To Vacuum Advance or Not to Vacuum Advance
I can not really find anything - just that Vacuum Advance helps fuel economy.
And economy usually correlates to efficiency.
In racing that can mean less fuel has to be carried, or maybe one less pit stop on a longer race. So its not just for street cars. But it does add another level of tuning.
And some comments that performance engines usually do not run vacuum advance
Drag racing it is certainly not needed. The whole race is run at wide open throttle.
When developing a tune for a modified engine, or even desmogging a stock one, first get the initial and mechanical advance figured out. So generally no vac advance until thats worked out. Nor on the dyno (unless you're doing part throttle tuning).
Butting in here...why ported vacuum vs full manifold vacuum? I've always thought that you should connect to full manifold vacuum.
It depends.
The purpose of vacuum advance: provide more lead time for lower density mixtures.
The elegance of using ported is then vacuum advance correlates directly with lower fuel-air density.
This leaves mechanical advance to correlate with rpm, and changes in burn rate related to rpm.
And when racing (where you care about timing at 4000 rpm and above) sometimes mechanical advance can also offset time lost in the electronics, etc
Sometimes a distributor can't be adjusted to get the initial timing wanted. And sometimes in these situations manifold vacuum can be set to provide it.
Sometimes on emmissions equiped engines, manifold vacuum was mixed in with ported vacuum during warmup. My AMC era jeep has this, called a non-linear valve. It also has a 'heavy duty' coolant temperature switch that adds switches to manifold vacuum if coolant gets too hot.
The above comments about mixture density makes sense only if you know that pre-emmissions era idle mixtures were relatively rich. The mixtures get leaner the more the throttle is open until some point near wide open, when relatively rich mixtures are needed.
