At 09:52 AM 10/10/00 -0700, you wrote:
>Theo,
>snip
>The 16 and 20 limit plates are for mechanical advance distributors. The
>vacuum
>advance units are generally stamped 10 (dist. degrees). I use a 10 distributor
>degree (stock) limit plate on the mechanical advance, 10 degree (distributor)
>on the vacuum adjustable stops on the ACCEL replacement unit with the limit
>stop adjustment through the vacuum hose inlet with an allen wrench (supplied).
>I don't really count all the 20 degree (crank) vacuum result, as that is under
>high vacuum (off the throttle, or low load) and won't cause pre-ignition. To
>the 20 degree (crank advance, which I have tailored to be all in by 2500 rpm,
>less if your system can take it) plus 9-10 degree static advance on the
>indexed
>vibration damper. This gives a total of 20 + 10 = 30 + about 7 vacuum crank
>degrees (guess). About what my engine tune (mild) and today's gas (poor) will
>allow me to risk. I believe that 39 is absolute tops, according to others and
>the Ford manual.
I think you may be a little conservative in your timing numbers,
particularly with the limited vacuum advance module. I know that most
owners with modified engines never consider the addition of vacuum
advance. I think the're losing out on some economy and performance. The
aftermarket vacuum modules are very good at limiting advance until the
manifold vacuum is high enough so that it avoids pre-ignition under heavy
load conditions.
If you start by setting your timing (distributor lockdown) at the
RPM that you get full mechanical advance, you can get into the 34-35 degree
range. Then drop to idle and remeasure the timing. If your not up to 12
-14 degrees, you should shorten your mechanical advance until you get it to
drop into that range. The hotter your performance engine is built and the
lower your gears, the higher you can set this initial timing. Then when
you hook up the vacuum advance, you can get up into the high 30's or low
40's under light loading conditions. When your cruising in the 14 -16
inch range, you not going to get any pre-ignition problems.
My 302 crate rebuild with a 550 cfm Holley consistently pulls 24
-26 mpg under 70 mph conditions at about 2200 rpm, and it's no slouch when
it's time to get up and go either.
Tom Hall
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