That topic got a little big, starting a new thread in case anybody has anything else to say.

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Chris Dagenais
'71 Malibu with a home built 454! 12.7@107
For everything you do, this burnout's (http://www.71malibu.com/Burnout.wmv) for you.
The Green Monster (http://www.71malibu.com)

Come now! Isn't 61 responses enough?

I think the real problem is there is confusion as to what is venturi vacuum and what isn't Find an old VW carb and look in it. There actually is a sort of extra booster right in the middle of the venturi for the vacuum draw to the dist.That is true venturi vacuum the amount of vacuum pull increases with the amount of air passing throgh the venturi. The small hole in the throttle bore wall just above the blades is not true venturi vacuum. It does not increase with the airflow through the venturi but acts just like manifold vacuum once the throttle blades start to open. Like I said about a week ago it will work either place but there will be no vacuum advance at WOT with either.

VW?? Arn't those the kits that come with a cardboard box and a lawnmore motor? Or was it a mouse on a wheel? I can't remember, maybe you guys could clear it up for me! http://www.chevelles.com/forum/biggrin.gif http://www.chevelles.com/forum/biggrin.gif http://www.chevelles.com/forum/biggrin.gif http://www.chevelles.com/forum/biggrin.gif
Brandon

Mike, I never once said that the vacuum connection that goes to a port right above the throttle blades is venturi vacuum. I said this is commonly called ported vacuum. Since that's what everyone else calls it I think I'll call it that too.

I was also hoping this thread would provide some general insight into which port works the best but it seems the discussion centered around what way was stock and also that you have to try both manifold and ported vacuum. No insight into why one way would be better.

Peter

PeterF you need to go back and read my reply that will explain why manifold is the prefered method...FRED

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Fred can you explain it again? I can't figure out what you were saying.

When you have vacuum you have advance ,no vacuum no advance.In other words with no vacuum you have retarded ignition like when you are at WOT or part throttle pulling on a hill.What is confusing is guys thinking you want more advance under hard accleration NO, BIGGER or more is not always better...FRED

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Hope I'm not gonna get flamed for re-opening an old wound, but I read the original thread, and then this one - they both really got me to thinking about ported/venturi vacuum v. full vacuum, in addition to why the factory specs say my '66 used the small port on the front of the Q-jet for spark timing (exactly the same port as shown in the pic someone posted with the red box around the port on their QJ - the one that opens to the primary throttle bore just above the throttle plate).

So I hooked up my vacuum gauge to that port and started the 396. Got it warmed up and idling at 750. The gauge was reading about 2 inches of vacuum. When I opened the throttle the vacuum increased. The more I opened, the more it increased. If I quickly opened it enough to crack the secondaries the gauged pegged at 30". And every time I released the throttle the vacuum decreased back to 2".

This carb port is obviously reacting to air flow past it's opening. The more air flowing past the more vacuum produced. Therefore, as this is the correct factory specified vacuum advance port on my '66 396/325 the only conclusion is that as the throttle is opened more and more, the engine sees more and more timing advance until the vacuum advance pot hits a mechanical limit. The stock set up is using the initial timing, the centrifical AND the vacuum. Finally, because I was seeing 2" at idle this explains why you are supposed to disconnect the line before setting the timing - even that 2" will pull in a certain amount of advance from the vacuum can.

Of course, the manifold vacuum - measured directly off the metal "T" screwed into the manifold just behind the carb, reacted opposite of the port on the front of the carb. At idle the gauge read 17", decreased then recovered back to almost 17 when I opened the throttle slowly then maintained that position. If I snapped the throttle open the gauge would plunge to zero - as expected.

So that explains it for me. Running the stock "ported" or "timed" vacuum starts with a modest amount of initial (good for a clean idle) pulls in more advance just off idle, and then even more is brought on by the mechical advance, and - as RPM's increase, even more is brought in. Under hard load - when you don't want a LOT of advance so you can avoid detonation, the stock set up - due to the ported vacuum source, is doing exactly what you don't want - adding more advance as the throttle is opened more.



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Awww, fer cyrin out loud, there's that IMPALA guy again.

http://photos.yahoo.com/bc/ss3964spd?d&.flabel=fld1&.src=ph

I just had the time so I put the vacuum guage on my holley. It has 3 ports, the big one for the PCV, one for the distributors vacuum advance (passanger side toward the front) and one connected to the choke. The one connected to the choke pulled at idle. The other had no vacuum at idle but the crack of the throttle caused it to pull. It's my belief that if there is venturi vacuum on this carb it's only on the choke port. I don't have a guage in the cabin so I cannot verify WOT. I don't know if a vac sec 780 holley came stock on a chevelle but it did on several camaros.

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...Dennis
The '69 (http://chevelles.com/showroom/fine69_03.jpg) & the '96 (http://chevelles.com/showroom/fine69_02.jpg)
Team Camaro (http://www.camaros.net)
Camaros Limited (http://www.camaroslimited.com)

So here is a question for you.
Where do you put the vacum line for your tranny? I have it on the front of the carb, small port (4150 carb)
Or should it go to the back of the carb, tee it off with the brake line?

Also, if the vaccume port on the manifold is on the runner, not the plenum, do you recommend using that for the Brake booster vaccum?

Thanks

Plenum or runner, if there is a port I don't think it would matter. Mines on a runner with a "Y" fitting and a large threaded port for the tube to the brake booster. One of the small fittings is capped off and the other feeds the tranny. I believe this is very close to the wat the factory was doing it.

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...Dennis
The '69 (http://chevelles.com/showroom/fine69_03.jpg) & the '96 (http://chevelles.com/showroom/fine69_02.jpg)
Team Camaro (http://www.camaros.net)
Camaros Limited (http://www.camaroslimited.com)

If it is on the runner wouldn't that cause problems with that cylinder?
Thanks

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www.yannis68chevelle.com (http://www.yannis68chevelle.com)

All my plugs look about the same. I'm not running anything like the manifold on your mill. Is it even plumbed for a manifold vacuum port?

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...Dennis
The '69 (http://chevelles.com/showroom/fine69_03.jpg) & the '96 (http://chevelles.com/showroom/fine69_02.jpg)
Team Camaro (http://www.camaros.net)
Camaros Limited (http://www.camaroslimited.com)

ss396 how can you tell what is going on if you don't have a timming light hooked up.Sure you can check for vacuum but whats going on with the timming...FRED

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Fred, because I found the previous post on the same subject so interesting I had to check my Q-Jet.

No, I didn't hook up the timing light - saw no need to, here's why:

The VA can on the distributor rotates (advances) the breaker plate in the distributor when vac is applied, and retards it (back to initial) when vac is removed. The VA can isn't an on/off switch, it moves incrementally according to how much vac is present. Of course, the advance can doesn't care what the vac source is.

Now, at idle, my engine is pulling about 17" of vac at the manifold fitting and at the PCV fitting on the front of the carb, but is pulling only 2" of vac at the factory specified distributor vac port on the front of the carb.

By logical extention then, if I plug the VA into a port that supplies 17" of vac at idle the ignition will have to advance more than it would if I plug it into a port that supplies only 2" of vac at idle. Please don't take offense or consider me smug, but I don't need a timing light to tell me this is so.

The manifold and distributor ports react opposite in response to throttle openings. While opening the throttle makes manifold vac drop, it makes vac at the distributor port rise.

All I was trying to accomplish in my previous post was to point out that the distributor port does in fact produce vac by means of increased air flow rushing past it's throttle bore opening.

Regards, Dan

Dan;

I wouldn't come to that conclusion unless you put a guage on the port while actually driving the car and can hold part throttle and WOT. The load on the engine may make a difference as to how that port reacts.

Peter

Yeah Peter, ordinarily I'd agree - opening, closing, and bliping the throttle in the garage is far less accurate than getting it on the road. In this case I'll have to disagree though; at idle that port has 2", open the blades a little and it sucks more, open them a bunch and it sucks a bunch. Bliping the blades enough to crack the secondaries made that port pull 30" - the limit of my gauge.

With a load on the engine, even more throttle would be required to obtain the same engine speed and therefore, more vac at that port.

An easy test though, as my ride is equipped with a factory vac gauge - which is currently hooked up to manifold vac. Acceleration from a stop makes the vac drop but it mostly recovers at cruise. Toe into it at cruise and it drops again - pretty proportional to how much you get into it. I have every reason to expect the gauge to do the oposite when hooked to the distributor port.

Dan

Dan;

I look forward to the results of that test. It'll be nice to know what that port actually does.

If it does increase as the throttle is opened it does exactly the opposite of what the distributor and motor wants. I was always led to believe it was like manifold vacuum except at idle.

Peter

Peter, I'm in the process of getting the car ready for a show this weekend. I connected the factory vac gauge to the port but haven't taken the car out yet.

Show is on Sat and probably an hour and a half drive one way. Should be more than sufficent to see what the little needle does.

Dan

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Awww, fer cyrin out loud, there's that IMPALA guy again.

http://photos.yahoo.com/bc/ss3964spd?d&.flabel=fld1&.src=ph