Vacuum or mechanical advance distributors?

[Jack21]

On the street, generally, you need BOTH mechanical, AND vacuum.
On the track, only mechanical is needed.

For Hi Po engines, both mechanical and vacuum MUST be adjustable so each engine can be dialed in.

I'm running 14 deg static, 20 deg mechanical, and 8 deg vacuum. Like many of you, I have the MSD vacuum distributor, but had to replace the vacuum canister with the Accel unit. MSD in their marketing wisdom neither makes one, or offers advice on installing their competitors adjustable vacuum advance cannister.

[mylesdw]

Mr Fixit,
my logic was that since the extra advance causes the fuel to be burnt more completely and more efficiently then for any given cruise the throttle would need to be slightly less open. Maybe the word 'leaner' was not the right one here?

[mr0077]

Yeah, not really "leaner", just seems that way, as with the lower mean effective pressures at cruise, the combustion process takes longer to complete, so more total advance is needed for a complete burn during cruise...more complete burn results in higher effiency and more power for a given throttle position (during cruise), so less throttle opening is needed to maintain a given cruise speed...that equals better mileage, so "leaner" on a per-mile basis...
Oh, well...a well set-up vacuum advance will improve mileage, but not having a vacuum advance won't hurt the motor if you don't want to run one.

[mylesdw]

Ken:
That seems to wrap things up beautifully!