vacuum or mechanical advance?
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Freshman User
Joined: Jul 2004
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vacuum or mechanical advance?
what are the pros and cons of the two? I've always used the vaccum advance distributors so I know nothing about the mechanical type. Is there that much difference. thanks
Posting Legend
Joined: Sep 2001
Posts: 30,255
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You need both for a street engine. The mechanical advance only systems are for full throttle racing applications and will give reduced power and fuel economy if used on the street. The mechanical advance is sensitive to RPMs only. Unfortunately at part throttle there is a varying mixture in the chamber that takes varying amounts of time to burn properly. The vacuum advance system is sensitive to varying engine loads and provides the optimum spark advance for light and part throttle operation. Of course the vacuum and mechanical advance systems must be set up properly too. Most OEM distributors have both vacuum and mechanical advance built in.
-Hope this helps!
-Hope this helps!
Posting Guru
Joined: Sep 2004
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Actually the question is should your run timed vacuum to the distributor or full vacuum, with a street engine. If you have emissions equipment and testing the timed is the way to go. If emission equipment and testing doesn't concern you, then you should really look into full manifold vacuum. I cannot believe the differnce it made, even with gobs of vacuum advance at idle, it still starts better and off idle performance is improved 100%.
When I did this with a few others, a couple reported better mpg, and all improved vacuum at idle along with improved acceleration and drivablity.
The concept isn't new, before emissions were put into place, many of the factory ignitons were set up using full manifold vacuum.
When I did this with a few others, a couple reported better mpg, and all improved vacuum at idle along with improved acceleration and drivablity.
The concept isn't new, before emissions were put into place, many of the factory ignitons were set up using full manifold vacuum.
Last edited by Motorhead351; Sep 23, 2004 at 03:27 AM.
Post Fiend
Joined: Jun 2001
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From: Running Springs CA
Lots of advance at idle and decel caused huge CO emissions. That is why different systems, including vaccuum retard on some distributors, time delay solenoids or carb venturi operated vacuum controls on others, were introduced in the late 1960's. The high-performance setup for old VW's is all-mechanical advance, even for the street.
Jim
Jim
Posting Guru
Joined: Sep 2004
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True but running the timed vacuum also caused a lot of heat which may be good for emissions equipped vehicles it leaves room for improvement for non emissons equipped vehicles. Kind of a weird concept, timed port is supposed to release less emissions. While full vacuum is supposed to release more, yet better fuel economy can be a side effect. I wonder if you took two identical engines, set one to run timed and one to run full, then set the timing properly on each, just how big a difference would there be?
Last edited by Motorhead351; Sep 23, 2004 at 12:02 PM.
