Was there a post on what port to use for distributor vacuum advance? If so, should I use vacuum at the base or elsewhere on a Holley carb?

You should use a ported vacuum source. On a Holley, it is usually on the passenger side of the front metering block. Test it to see that it has no, or very little vacuum at idle, and it should show manifold vacuum at part throttle and above.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

you need to use what your disributor's advance curve is set up for.

IMHO ported/timed is not as good as manifold vacuum, if you do not have an EGR. if you have a EGR set up you need to use ported/timed vacuum.

do a search on this subject on this board you will find some very good info on it.

like i said it depend's on what the distributor is set up for, no offence Grape Ape but i feel you can not make a blanket statement like you did. if you use ported/timed on a distributor set up for manifold vacuum it will not perfrome up to it's pontial, and if you use manifold vacuum on a distributor set up for ported it will mostlikely ping and run like crap.

i guess you need to deside what you want to use, i would recomend you set it up for manifold vacuum if you have no EGR or cat on the car.

do some resurch and make a desion for you self. there is alot of info on this on the team camaro site and it has been a hot topic for a long time.

Thanks guys. I don't have a fitting on the metering block on my new Holley HP model 750 Vacuum Secondary carb. Just have the fittings on the base below the fuel bowls. Can't find anyone that has a distributor machine to set up the distributor curve. People around here are afraid of the "smog Police".

No offense taken, I shouldn't have assumed. My personal feeling is that ported workes best for high vacuum engine (14 inches and up). Even without an EGR I don't see the reason for full vauum advance at an idle. Many people used full manifold which helps out when the idle is on the rich side, which is common of aftermarket performance carbs.

Radical cams with low idle vacuum like more advance, so many people use manifold vacuum to help so they don't need to add too much initial and strain the starter.

My personal opinion is to set up the dist for ported vacuum unless you cannot get enough initial advance due to a radical cam. If your car does run better with manifold vaccum and you had plenty of idle vacuum, check the curve in the dist and you're probaly a bit rich.

But that's just my opinion, both ways can work fine. I've had the best luck using ported on mild cams and full manifold vacuum on radical cams.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

Grape Ape don't argue with these guys I had a war going on they believe what they believe. I agree with you except I will never use manifold vacuum even with a big cam you can bring in the advance guick enough to make it work right.

Thanks oger, I've tried many curves, and vacuum advance combinations while dyno testing and I've come to the conclusion that full manifold vacuum is a band aid fix for another problem (usually a mismatched timing curve or an overy rich idle curcuit).

No offense to anyone, there are many opinions and this is just mine from my personal experiences.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

None taken! oger and I have different views but that keeps life interesting. If there was only one way or one type or one paint color it would get boring and all cars would turn the same times in the 1/4 and look the same too...

I think we all agree that cars setup just for the track don't use vacuum advance at all. What is the reason we use vacuum advance on the street? Why have it only come on after starting to accelerate and push it over the maximum mechanical that is built into the dist? And why do stock vacuum canisters add as much as 20deg's of advance?

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...Dennis
'69 RS Convertible w/SS trim (http://www.camaroslimited.com/memberscars/den.htm)
'96 Z28SS #1679 of 2410 (http://www.camaroslimited.com/graphics/memcars/96ss.jpg)
"The Club" (http://camaroslimited.com)

DJD asked,

"I think we all agree that cars setup just for the track don't use vacuum advance at all. What is the reason we use vacuum advance on the street? Why have it only come on after starting to accelerate and push it over the maximum mechanical that is built into the dist? And why do stock vacuum canisters add as much as 20deg's of advance?"

the answer is, they are set up for emision's on very mild cam's. with ported/timed vacuum it is used as part of the timeing curve, that is why most emsion distributor's have a very lazey mechanical advance, they need the vacuum advance to be part of the curve at part throtel and during aceleration.

i am not trying to start anthor big argument, what i have said is a fact. if you have ever worked on emsion's, like i have, or have been certified you would know this, i was certified about 10 year's ago.

alot of people have used manifold vacuum wrong or have not set there disributor's up for it and get very poor result's. i use it on my motor, not to mask a rich idle circit, but because my car run's better with it.

it is a great help when you have a auto tranny and drop it in gear. you can pull agenst the tourque converter better, and have a smother idle while doing it.

if you look at factory can's on motor's that used manifold vacuum you will only see between 10* and 18* of advance at the crank from the can's. with a ported/timed can you will see up too 40* at the crank.

a mechanical advance curve start's around 1000RPM and is all in by 3000RPM when you use manifold vacuum. with ported/timed it start's around 2000RPM and is all in by about 4500RPM. from this you can see how the ported can has to be part of the curve at low motor RPM's and under a load to fill in the nessary advacne to get power and a clean burn.

oh well now there i go again i am starting something i know it! http://www.camaros.net/forum/biggrin.gif i can never stay out of trouble!!!! did that help ya out DJD?

Geez guys, enough! Got the point! Figure initial advance with port or manifold (even though "manifold" can be on the base of Carb) and add centrifugal full to get total advance.

Ken,

There is so much info that gets rehashed over time. It's a good idea to search a few of the forums before posting a question. Even if your question isn't 100% answered you may have a better insight before asking it. This topic is one of those that folks are set in their ways about as is evident here.

ilbl8 - I was kinda hoping to hear from Grape or oger on my questions. http://www.camaros.net/forum/wink.gif

To all - If we can state our ideas and opinions when replying to a question and leave out the comentary about other replys, it will benifit all of us. The person doing the asking can then gleen from the thread what he feels is the way he/she will approach the problem. Somebody looking at things different than you doesn't equate to a war...


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...Dennis
'69 RS Convertible w/SS trim (http://www.camaroslimited.com/memberscars/den.htm)
'96 Z28SS #1679 of 2410 (http://www.camaroslimited.com/graphics/memcars/96ss.jpg)
"The Club" (http://camaroslimited.com)

I can't seem to understand why everyone is hashing about anything but idle. Who cares what total timing is and what the mechanical advance comes in at. A ported vaccum source is manifold vacumm when the port is uncovered, so after that they do the same thing (nothing to argue about). On every carb I've seen this port had vacuum just above idle (when driving). It is not dependent on rpm at all, it is totally dependent on throttle position and manifold vacuum. When cruising, there should be no difference between the two.

PCV's and EGR's are to things that will alter the air/fuel ratio. If you remove these you can band aid fix it by altering timing, but you should bring the carb back into tune. Timing did not change, the fuel ratio did.

The reason why you have vauum advance has nothing to do with emissions at all, you change both vaccum, total and the whole curve for emissions resons.

The vacuum advance is there to compensate for the change in speed of the burn due to a change in dynamic compression ratio. We should all agree that cruising at a steady speed of 1500 rpm will have slow burn rate due to a low dynamic compression ratio, you need more advance for this slow burn or the fuel will still be burning as it exits the exhaust. WOT at 1500 rpm will have a much higher vloumeteric efficiency (more dynamic compression), the speed of the burn will increase, to maintain the best point of peak cylinder pressure (about 20 ATDC), you will need to reduce timing.

Here's the idle part, there is no change in load at an idle, so there is nothing to compensate for. If you change the idle load, you accerlate past idle. You don't need to compensate for that. If you do, your dist is set up wrong and less effecient.

The only thing we should be arguing about is idle, whether you want full vacuum advance or not.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

[This message has been edited by Grape Ape (edited 05-07-2001).]

Grape,

Can we consider it a discussion instead of an arguement? When folks argue they don't listen to what the other is saying and I for one do read and try to apply what others say. I'm inclined to agree about it being about idle but because the tips I have followed and passed on in the past also recomend using an adjustable vacuum canister and limiting the amount of vacuum advance it allows the engine to run a bit more like the performance engines found on the track.

I offer up something to read www.autoweb.lycos.com/garage/subsys/baigni08.htm#Chap8 (http://www.autoweb.lycos.com/garage/subsys/baigni08.htm#Chap8) and will quote as well. <BLOCKQUOTE>quote:</font><HR>On some models, the hose from the vacuum advance is connected directly to the intake manifold, whereas on other models this hose is connected to the carburetor just above the throttle plate.<HR></BLOCKQUOTE> Both can be acceptable! And; <BLOCKQUOTE>quote:</font><HR>If the vacuum advance hose is connected above the throttle plates, the vacuum advance does not provide any spark advance when the engine is idling. When the vacuum advance hose is connected directly into the intake manifold, the vacuum advance is fully advanced at idle speed, which provides lower engine coolant temperature. This action may be necessary during high atmospheric temperatures especially on cars equipped with air conditioning (A/C). <HR></BLOCKQUOTE> so this can be used to reduce engine temps and overcome extra loads placed on an engine. A/C is the example given but it can also apply to an engine that pulls against the converter because of the addition of a performance cam.

Now with all the vacuum timing in at idle when you accelerate the mechanical curve (if setup correctly for this) takes over much like an engine designed for the track. For the street I would think this a much better solution than a full mechanical advance distributor that so many seem to be running at cruise nite...

I didn't not invent this, it seems to work on my 383 and I don't think I am using it to cover up a carb. problem. The one thing I have noticed with this change is the throttle is more responsive under light acceleration and when I switch the vacuum off of the manifold source it is a bit less responsive when leaving from a stop. Thank you for taking the time to read this.

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...Dennis
'69 RS Convertible w/SS trim (http://www.camaroslimited.com/memberscars/den.htm)
'96 Z28SS #1679 of 2410 (http://www.camaroslimited.com/graphics/memcars/96ss.jpg)
"The Club" (http://camaroslimited.com)

[This message has been edited by DjD (edited 05-07-2001).]

I would have to say that if you cannot get enough initial advance to give a good idle and off idle performance without manifld vacuum that there is a problem you are covering up with more timing. I read that article, but if the air fuel ratio is correct and the idle spark advance is set right, that is when the enigne will run the coolest. If the carb is on the rich side, which most performance carbs are, the engine will run hotter, at that point more idle advance will cause the engine to run cooler, but a well set up carb and idle advance will do better.

Can you get a nice smooth idle with no vacuum advance? If you can, and off idle response is poor, the problem is too large idle feed restrictions in the carb causing a rich condition off idle. More timing can make this dissapear, but it's not optimum.

If you cannot get a smooth idle with no vacuum advance, it could be few things. You might need more initial advance, or a leaner idle. Also overcamming can have this problem as well, not that your overcammed, I'm just giving examples. If you need much more than about 14 initial, I'd bet you are rich at idle.

The last time a looked deep into this was dyno testing a tunnel rammed small block street/strip motor about 6 years ago. The first step was to set up the mechanical advance and carb for peak power. That engine liked a lot of initial (typical of tunnel rams), 16 initial and 34 total in by 2700 gave the best power curve at WOT. This car was built for some street use, so a vacuum advance would help. Full manifold vacuum worked with no problems if I richened the idle up some. Originally the idle was rich, but ran fine, after leaning the idle feed restrictions I was able to get it to work with a little less initial advance (16* from 18*). Then I used the ported vauum signal to the advance. This gave the same feel of throttle response and off idle performance was nearly the same (not out of the margin of error of the dyno), but the BSFC #'s were better. This indicates a more efficient set up that would give better fuel economy. Engine operating temps did not really change either way (when the advance was tuned in.

I believe in my testing that I set the distributor and carb up for optimum results for both full and ported vacuum signal.

I believe you that your engine works fine with full manfiold vacuum, but I don't think that it's the most efficient set up.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

[This message has been edited by Grape Ape (edited 05-07-2001).]

Great discussion here!! To all, many thanks. DJD - If I would have found the answer in the older posts I wouldn't have asked the question. That is what this Forum is about, asking questions and talking about the answers received. I belong to three other forums as I own different makes of vehicles. There are all kinds of people and personalities in these forums, however, even the people we perceive as "idiots" smetimes can offer good advise and food for thought. If readers of these posts don't like what is said, they're not stupid and will go on to another question. Grape Ape and ilbl8 have differences in opinion and both are articulate and knowlegible and have alot to offer. If I get bored with this forum, there are always others to go to.

Damn,

I though this thread was going to have something to do with house cleaning! Since my wife left, its gettin messy.....

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Mark

1992 Firebird 355/Six Speed (Soon to be LT1)
1991 RS 350 / 700-R4
1987 Toyota Pickup 383 / 500 + HP 10.963 @ 119.95 Slicks / 11.997 @ 114.23 Radials
"Speed KILLS, so drive a FORD and live forever!"
http://personal.lig.bellsouth.net/~racer383/

Member of the "unwanted" 3rd Gen group.

Ken,

I thought since you orig. asked, "Was there a post on what port to use for distributor vacuum advance?" you might not have known about the search feature. It is suggested to use it because those that have been around send email about seeing the same questions over and over. The administration at Team Camaro is not about to tell a long time member to go else where if they don't like seeing the same questions over and over. It also creates a housekeeping problems that the average new member doesn't think about. The fact that other sites allow something or are administered differently has no bearing on how this site is run. I'm sorry you percieve some people as "idiots"!!

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...Dennis
'69 RS Convertible w/SS trim (http://www.camaroslimited.com/memberscars/den.htm)
'96 Z28SS #1679 of 2410 (http://www.camaroslimited.com/graphics/memcars/96ss.jpg)
"The Club" (http://camaroslimited.com)

Grape, I agree with much of what you say. I thought a lean condition causes and engine to run hotter not a rich one though. I may be incorrect there and will do more research.

We are for the most part non-professionals without expensive dyno's and distributor machines or other tools necessary to tune our cars. A timing light, a tach and maybe a vacuum guage if lucky. And no real formal training on what we do have. The way we are bombarded with commerical hype for the most HP for a buck you do find many of us with over carb'd over cam'd improper torque converter'd combinations. Getting the car to idle in park is one thing but take a less than perfect combo and put it in gear and set it at a stop light. An overall slight rich condition though not optimal is much safer on the engine than a lean one.

You have a great deal of experence under your belt and all your input is appreciated. I look forward to reading more from you and conversing further in the future.


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...Dennis
'69 RS Convertible w/SS trim (http://www.camaroslimited.com/memberscars/den.htm)
'96 Z28SS #1679 of 2410 (http://www.camaroslimited.com/graphics/memcars/96ss.jpg)
"The Club" (http://camaroslimited.com)

Running rich or lean can cause excess heat. The simple short reason is simple. An engines job is to turn heat energy into machanical energy. When an engine is at it's best efficiency, it wastes the least amount of heat (which is still a lot). The most heat is used to make power, and the cooling system and exhaust temps are lower. Run it too rich or too lean, the efficiency goes down, more heat energy is wasted. The cooling system takes more heat from the engine and the exhaust temp is higher for the same power output. This is when you see BSFC #'s go up.

Running lean tends to heat up the cumbustion chambers more, which transferes more to the coolant. Running rich heats up the exhaust valves and ports more, which is tranfered the coolant as well. Lean will heat up the coolant more than the exhaust (it will increase temps of both). Rich will increase the temp of the exhaut more the the coolant.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

Thank you Grape,

I have a better understanding of rich and lean and how it effects the engine.

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...Dennis
'69 RS Convertible w/SS trim (http://www.camaroslimited.com/memberscars/den.htm)
'96 Z28SS #1679 of 2410 (http://www.camaroslimited.com/graphics/memcars/96ss.jpg)
"The Club" (http://camaroslimited.com)

all so a rich condetion will "wash" the clynder's of oil and cause oil burning.

lean allso causes ring over heating and piston over heating.

a ported/timed set up was used originaly on some hi-pro motor's in the 60's as a way to give more advacne to the motor under light load and hi RPM because of overly rich calabrated carb's. the gas was much better then so the extra advance was ok.

in the 70's it was used in what is called a lean burn motor. this is how the maker's passed emision's and fuel econamy standard's.it was allso used to creat "heat" the life blood of the emsions sytem. a cat need's heat to work right and so dose the EGR and air pump.

if you run a motor on the lean side you need to realy watch when the advance come's in and how much. most motor's that use timed/ported advance have a very lean idle and cruse circit in the carb, in fact you need to use propain to set the idle mix. if you use manifold vacuum on this motor with the OEM calabrated carb you will get over heating and pinging.

with a non emision after market performance carb you normaly have slightly rich idle and cruse circit. this is because most performance motor's have a low idle vacuum and that mean's a weak signal to the carb. you need a rich idle circit to work with them.

what grape ape said about masking a rich idle circit can be true if you do not set up every thing corectly, hell with some cam's you have to have a way rich idle or it will not run.

by seting your base timing between 8* to 15* BTDC you get a easy starting motor, add the 10* to 15* from manifold vacuum advance you get the idle timing in the 18* to 32* BTDC area, this is what most small block's like the best by the way it has been proven over and over again by motor builder's and racer's. you will get a stronger signal to the carb and higher idle vacuum reading's.

you see people a carb is just a dumb air fuel flow device. it respond's to motor vacuum and the flow of air. anything you can do to get it a strong signal, air flow or vacuum, make's it work better. all the working princaples used in a carb are over 100 year's old. it is not a hi-tech part, it is realy a stone age part that dose it's job very well normaly.

now distributor advance is a needed part in a good strong running street motor. vacuum advacne is need on a street motor to aid in idle and cruse econamy, part throtel drive ability and to keep the plug's clean. a race motor dose not need it, they spend most of there life above 3000RPM and have a very agresive mechanical advance. a street motor live's most of it's life in the 1500 to 3500RPM zone. there is a big diferance in the the motor's life's.

now here is a fact, untill the advent of emision control's cat's and EGR's all hi pro factory motor's used manifold vacuum, just check out a corvett from the 60's to the early 70's this is a race ready car off the show room floor basicaly ya know.

now to each his own on the advance sceam you use, the carb and the cam and distributor all have to work together. most cam's have a 4* to 7* advance built in to them too, you need to rember this allso.

oh well i am just trying to give you info here not get into a cat fight. i was a ASE master tech in the late 80's early 90's and i learned the trade the hard way, from the ground up. my teacher's where old rodder's and racer's. that had forgoten more about building small block's that GM ever knew, hehehehe.

enjoy!

ilbl8,
When you say that some cams need it way rich or it will not run, that's a good example of too much cam. If the cam is matched to the compression ratio, you should be able to run at 13.5 to 14:1 or leaner just fine. The two biggest mistakes of hot rodders, I'm sure you'll agree, is too much cam and to much carb. I found that most poeple don't really know how to decide what compression to run with a cam. Cams wil have a recommended comp ratio range. Where in that range is right for you depends on your engine. For pump gas, you want a 8:1 compression ratio, not static compression. What you want to do is find the intake valve closing point after TDC. Then when you figure your compression ratio used the swept volume of the cylinder from where the piston is at the intake valve closing point, use the 0.005" lift point, to TDC. This is much more accurate than static #'s. This gives a better comparison of compression ratios for many different cams.

back to the topic, you are right about he old Corvette's using full manifold vacuum. But that was also 40 years ago. Cam technology has come so far since then. If I recall correctly the cams in the hi po Corvettes were in the area of 320 degrees advertised duration, and were only in the 240's duration at 0.050". They had to idle rich to get it to idle. The 0.050" #'s were good for the power they made, but adv duration was way too long. Today you see much better profiles, the advertised and 0.050" #'s are closer together. The last motor I had in my camaro was a 397 cubic inch small-block. It made 475 hp at 6000 rpm, idle at 900 rpm quite smoothly and pulled 16 inches of idle vacuum with 14* initial. I mention it because it is an example of how much better an engine can run with a modern profile. Also, that engine idle in closed loop (14.7:1). That cam had an adv dur of 284/292, dur @ 0.050" was 222/230 with a .509"/.528" lift. It was ground on a 112* lobe separation.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

[This message has been edited by Grape Ape (edited 05-08-2001).]

VACUUME SUCKS!

The HP Vette motors of the 50s and early 60s used no vacuum advance. In the mid to late 60s when smog control started vacuum advance was used on HP motors and it was ported.

joe, yes it dose! i bet you like that! you silly diry boy you! http://www.camaros.net/forum/biggrin.gif

on the corvett's that used ported in the 60's, not all did by the way, the mechanical advance curve is the same as used on a manifold vacuum set up, this was to give the motor better light load and hi-RPM performance as the gas back then could handle the advance better then today.

today's gas like's a cooler intake then the gas in the 60's. cooler motor temp's and lower CR's. there is allso less power per gallon with today's gas then the gas in the 60's. it burn's diferant and react's to heat diferant then 60's gas.

there is a bigger picture then you all are looking at, yes cam's are better now, gas is not. but the prinsipal's of vacuum advacne and mechanical are the same now as then, just the amount you use and when it come's in are the factor's.

with a cast iron head you have to use less advance then with aluminum. air and motor temp are big factor's too.

i am not trying to make a blanket recomendation. just give some fact's about the diferances. you have to take every thing into acount. and match it all up. if you just bolt on this and that odd's are you will not be happy. i have used both manifold and ported on varrious motor's, 99% of the time manifold worked better.

grape ape, on a long duration high overlap cam you need a rich idle to run, no matter what the CR is. this is because of the overlap. this causes dilution of the A/F charge. this is a fact talk to any pro builder, or racer and they will say what i have. you also need more over lap with higher CR's or you will get detonation on pump gas, and even some race gas. this go's for aluminum or cast iron head's.

oh well i am not going to change your mind's, and i am not trying too. just trying to give info is all. rember 99% of the motor's are going to live in the off idle to 4000RPM area. this is not where a race motor live's, they live in the 4000 to 6000+RPM area. what work's on them is not going to work the same on a street motor. it is like trying to get a apple to be an orange, not gona happen.

What HP Vette motor used manifold vacuum in the 60s.

What you said below makes no sense. It is true that high overlap cams need a richer idle, but not as rich as you seem to think and not even close to the reason you say why. Why are you wrong? Because if you dilute the intake charge with exhaust from reversion, you then have less oxygen available to burn, so less fuel can be burned, adding more will do more harm than good. But that is not really the case, so tell me really why a richer idle helps radical cammed engines idle?

<BLOCKQUOTE>quote:</font><HR>Originally posted by ilbl8:
grape ape, on a long duration high overlap cam you need a rich idle to run, no matter what the CR is. this is because of the overlap. this causes dilution of the A/F charge. this is a fact talk to any pro builder, or racer and they will say what i have.<HR></BLOCKQUOTE>

I will post the right answer tommorow if no one else does. The reason you cannot change my mind is from statements like you made above, you state wrong theorys as facts. I'm not saying you're dumb, you seem to be more educated than the average hot rodder, but don't always take the simplest solution, look deeper. All I need to change my mind is proof. If you can't prove it on the board, tell me how to test your theories. I want to see things happen and know why they happen.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

[This message has been edited by Grape Ape (edited 05-08-2001).]

Geez, I didn't want to get back into this discussion again... http://www.camaros.net/forum/biggrin.gif

Theres a lot of good info here...now I am going to give my opinion on the subject. There is no way anyone can make a blanket statement that one is better than the other, period. There are too many variables, too many different combos, and too many different methods of tuning an engine. Personally, I prefer ported vac, and most of the engines I have built ran better with it. I suspect a lot of that is because after I build an engine, I dont leave it alone. I may spend months tuning it to perfection (or my opinion of it)...timing curves, jetting, etc. Not just for best power, but also for best economy and part throttle response. On any engine I have really taken time to fine tune, I have never gotten better performance or economy running manifold vac. As a matter of fact, of all the vehicles I have ever owned, only 2 ran best with manifold vac...one was my old 79 4 door impala with a mildly built 305 and the other just happens to be the 77 nova I currently own. I know why the nova runs better with manifold vac...it runs a bit too rich, but I havent had time to mess with it much since I got it in january. It has a 600 holley vac sec carb, performer intake, headers, brutally loud 3" exhaust, and an old recurved HEI I had laying around (replaced the bad HEI that was in the car). It is a low compression engine...cast dished pistons and 882 heads which makes about 8 to 8.5-1 compression, and it has some sort of mild aftermarket cam in it. It is more responsive part throttle with manifold vac and 14 degrees initial. With more tuning I am almost positive that it will work better with ported vac. The impala had a stock 8.0-1 2 bbl 305 that I did a cheapo rebuild on...fresh 434 heads, rings, bearings, timing chain, oil pump, and gaskets. I did add a cam dynamics 266 cam...single pattern 210@.050/.440 lift. It also had a recurved HEI and dual exhaust. I spent next to no time tuning it...it was a daily beater and that was it. It liked LOTS of initial timing and a very fast advance curve, and it liked ported vaccuum. It got lousy mileage considering the 2.56's in it, but it ran very well for a 305 in a 4000 pound car. The point here is that neither car was jetted properly. The impala had too much cam in it and ran a bit lean, and it had a restrictive intake, carb, and exhaust system on it (manifolds with duals). The nova, on the other hand, is a bit too rich and has too much exhaust system on it. Both cars were low compression and had low cranking pressure (around 130-140 pounds max).
Heres something you guys may or may not know...retarding your timing has about the same effect as richening your jetting, and vice versa. I learned this in advanced auto class back in college from a certified mechanic, race car builder, and instructor...after all these years of tuning I believe it more now than I did then. That is part of the reason why so many aftermarket carb equiped vehicles run so much better with 14 degrees or more initial timing (theres more to it than that)...it is a crutch for the slightly rich jetting that these carbs come with. Ported vac has the same effect while the engine is at idle...it is a crutch for rich jetting, and it works. That doesnt mean it is optimum though, but it works, and it can work well. I have seen many, many times bone stock cars that run manifold vac have a low speed miss to them...its simply too much idle advance. My truck, which I believe i have finally got tuned in to perfection (it wont cover the 1/4 mile any quicker) runs ported vac...when I switch to manifold I get a part throttle stumble. By fattening up the idle mixture a bit and slightly slowing down the advance curve I was able to get rid of the stumble, but I also lost 1 mpg on the highway. If manifold works best for you, great...for the way I tune it just doesnt work best for me.
Dennis, the reason for vac advance is for fuel economy...at part throttle cruise, your engine is running nowhere near max volumetric efficiency. Consider that the formula for figuring the required carb size is calculated for max engine speed and usually figured with a VE of 85-90%. At part throttle cruise, you are running at 50% or less VE. What this means is that you are not getting a cylinder full of air and fuel, more like half a cylinder full (I hope that makes sense) because the throttle blades are not wide open. Also, typical cruise A/F ratios are on the lean side (to promote better economy)...somewhere around 14.5-15.0-1. Your dynamic compression at this point is considerably less than at full throttle, so the mixture burns much slower. Vac advance starts the process earlier so that you get a good complete burn, thus you get maximum power from each combustion cycle, requiring less fuel to keep the vehicle at speed. The vac advance acts as a load sensing device...as the throttle is opened more, the dynamic compression rises, the mixture burns faster, and less initial timing is needed. When the throttle is whacked wide open, you get max compression and the quickest burn rate, so you dont want or need all that vac advance added to your mechanical advance. This same principle is why 14-1 compression engines can be run on the street on pump gas...they can only run at part throttle but they can run...and they have to do it without any vac advance. The reason many cars (especially mid-late 70's) have 18-20 degree vac cans on them is because they use low compression, lean A/F ratios, and poor 76cc chambered heads...all of this slows the combustion process to a point where a lot of vac advance is required for decent part throttle economy and performance.
Moving on...rich A/F ratios do not overheat anything...running a rich A/F ratio has been used for many years as a crutch for engines that are borderline detonation prone...it cools the combustion process, and it also hurts power. The reason is simple...the combustion is weak. It can cause your headers to glow if you have an efficient exhaust system that overscavages (too big of header or exhaust tubing or too much exhaust duration for your application). What overheats parts and plays hell with exhaust valves is running lean. Remember, lean burns slow. This slow burn causes a longer duration of heat to be transfered into exhaust valves, head gaskets, pistons, and exhaust systems. This is one reason so many late 70's/early 80's 305's and 350's have warped exhaust manifolds...nearly all cars and trucks made during this era are calibrated lean out of the factory.
Everything a person does to their engine, no matter how small or insignificant, changes that engines tuning requirements, assuming it was right on in the 1st place. It is physically impossible to get the absolute best timing and jetting for every possible situation...throttle opening, engine load, fuel quality, etc play a part in this. This is why computer controlled, fuel injected cars run so efficiently, but even the best computer, ignition, and fuel delivery system will never deliver optimum results all the time. Too many variables. Everything is a compromise one way or the other.
Ok, I am going to get off my soapbox for now...this post is getting too long and I've got work to do...I've got more to add to this but I'll save it for now. I hope I didnt step on anybodys toes or offend anyone...not trying to add fuel to the fire, just sharing some of the experience and knowledge I have gained over the last 13 years. You guys can begin bashing now! http://www.camaros.net/forum/biggrin.gif

Geez, I am not going to be able to type for the next 3 days...finger cramps! http://www.camaros.net/forum/biggrin.gif
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375hp 78 Chevy truck
77 Chevy Nova
95 Chevy Lumina 3.4L
and building a 78 Nova

[This message has been edited by travis (edited 05-09-2001).]

travis in the middle of your book you made a statement that is the point of most of the disagreements {the reason for vacuum advance is fuel economy}. It has nothing to do with performance at WOT. If you run a car at the strip. Try ported then manifold then none unless the are problems with runs {tracion etc} it will run the same. Using manifold vacuum is just a way of covering a tuning problem. It is the easiest way to do it but not the best.

travis,
I aggree with just about everything you said, especially the ingnition advavnce and carb tuning part http://www.camaros.net/forum/smile.gif.

I have to dissagree about running rich not making more heat. The example you stated is true, at WOT a slightly richers mixture will reduce combustion chamber temperatures, but will increase exhaust temps, you are not at WOT long enough for a lot of that heat to make it through the to the coolant, but exhaust temps will go higher. There will be a rich limit, about 12:1 or so (depending on the engine), that the exhaust temp will come down, at this point power will start to fall drasticly. There will still more heat wasted per hp made.

My example was idle. You need a certian amount of hp to maintian an idle. If you go rich and loose power, you need more throttle to keep that same idle speed. In that situation the rich idle will run hotter. At WOT you cannot add throttle to get that power back, if you could, it would run hotter, that power is lost.

This is why I wanted to keep the topic down to an idle, but you made some excellant points in there.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

[This message has been edited by Grape Ape (edited 05-09-2001).]

travis,

good post buddy! like i stated i am not makeing a blanket recomendation.

if you take the time to tune for manifold vacuum you will get awsome result's. same go's for ported/timed. but i have found manifold vacuum is easyer to tune for.

at WOT you do not get any vacuum advance, unless you have a carb that is too small for the motor. say like a 500CFM carb on a mild 350SB, you need a 600 to 650CFM on this motor. with a small carb you get higher air velosity's in the carb bor's and that make's vacuum.

all after market performance carb's a jeted and tuned out of the box rich. this is so that they will start a motor, and because of the idle carictoristic's of most performance cam's like a rich idle. thye are ment as universal carb's so to be rich is good for the maker.

my motor is very simular to travis's, the one he has in his pick up. i have it in my K-5 blazer. i have the same gear's in the diff but i am useing a 700R4 tranny. i get better performance and mileage with manifold vacuum then ported/timed, but i have tuned it for that. but i need 3.73 or 4.11 gear's to work better, i have 3.08's now.

oh well i hope we all learn something, and oger i do not rember exactly what motor's used ported/timed or manifgold vacuum in the corvett's ok. it has been over 10 years since i was a tune up tech. but i do rember the vast majority of 60's motor's used manifold vacuum. this GM, Ford and Dodge. both V-8's and inline 6's. but it allso depended on the tranny and gear's that they came with. the auto maker's back then wanted power and drivability over econamy and emision's.

Let me explain without writing a book.... Let's learn from the newest technology out there. The LT1 is computer driven.(we all know that). It has timing at idle between some set parameters which happen to be 25 -30 degrees at idle. I have switched all my cars to manifold vacum into the HEI can cause I know of no other way to pull that much timing into the car at idle. Does anyone else run a ported vacum motor that idles with 28 degrees? I will answer the question with one word; "NO". The reason I bring up the LT1 is cause my 95 Z runs like a banshee using the factory timing while it never overheats at idle. (result from the major high number of timing degrees at idle). This reply considers only idle, but is a major push for everyone to rework their dist to utilize Manifold vacum.

What in the world does where I hook my vacuum advance up have to do with how your computer controlled LT1 runs

tufz,
You made a blanket statement that everyone so far said cannot be done, and it can't. "My LT1 works like this, so we all need to do this." WRONG.

Your LT 1 is multi-port fuel injection with computer controlled timing. MPFI has a much better air fuel mixture than any carb out there today. The fuel droplet size passing the intake valv is about 1/10 the size of the average street perfromance carb set up. This allows very lean mixtures to be used with a lot of advance. Two things to reduce emissions. Take that EFI off, throw a carb on then tell us what's the optimum timing. Your LT1 also adds more MAP sensor timing under cruise conditions above idle, which actually simlulates a ported vacuum source.

I have no problem talking about new technology, but lets get the facts straight. There are too many people out there that only look at what is happening and take it no further. Look deeper and find out why things do what they do. None of your reasons make any sense, you are taking the easy way out. One for example is my car never overheats at idle because it has 25-30* idle timing???????? Try and prove that. What tests do you do to come to that conclusion?

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www.grapeaperacing.com (http://www.grapeaperacing.com)

[This message has been edited by Grape Ape (edited 05-10-2001).]

I forgot about the radical idle/rich mixture thing.

To understand why you need a richer mixture, you have to look at what is happening in the intake manifold durring idle with high overlap cams. The most obvious thing is the piston pushed some air/fuel charge back toward the carb (reversion) before the valve closes. This will kill some vacuum signal to the carb. Now weed need small idle air bleed and or larger idle feed restrictions to make the carb respond better to the lower vacuum. If that were all that happend, you could still get an optimum idle air fuel ratio, so there's other things happening.

Along with reversion of the intake, there is also reversion in the exhaust. Slow moving exhaust gasses at the end of the exhaust stoke will circle around and come back through the exhaust valves on the short side radius. The amount of exhaust gasses that enter the combustion chamber will depend on exhaust gas speed and the end of the exhaust stroke and also on the pressure of the combustion chamber. If a low pressure induction pulse left a low pressure are in the chamber, there will be more exhaust reversion.

Lets look back at the intake side again. The reversion takes place in pulses, 4 per revolution to be exact. These pressure pulses will confuse any carb. The magnatude of these pulses are effected by the exhaust reversion changing the power output of each cylinder cylinder. Sometime the cylinder power goes up and others it goes down. This gives a unsteady or radical idle. The intake pressure pulses reach the carb, which can only sense pressure differential, not density. Will not react fast enough to keep the idle air/fuel ratio constent. Even if it could, correction would be based on pressure differential, not actual density of the air.

These inconsistencies (I'm just pointing out the largest ones, there are more) add to a rough idle. Any engine will run the smoothest with the optimum air/fuel raio and timing, however, I just said that the air fuel ratio is inconsistent. Now, we should all know that if an engine is run too lean it will surge, adding to the problem. We can put a band aid on it and run it on the rich side. We can also add more timing to keep the plugs clean with the overly rich mixture (put a band aid over a band aid).

As I said before, if the idle air/fuel ratio is richer than 13.5:1, you have other problems that you are covering up.

Another poster said that compression didn't matter at idle. This is wrong, changes in static compression has a small affect on idle dynamic compression ratio, but small changes help at low engine speeds. More compression will increase exhaust gas velocity at the end of the exhaust stroke, which reduces exhaust gas reversion. This reversion is one of the reasons high overlap cams need a rich idle. Reduce it and you can run leaner. Porting intakes, heads and headers with antireversin dams helps this a lot also.

Another way to smooth out the idle is to use EFI. The electroncs can compensate much faster than a carb and also has idle timing correction to add and take out timing when the engine speeds up and slows down.

This post may seem long, but really it's just the tip of the iceburg on idle quality.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

[This message has been edited by Grape Ape (edited 05-10-2001).]

i belive what Tufz was trying to get across is this.

on computer controled FI car's, i know this, the static idle timing is TDC, for the vast majority with a computer controled advance curve. when you hook the puter back up to the distributor, most not all but most, jump the timing to between 18* and 30*BTDC, just like manifold vacuum dose.


the timing curve a FI car uses is basicly what you use when you use manifold vacuum. now not all FI system's do this. but most do. they allso use a knock sensor to retard timing when you get pinging, or you get too hot they retard timing too.

what grape ape said is true. i hinted at it when i stated a carb is just a dumb air flow fule flow metter. a FI system is a active control system, a carb is pasive. in other word's a carb respond's after the motor dose, and is very limited to what it can do, and a FI system respond's with the motor and is very dynamic.

baciscaly i have found on motor's with CR's of 9.1:1 and higher respond to the use of manifold vacuum best. low CR motor's, 8.9:1 and lower, prefer timed/ported. all EGR motor's need to use timed/ported. this is a general statement not a flat out fact. every motor is diferant, so you need to experimant sometime's.

Ya thats why my L72 with 11 to 1 compression used ported vacuum.

Thanks for your imput DJD. Please read the my post and don't quote me out of context re: "idiots". Look at the response and all the wonderfull info we all got from everyone. You got 36 responses so far, so let's let it go for now. Apparently there was enough interest in this subject!

Ken,

I'm really not trying to start anything with you but I can show you a dozen threads on this same subject. The folks that posted before have not taken part as it has gone on and on like the Eveready Bunny. With hundreds of active members only three have perpetuated this one.

I would never say not to post but I will always ask that you read up on some of the past posts to help yourself. I'm just a shadetree mechanic and wouldn't be able to tell if these guys were BSing or if they really know their stuff without some of the past info I have read. (I'm not saying anyone is BSing!!)

I will be the first to admit it's gotten to a different level this go around!! Thanks for keeping it interesting guys.

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...Dennis
'69 RS Convertible w/SS trim (http://www.camaroslimited.com/memberscars/den.htm)
'96 Z28SS #1679 of 2410 (http://www.camaroslimited.com/graphics/memcars/96ss.jpg)
"The Club" (http://camaroslimited.com)

I 'm one who's guilty of not reading past posts http://www.camaros.net/forum/rolleyes.gif. I did do a search on the subject, but I didn't find too much. I never thought to look in troubleshooting. I mught be better if you cold serach all the forums? Or can you? Once I did find that thread is looked like many poeple were just mad at each other and not much was making any sense.

My views differ from a lot of people, but people who know me, also know that I do not argue abount engines unless I have have done or seen some convincing tests on the subject. I will never take the simple quick solution wihtout looking deeper.

My advise to everyone is, when you think about why an engine does something or work a certian way, never assume that there will be a simple or cut and dry answer. There is nothing simple about what is happening inside a running engine.

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www.grapeaperacing.com (http://www.grapeaperacing.com)

Use this http://camaros.net/search.html and try "manifold vaccum" You will get 86 threads of which many have to do with vacuum advance. My whole point to to educate folks to the ways of this site. Some folks post the same thing that was posted last week. In fact this thread started as a rehash of threads from the very recent past. On 05/01 this one was started. www.camaros.net/forum/Forum4/HTML/001646.html (http://www.camaros.net/forum/Forum4/HTML/001646.html)
And then there are these 2, www.camaros.net/forum/Forum5/HTML/001029.html (http://www.camaros.net/forum/Forum5/HTML/001029.html) $ www.camaros.net/forum/Forum4/HTML/001579.html (http://www.camaros.net/forum/Forum4/HTML/001579.html)

Someone ultimately will have to clean up the forums, build archives, keep the good threads and whack the bad! When the same topic is fragmented in different threads and in different forums it's an administrative nightmare! We want the site to be as informative as possible is that too much to ask?

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...Dennis
'69 RS Convertible w/SS trim (http://www.camaroslimited.com/memberscars/den.htm)
'96 Z28SS #1679 of 2410 (http://www.camaroslimited.com/graphics/memcars/96ss.jpg)
"The Club" (http://camaroslimited.com)

I'm a man of few words who lets my cars do the talking/performing. but kudos for iLbl8... you do know your stuff. I brought in the LT1 computer-controlled information to demonstrate that cars need more than 4 or 8 or 12 degrees at idle. The only way to achieve this is to set idle timing at the 4-10 range for easy starting and let the manifold vacum pull in another 10 - 16 degrees. My experience is with 327 10.5 compression fuelie heads, 280 magnum cam headers, with M20 4.10 out back. Also my newest car is 383 SBC Edelbrock heads 270 magnum cam. Someone asked to prove that the overheating can be prevented with more timing at idle. This is exactly what I've proved on both of my cars. I tried everything (new victor water pump, modifying a/f ration, new 4core radiator, flex and clutch fans, different thermostats, etc.) I finaly changed timing from 8degrees ported to 8deg plus 10 manifold vacum at the can gives me 18 at idle and solved the problem. Now don't cry foul cause I know some will say this is blanket statement and not everyone runs the 383 with 270mag, alum heads, etc. But it worked for me on one car and I also changed the 327 to same Manifold Vacum. I'm just saying this to help the nonbelievers. For those of you saying this is old hash,... whatever, you must realize that nothing stays the same. heck... even today, I recieved my new Chevy Hi Perf mag. (I think July 01) and it has an article on this exact subject. They built some 355 Hot type motor to run on 87 octane. Their quest to improve dragstrip ET brought into the mix the timing. They finally settled at 22degrees idle AND PLUS they run ported vacum advance. (total is still limited to 36) So to my disbelieve (remember I asked the question yesterday - Who runs ported with 28 Degrees at idle? - Well, here I have my answer. So this scenario leads back to my inclusion of the LT1 --- it is most benefecial to run extreme timing at idle. That what the computer does for the fourth gen LT1's.

the overheating at idle when useing ported vacuum is because of incomplete combution of the A/F mix, it tend's to heat up the exuast more and the motor. by advanceing the timing to between 18* to 30* BTDC at idle you get a more complete burn and lower motor temp's. the motor allso run's smoother because it dose not have to work as hard to keep it self running, and there is less dilution of the intake charge by un burnt exaust gasses in the cylender. it allso aid's in evacuationg the exaust, if you have a copmletly burned charge i exit's more readely than one that is not. oh and plug's stay cleaner and get to operating temp faster.

on an emsion motor you want the heat! for the cat and the AIR system to work right. allso the EGR need's the hotter temp's too.

now like i have said before, you need to deside what you want to run, i prefer manifold, but ported will work if set up right. just not as well IMHO is all. these are my thought's is all, every one think's diferantly about this.

Can we close this post if we have degenerated to quoting magazine articles it is time to go on.

Amen!!