Vacuum Advance Distributor? Can someone explain this?

[Fiddler's Gold]

I am contemplating going over to a true MSD distributor system and the Pro-Billet Street has the same type of cone shaped vacuum advance system that is currently on my distributor. I may just stick with mine as it is brand new, but my question is this:

What is the vacuum advance system for and is it better to have with my current configuration or not? (I am using for performance and not limited nor concerned with the economy or state laws) I couldn't ascertain from the MSD literature what they meant by the manual or vacuum advance for the carb, I am assuming. Currently is hooked up via lines to the carb etc.

Thanks!!!

Fiddler

 

[AKRAY4PLAY]

the vaccum advance is for fuel economy. if set up right, you will never notice a drop in performance, only a slight gain down low. the MSD has optional vaccum advance, and it is adjustable to work with many different cam profiles. it is not needed, but will save some cash at the pump. i am running the pro-billet and 6al with mine and love it. brian

 

[JohnZ]

Vacuum advance also improves driveability and throttle response, and lets the engine run cooler at idle. Lean mixtures (as seen at idle and steady cruise) burn slower than rich mixtures, so the spark needs to occur earlier in the cycle; these conditions have high manifold vacuum, so the advance diaphragm on the distributor advances the spark timing so peak cylinder pressure occurs correctly just after TDC for maximum efficiency. Rich mixtures burn faster and don't require that extra spark advance, so when you accelerate (and get a richer mixture right away) and manifold vacuum drops (due to engine load), a spring on the other side of the advance diaphragm pulls the vacuum advance back to zero so maximum cylinder pressure still occurs at the ideal point after TDC. Vacuum advance is engine load-sensitive, whereas centrifugal advance (the weights and springs under the rotor) is solely rpm-sensitive. Properly-calibrated vacuum advance is essential for a street-driven car; centrifugal-only distributors are hyped by the speed equipment catalogs, but they're race pieces and are inferior on a street-driven car.

 

[Fiddler's Gold]

Thanks for all of the info.

Wow, I feel stupid most of the time.... ;stupid
especially when I come to the CAC forum! hehe. Thanks so much for all of the advice and insight!

Fiddler