Steve, and Listers...
On this side of the border, 13 degrees initial plus 25 degrees
mechanical advance will get you 38 degrees total, which is probably what
you intended to say...
The vacuum advance is only active during light-throttle operation. At
full load the vacuum advance is zero, and then the distributor has to be
set correctly to give the desired advance for maximum power without
detonation, based only on the initial and mechanical advance. The vacuum
mechanism allows additional advance when the cylinder pressure is not so
high, and that will give better mileage and driveability on the street.
Based on my bench testing of the two systems, I'd recommend the
Pertronix (specifically the Pertronix II) over the Unilite, as far as
stock-distributor-upgrade type ignition parts go. The Unilite uses a
light beam to trigger the coil discharge, which is fine, but then it
allows coil current to flow anytime the light beam is interrupted (which
is about 90% of the distributor rotation). What this means is that if
you're doing something to the car and you have the ignition on, there is
a good chance you're heating up the coil (and the distributor wiring) to
some pretty decent temperatures. The original Pertronix did this also,
according to what I've read, but the PII solves the problem by
(apparently) detecting whether or not you're getting repetitive trigger
events from the reluctor assembly. Another nice feature of the Pertronix
is that you install it in the factory distributor, which means you get
to keep the vacuum advance and distributor curve that was part of the
original system - or else you can muck around with it using Ford parts
instead of trying to get Mallory distributor guts.
For anyone not happy with what they get from these systems, I've had
very good success with the MSD-6AL. You get rev limiting, it's easy to
install, and it gives great performance from idle to $000 RPM. The only
thing I'll be replacing it with, will be a crank-triggered
distributorless multi-coil system. If I ever get the time.
Best regards,
Theo
|