> There is a rolling road dyno that comes to APT when David gets enough
> requests and I will take both distributors with me so I can tell you which
> one is better for performance.
The rolling road will only tell you which one distributor's mechanical
advance is currently curved and set better for performance on your engine.
The vacuum advance will not even enter into the picture.
I also find his philosophies of vacuum advance rather bizarre. The only
reasons for not using vacuum advance are that either you never cruise at
part-throttle (you use the throttle as an on/off switch) or that your cam is
so wild that you don't have any vacuum to speak of. I have never heard of
anyone trying to tie it to compression ratio, because that doesn't affect
manifold vacuum nearly so much as your cam overlap does. True, as your CR
goes up, generally the cam overlap selected will go up as well, but the
fator involved is the overlap.
The only reasons I can think of for using manifold vacuum rather than ported
vacuum, assuming that you have access to distributors that have a choice
(the advance modules are different), are that your carb has no ported vacuum
tap or that you simply don't generate enough manifold vacuum to properly
activate the heavier diaphragm and use the lighter one to color-rig it (as
it sounds like Anton does, at least that is how I am interpreting the
remmark about switching to ported).
Ported vacuum is generally agreed to convey more info about the actual load
state and throttle position of the engine than manifold vacuum does.
Attaching a vacuum module that expects ported vacuum to manifold vacuum is
not recommended. Classic Motorsports had an interesting article about this
sometime last year.
David Lieb
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