Vacuum Advance - Manifold or Ported

[Excaliber]

This is not an easy question to answer, lots of variables. ONE thing is clear: Generally speaking the SOLE PURPOSE of having a vacuum advance on your distributor is to increase gas mileage, period. It is NOT a 'race item', it is a 'highway cruising' item.

Dist vac advance units respond to a vacuum signal, it doesn't care WHERE it comes from. IF it is set to advance at 11" of vacuum signal by 8 degrees of ADDITIONAL timing and stop advancing at 16" of signal and 15 degress of advance timing, it will. It has a 'curve', just like the mechanical advance has a 'curve'. The advance curve in terms of signal required and amount of advance vary widely among distributors.

Say your 'cruising' down the road at 2500 rpm and 15" of vacuum. Your vac advance unit starts to advance at 11". This will GREATLY increase the amount of TOTAL timing your engine now has. Mechanical advance PLUS vacuum advance. Your engine THRIVES on LOTS of advance during 'light load' cruising, loves it, mpg can go up dramatically. When you step on the gas to pass, the vacuum signal falls the vac advance stops working and the advance timing is now strictly mechanical. This will happen regardless of WHERE the vac signal comes from (carb or intake).

Dirtect intake vacuum provides a FASTER response time than the carb. It also offers a stronger, cleaner signal. It is, personally, my preffered method, but I acknowledge that for SOME applications it does not work well. In such applications I would also question how will it works (if at all) with a carb (ported) signal.

One interesting side effect of a DIRECT intake vacuum signal can be to limit 'dieseling' or 'run on' when you shut off the engine. With direct vacuum as soon as the engine starts the timing is immediatly advanced (assuming your engine idle vacuum signal is GREATER THAN or EQUAL TO your distributor vacuum signal requirement). This advanced timing subsequently increases the engine idle rpm, in some cases by a BUNCH! You then have to 'back down' the idle speed by closing the carb throttle opening (adjust idle speed). Now when you turn off the engine the throttle plates are set to such a LOW IDLE SPEED the engine can't draw in any air to support the 'dieseling' or 'run on' condition. The engine simply 'dies' cleanly.

I USED TO use this method to prevent 'run on' with my dual fours 427. The BIG cam, intake, heads and the dual fours combine to require a fairly high idle speed (1000 to 1100). This causes a tendancy toward 'run on' when shut down because of this. NOTE: 100 octane fuel solves that little problem nicely, but I run 93 octane pump gas. But my NEW MSD vac advance dist requires such a high vacuum signal it does not 'advance' at all at idle speed. I idle at about 10 to 11 inches of vaccum, MSD requires about 15". I use it only as a means of increasing mpg during light cruise rpm and it is direct intake vacuum. However, at times it is difficult to even get 15" of vacuum at cruise rpm\load (with a radical motor). Thus rendering the dist vacuum advance 'useless' for any application!