Ptegler wrote:
> ...snip
>
> So ...just because you rebuilt everything to spec for your model year
> doesn't mean it's the best setup for your aging toy.
This is EXACTLY my point in asking the original question.
I want the BEST settings, not the period CORRECT settings.
My distributor on my '70 B-GT with the 18GH engine is a
25D4 #41228 using a 5-17-10 vacuum unit. The correct vacuum
unit is supposed to be a 54411985 module with a 5-13-10 profile.
My module will provide LESS advance per given unit of vacuum
(over the minimum) but reach the same maximum advance, 20
degrees (crankshaft), at a higher depression just like the 'correct'
unit. Remember, this 20 degrees is added to the static and the
centrifugal advance to equal the total timing observed with a light!
FWIW, it appears that there are a great number of 25D4 variants
out there. It also follows that the only functional difference, allowing
the vacuum module to vary independently, is the weights and springs
in the centrifugal advance system. I have noticed that the
centrifugal advance has a number stamped on it and on the two
different distributors that I have, the number is different -- and the
advance springs are different. My current distributor has a "10"
stamped on the centrifugal advance weight mounting. So this all
begs the question; What are the variables and what is the optimum
setup?:
External:
compression ratio
cam timing
static timing
ported vs. manifold vacuum source
Internal:
centrifugal advance profile
vacuum advance profile
From this list, an optimum set of choices can be had for the style of
driving you anticipate. Easier said than done, of course. :-)
Rick Lindsay
Diamond Geoscience Research
5727 S. Lewis Ave., Tulsa, OK
Voice: +1 918-747-3456
Fax: +1 918-747-8599
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