Expert: Dan Liddy - 8/18/2011

Question
I read in a magazine recently that the ultimate distributor for tuning racing engines (gasoline) do not have vacuum advance mechanism. How does such distributor work and what is the advantage of this type of distributor over the normal type?

Answer
I'm a little vague on this myself. The vacuum advance works to gradually advance timing as the driver accelerates the car. The spark , in effect, starts occurring a little bit earlier in the cycle, which allows the engine to make more power faster.

The mechanical advance distributors used in racing just do it more positively, and usually no mid range is used so they're either "all in" or back to their idle setting. Some engines almost never run at idle and the static timing setting is never used. So the distributor is locked in the full advance position, This is OK in most cases, but makes the engine difficult to start.

In my own engines, I use a modified distributor with mechanical advance and then rather than setting the static timing, I adjust the distributor at high engine speed to offer about 34 degrees of spark advance at about 2500 RPM. This gives me satisfactory acceleration, and it still idles and starts reasonably well. These adjustments are set with a xenon strobe type of timing light and I time it to marks I've established myself which are more accurate than factory marks.

Hope that's a help !

Dan Liddy
Sarasota, Florida USA