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68gmsee · 01-05-2010, 10:21 PM

When you are at idle or real low rpms, the engine needs to advance the spark to properly ignite and burn the fuel. This is where the vacuum advance comes into play.

As the rpm's increase the vacuum pressure in the intake diminishes to the point where there is "0" vacuum pressure at wide open throttle. That's because the butterflies are open allowing air into the intake, therefore, equlizing intake with outside pressure. This is where the mechanical advance in the distributor comes into play because the engine still needs to advance the spark for proper combustion.

A lot of drag racing cars don't have a vacuum advance since they are mostly run at wide open throttle.

01-05-2010, 10:21 PM	#4
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: HEI Ignition question When you are at idle or real low rpms, the engine needs to advance the spark to properly ignite and burn the fuel. This is where the vacuum advance comes into play. As the rpm's increase the vacuum pressure in the intake diminishes to the point where there is "0" vacuum pressure at wide open throttle. That's because the butterflies are open allowing air into the intake, therefore, equlizing intake with outside pressure. This is where the mechanical advance in the distributor comes into play because the engine still needs to advance the spark for proper combustion. A lot of drag racing cars don't have a vacuum advance since they are mostly run at wide open throttle.