Tod,
Under little or nearly no load an engine can handle more spark advance.
This helps economy and to some degree cruising power (the rest of the more
intelligent folk on the list will have to explain "how" this works). The
advance is junctioned into the carburetor with a special connection that has
no vacuum at idle, but shortly after move past idle the vacuum engages.
After this point the vacuum is similar to manifold vacuum in that the more
you press on the pedal the less vacuum you get (in general theory).
Hence, as one attempts to accelerate the vacuum and vacuum spark advance
will both decrease to avoid engine knock. Since a race type engine will
often be driven under full throttle there is not likely to be any vacuum
advance due to the lose of vacuum (at least enough to disengage the vacumm
advance). Additionally the average vacuum advance is not the most robustly
designed item. Often it can cause errors in the timing due to its plate
movement. Lastly in a race engine a degree or two in timing can mean the
difference between a fast engine and a broken engine. By eliminating the
vacuum advance (which as I described above isn't effective in a race motor
anyway) you remove a variable that could destroy a pricey engine.
I would think that vaccum advance distributors would be a dime a dozen.
Depending on how you drive it could help. If this is any insperation to have
a functional vacuum advance, a cheap smog device of years ago inhibited the
vacuum advance to decrease NOX (It was a dual vacuum hose and temperature
sensor on the upper radiator hose). See if you can pick up a vacuum advance
distributor on Ebay or a self service wrecking yard. Hopefully the Pertronix
will fit and give it a try. I think I've even seen Mallory vacuum advance
distributors on Ebay for around $35-$50???
Tom Witt
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