View Full Version : Vacuum Advance

08-20-2003, 11:53 AM
Confused here with some of the posts. I run a 73-454 with a Q-Jet, MSD mag pickup distributor and box. I have always run ported vacuum to the advance. I see several posts from the techs that recommend using manifold vacuum. Will this not give me too much initial advance?

08-20-2003, 11:10 PM
When you time your engine you disconnect the vacuum advance. The setting that you put the timing at is considered your initial advance. The mechanical timing in the distributor comes in as you increase the rpm of the engine. If you add the initial timing plus the mechanical timing that gives you your total timing. Example: 12 degrees initial + 25 degrees mechanical = 37 degrees total. The vacuum advance is designed to advance the initial timing when there is no load on the engine and vacuum is present. An example of this would be when you pull up to a stop sign and the engine returns to idle. At that point you have high vacuum and the vacuum advance would be added to your initial timing. As soon as you accelerate from the stop sign and increase the load on the engine then the vacuum drops and the vacuum advance is removed. The purpose for vacuum advance is to improve your gas mileage and exhaust emission while at idle. This is why you want to use a manifold vacuum instead of a ported vacuum.

MSD Tech1