM #8
BarryK
CF Senior Member
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My Corvette PhotosMember Since: Apr 2004Location: Newark DE
Quote:
Originally Posted by pws69Nice post, BUT, I disagree with this statement:
The ported vacuum is drawn off the carb higher up ABOVE the throttleblades.
It still comes from below the throttle plates, but it IS (as your dissertationstates) switched.
Vacuum ABOVE the throttle plates is HIGH at WOT - period.
Care to explain your reasoning?
To clarify, I'm NOT referring to Venturi vacuum which would be higher at WOT, butrather, simple intake vacuum.At WOT the motor is producing no vacuum with the throttle blades open so how areyou reasoning that vacuum is high above the throttle blades at WOT? There is NOvacuum (intake vacuum) at WOT.
A Ported or Timed vacuum source, which is located above the throttle blades,operates the same as a full manifold vacuum source except at idle when it shows novacuum - crack the blades open slightly and get nigh vacuum as as the blades openmore the vacuum decreases until you get to WOT at which point there is novacuum.
Ventura vacuum on the other hand does increase at WOT simply because you haveincreased airflow inside the carb. Venturi vacuum is completely independent andseperate from intake vacuum. Intake vacuum is based on engine load and venturivacuum is based on airflow. There is not even a connection availabe to try to utilize venturi vacuum and it's notrelevent to our discussion on timing.
If you would like to understand more on vacuum and this distrinction between intakevacuum and venturi vacuum, an excellent paper is another one by Lars also on mywebsite tech pages and it's called : Vacuum_Explained.pdf
06-28-2006, 03:20 PM #9
pws69CF Senior Member
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Quote:
Originally Posted by BarryKCare to explain your reasoning?
To clarify, I'm NOT referring to Venturi vacuum which would be higher atWOT, but rather, simple intake vacuum.At WOT the motor is producing no vacuum with the throttle blades open sohow are you reasoning that vacuum is high above the throttle blades atWOT? There is NO vacuum (intake vacuum) at WOT.
A Ported or Timed vacuum source, which is located above the throttle
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A Ported or Timed vacuum source, which is located above the throttleblades, operates the same as a full manifold vacuum source except at idlewhen it shows no vacuum - crack the blades open slightly and get nigh
vacuum as as the blades open more the vacuum decreases until you get toWOT at which point there is no vacuum.
Ventura vacuum on the other hand does increase at WOT simply becauseyou have increased airflow inside the carb. Venturi vacuum is completelyindependent and seperate from intake vacuum. Intake vacuum is based onengine load and venturi vacuum is based on airflow.
There is not even a connection availabe to try to utilize venturi vacuumand it's not relevent to our discussion on timing.
If you would like to understand more on vacuum and this distrinctionbetween intake vacuum and venturi vacuum, an excellent paper is anotherone by Lars also on my website tech pages and it's called :Vacuum_Explained.pdf
I fully understand the terminology and the priciples.
The statement is wrong, as are some of the points you made above - I simply don'twant people to be mislead and I really am not interested in arguing about it.
06-28-2006, 03:26 PM #10
BarryKCF Senior Member
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Quote:
Originally Posted by pws69I fully understand the terminology and the priciples.
The statement is wrong, as are some of the points you made above - Isimply don't want people to be mislead and I really am not interested inarguing about it.
I'm not arguing at all, I was asking you a legitiment question on your reasoning howintake vacuum is high at WOT. Unless you can explain to me how intake vacuum is high at WOT, which goescontrary to everything I've ever learned and read, and which goes against theentire theory of how full manifold and ported vacuum ports operate, I stand by mystatement.
I hope he won't mind me quoting him, but here is a direct quote from Lars from oneof his tech papers. It was used in the context of explaining the differences betweenfull manifold and ported vacuum but it covers the exact thing we are nowdiscussing."At both cruise and Wide Open Throttle (WOT), manifold vacuum and portedvacuum are exactly the same: There is high vacuum at cruise, and virtually novacuum at WOT. The difference in vacuum occurs only at idle."
John Hinckley states the same in his tech papers also.Both are people that are/were engineers for GM and are true experts in thisinformation.
I stated at the beginning of the initial post that I am not an expert so if you can
explain how you calculate that vacuum is high at WOT I'd be most interested in
learning but as mentioned that goes against everything I've ever learned or hasbeen stated by people such as Lars and John
Last edited by BarryK; 06-28-2006 at 03:36 PM.
06-28-2006, 05:09 PM #11
pws69CF Senior Member
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Quote:
Originally Posted by BarryKI'm not arguing at all, I was asking you a legitiment question on yourreasoning how intake vacuum is high at WOT. Unless you can explain to me how intake vacuum is high at WOT, whichgoes contrary to everything I've ever learned and read, and which goesagainst the entire theory of how full manifold and ported vacuum portsoperate, I stand by my statement.
3/10/13 Info on Timing - long post! - Corvette Forum
forums.corvetteforum.com/c3-tech-performance/1430339-info-on-timing-long-post.html 10/14
My Corvette PhotosMember Since: Nov 2001Location: Marlborough MA
operate, I stand by my statement.
I hope he won't mind me quoting him, but here is a direct quote from Larsfrom one of his tech papers. It was used in the context of explaining thedifferences between full manifold and ported vacuum but it covers theexact thing we are now discussing."At both cruise and Wide Open Throttle (WOT), manifold vacuum andported vacuum are exactly the same: There is high vacuum at cruise,and virtually no vacuum at WOT. The difference in vacuum occurs only at idle."
John Hinckley states the same in his tech papers also.Both are people that are/were engineers for GM and are true experts inthis information.
I stated at the beginning of the initial post that I am not an expert so ifyou can explain how you calculate that vacuum is high at WOT I'd be mostinterested in learning but as mentioned that goes against everything I'veever learned or has been stated by people such as Lars and John
My point is that MANIFOLD VACUUM and PORTED VACUUM come from the SAMESOURCE - and that is the MANIFOLD and it is BELOW the throttle plates - NOTabove the throttle plates. Perhaps you are confused by the fact that the NIPPLE forported vacuum is physically located HIGHER on the carb than the throttle plates -but the actual channel routes BELOW the throttle plates - with a "valve" if you will,that opens immediately "off idle". It is "black and white" - Venturi vacuum above thethrottle plates, manifold vacuum below the throttle plates - period!
The quoted statement above (bold, italics) completely supports my point!
Look, I am not trying to be an azzhole or blow you away or anything like that - youhave done an outstanding job and your post is a GREAT resource for people who aretrying to understand all this. However, this IS a very important point for people tounderstand correctly because behavior of MANIFOLD vs. VENTURI is essentiallyopposite (as you so eloquently have stated!). That's it - no hidden agenda here!
06-28-2006, 05:31 PM #12
larsCF Senior Member
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Quote:
Originally Posted by pws69My point is that MANIFOLD VACUUM and PORTED VACUUM come from theSAME SOURCE - and that is the MANIFOLD and it is BELOW the throttleplates - NOT above the throttle plates. - but the actual channel routesBELOW the throttle plates - with a "valve" if you will, that opensimmediately "off idle".
That's not correct. Ported vacuum is pulled from a hole located just above thethrottle plate when the throttle is closed - flip any carb with ported vacuum upside-down, and you can see the ported vacuum hole drilled in the throttle bore justabove the throttle plate. The ported vacuum hole is not exposed to manifoldvacuum when the throttle is closed. As the throttle is opened, the ported vacuumhole is exposed to manifold vacuum - the throttle plate itself acts as the switch -there is no separate "switch."
Last edited by lars; 06-28-2006 at 10:58 PM.
06-28-2006, 05:35 PM #13
BarryKCF Senior Member
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Quote:
Originally Posted by pws69My point is that MANIFOLD VACUUM and PORTED VACUUM come from theSAME SOURCE - and that is the MANIFOLD and it is BELOW the throttleplates - NOT above the throttle plates. Perhaps you are confused by thefact that the NIPPLE for ported vacuum is physically located HIGHER on thecarb than the throttle plates - but the actual channel routes BELOW thethrottle plates - with a "valve" if you will, that opens immediately "off idle".It is "black and white" - Venturi vacuum above the throttle plates, manifoldvacuum below the throttle plates - period!
3/10/13 Info on Timing - long post! - Corvette Forum
forums.corvetteforum.com/c3-tech-performance/1430339-info-on-timing-long-post.html 11/14
Actually, to be more precise, vacuum is created by the piston's downstroke and thethrottle blades in the carb. The blades create a resitriction in the air flow. If theblades are closed they create more of a restriction in the "leak" of air so morevacuum is created and when they are opened they create less of a restriction soless vacuum is created.
I believe the confussion seems to be from my use of the word "Source". Excuse meif the terminology was confusing, I used the word source to try to simplify matters,not to make them more confusing. By the word souce when referring to either fullmanifold vacuum or ported manifold vacuum I was referring to which nippleconnection someone would utlilize to draw vacuum off of for use with the vacuumcan on the distributor. Full manifold vacuum is typically located below the throttleplates and ported vacuum connection is located above the throttle plates - it's thatreason that at idle the ported vacuum connection has no vacuum but it does oncethe throttle plates are cracked open. My usage of terminology was meant to make it easier for others to follow, not
meant to confuse them - more laymans terms if you will...... for beginners I felt it
would be easier for them to follow the procedure this way and for more experiencedpeople I assumed they would have understood what I meant.
Regardless, I'm still interested in your statement that manifold vacuum is HIGH atWOT.............. Unless we have another terminology confusion issue here I can notfollow how that can be.
06-28-2006, 05:37 PM #14
BarryKCF Senior Member
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ahhhh, Lars has responded faster than I did and with a better explanation. Nosurprise there!
06-28-2006, 06:16 PM #15
pws69CF Senior Member
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Quote:
Originally Posted by larsThat's not correct. Ported vacuum is pulled from a hole located just abovethe throttle plate when the throttle is closed - flip any carb with portedvacuum upside-down, and you can see the ported vacuum hole drilled inthe throttle bore just above the throttle plate. The ported vacuum hole isnot exposed to manifold vacuum when the throttle is closed. As thethrottle is opened, the ported vacuum hole is exposed to manifold vacuum- the throttle plate itself acts as the switch. No - the ported and manifoldvacuum do not come from the same source. And there is no "valve" thatopens off-idle - that's complete nonsense.
OUCH - you are harsh, Lars. I did say "if you will".....
Well, I stand corrected. I did not think that hole was for the ported vacuum -thought it was related to the idle circuit. I understand the principle, just went downthe wrong hole!
Thanks again, Lars for clearing up my misunderstanding!
Barry - I owe you a cold one!
06-28-2006, 06:21 PM #16
pws69CF Senior Member
Quote:
3/10/13 Info on Timing - long post! - Corvette Forum
forums.corvetteforum.com/c3-tech-performance/1430339-info-on-timing-long-post.html 12/14
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Quote:
Originally Posted by BarryKRegardless, I'm still interested in your statement that manifold vacuum isHIGH at WOT.............. Unless we have another terminology confusionissue here I can not follow how that can be.
BTW, I didn't say that!
I said: "Vacuum ABOVE the throttle plates is HIGH at WOT"
If I said it in some other place, it's bunk and I owe you 2 cold ones!
06-28-2006, 06:33 PM #17
BarryKCF Senior Member
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Quote:
Originally Posted by pws69Barry - I owe you a cold one!
no harm, no foul
discussions like this are very helpful as everyone learns from them.The entire thread was meant to be helpful and this just brought up additional pointsthat got clarified so I'm sure it was beneficial.
06-28-2006, 07:26 PM #18
larsCF Senior Member
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Quote:
Originally Posted by pws69OUCH - you are harsh, Lars.
No harshness intended - sorry 'bout that, it did sound a bit brutal, so I edited theresponse and toned it down to where it should have been to start with. I just typefast and short. For a detailed explanation of manifold, ported and venturi vacuum,just drop me an e-mail request for the "Vacuum Explained" paper - it's beenpublished in a couple of professional newsletters, and it's technically accurate. Thepaper goes over in detail how the different vacuum signals are generated and howthey are used.
Last edited by lars; 06-28-2006 at 11:00 PM.
06-28-2006, 07:34 PM #19
BarryKCF Senior Member
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LarsI have the paper up on my website tech articles page here that anyone is welcometo go to:http://lbfun.com/Corvette/Tech/vettetech.htmlThe posted file is called: Vacuum_Explained.pdfit's under the "Timing and Vacuum advance header title
I know you tend to update most of your articles so I hope this version is an up-to-date one.
Last edited by BarryK; 09-20-2008 at 07:16 AM. Reason: corrected outdated link
07-01-2006, 01:21 PM #20
76 stingCF Senior Member
Garage is empty, add nowBarry,
3/10/13 Info on Timing - long post! - Corvette Forum
forums.corvetteforum.com/c3-tech-performance/1430339-info-on-timing-long-post.html 13/14
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Thanks a lot!! In the past I have read all the papers and threads, but usually hadmore questions, but did not want to bother anyone. I have the new MSD E-Curve,which was confussing to me because it is different than the normal disty.
After reading the article above you must have worded things differently because itnow all makes sense. I am going to give this another try before I send it out tosomeone else.
Thanks to all three of you (BK, LG, and JK) for constantly working on an area thatcan greatly impact our cars and how they perform.
I wish you were all closer so I could buy you a beer to say thank you for all yourefforts.
Cheers!
07-01-2006, 01:21 PM
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