Vacuum advance
Thread Starter
Melting Slicks
Joined: Dec 2009
Posts: 3,233
Likes: 657
From: Thunder Bay
Vacuum advance
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?
Le Mans Master
Joined: Aug 2006
Posts: 7,426
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
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
If you only drag race then you do not need it.
Have you seen this post?
https://www.corvetteforum.com/forums...long-post.html
Racer
Joined: Sep 2011
Posts: 444
Likes: 35
From: Michigan
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.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
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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.
Instructor
Joined: Sep 2011
Posts: 145
Likes: 22
From: San Francisco CA
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 : )
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 : )
Le Mans Master
Joined: Dec 2006
Posts: 8,837
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Racer
Joined: Apr 2016
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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.
But as of now, it's happiest with no vacuum at all, obviously timed correctly also.
Safety Car
Joined: Apr 2006
Posts: 4,057
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From: BC
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.
But as of now, it's happiest with no vacuum at all, obviously timed correctly also.
Le Mans Master
Joined: Aug 2006
Posts: 7,426
Likes: 1,559
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
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.
Racer
Joined: Sep 2011
Posts: 444
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From: Michigan
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.
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
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..
Tech Contributor
Joined: Jun 2004
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
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.
But as of now, it's happiest with no vacuum at all, obviously timed correctly also.