drwet

I'm helping a friend set up the distributor in his small block. While we're working on it, he asked me why some guys like to set up their engines without vacuum advance and I realized I don't know. What is the advantage of not having vacuum advance?

Peterbuilt

For street use, you need the vacuum advance.
If you only drag race then you do not need it.

Have you seen this post?

https://www.corvetteforum.com/forums...long-post.html

drwet

Wow. Timing 101. Great post. Unfortunately it doesn't answer my question. Why do some guys feel that there is an advantage to not running vacuum advance on the street?

Jims427400

My 68 427 had an overheating issue that I couldn't figure out. Tried many things but then someone explained the true purpose of the VA. I took mine off and discovered that the previous owner drag raced my car and he had welded the VA in a specific spot. I ground off the weld so it would function correctly and no more overheating.I'm sure someone else will be able to explain it better.

Big2Bird

There isn't any really if it's done correctly. Vacuum advance adjusts the timing for engine load. It drops out under WOT conditions.
If your racing, pinning it is fine. The vacuum advance never comes into play. (No vacuum).
On the street, the extra added advance gives better millage and runs cooler when cruising. It can also add crisper engine response. No cons about it.

Sunracer

the vacuum advance as mentioned helps with part throttle drivability. It only functions at part throttle and cruise, which are rare or momentary events when racing. The fact that the vacuum advance mechanism moves the timing leads to an opportunity for the centrifugal advance to move or vibrate etc., which leads to the timing possibly being less stable. Timing stability is very important and more likely to be affected when constantly running to high RPMs as in a race application.

Racers remove it because it can lead to this instability/lack of perfect control, and it provides no advantage when racing.

This is my understanding, subject to correction by those more knowledgeable : )

Bloodzone

Check into using manifold instead of ported vacuum. I've used both on same engine and like manifold much better.

Big2Bird

Correct. Ported vacuum is a smog thing. Manifold vacuum is correct as designed.

USAFVeteran

My 71 LS-5 454 seems to be happier with no vacuum at all. But, I'm still playing with it, and only have 200 miles on the motor. I'm checking with an O2 sensor, to keep tabs on the F/A ratio.


But as of now, it's happiest with no vacuum at all, obviously timed correctly also.

CanadaGrant

How much initial and how much mechanical are you running?

Peterbuilt

If you read VACUUM ADVANCE in Timing 101 and it is still not clear to you then you might want to contact Lars and ask for his "Distributor Vacuum Advance Control unit Specs and facts for GM distributors' as it goes into more details.

Jims427400

JohnZ has suggested to many members here that manifold vacuum is buy far better. I've switched over both my tripowers over to manifold and runs much better.

fishslayer143

Theres NO advantage to not using it on a street car.. it helps MPG, it improves low speed response and at WOT, since there is zero vacuum, it has ZERO effect .. Why would you NOT use it? only because you don't know any better..

69427

Unless you've got that one single engine on planet Earth that somehow has a combustion process where the burn rate is independent of cylinder pressure, your engine isn't timed correctly.