On the street, vacuum advance is all about part-throttle economy. The vacuum
advance module adds a certain amount of advance on top of what the
mechanical advance gives you based on the vacuum signal it sees from the
engine. This gives you better gas mileage, especially under conditions of
extended part-throttle cruising. There is NO reason not to use vacuum
advance on a street machine, except for those who have distributor looking
for a ported vacuum signal together with a Weber carb that does not provide
a ported vacuum signal.
Racers are all about wide open throttle, so vacuum advance brings nothing to
the table. Since the vacuum advance implementation involves allowing the
points plate to rotate, there is a potential for less accuracy, so using a
distributor NOT DESIGNED for vaccum advance (as opposed to simply not
connecting it) can allow for more precise timing.
The Cooper S distributor is prized for this reason. In fact, people will pay
far more for one than it is really worth because of the mystique. A large
portion of that mystique is due to the legendary status of the Cooper S
rally cars. Careful analyses of the advance curve and extensive dyno testing
have since de-bunked this distributor. The curve is, in fact, less than
optimal, and costs you performance rather than enhancing it. The legend is
so strong, however, that I believe even the Mini/123 distributor included it
as an option rather than try to argue with people who wanted it.
David Lieb
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