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356-930 356-930 is offline
Acceleration Junkie
Join Date: Aug 2007
Location: Laguna Beach, CA
Posts: 263
Originally Posted by RarlyL8 View Post
As you have seen the 930 dizzy does not have a vacuum advance. It also does not have a boost retard. For the 2 pot units one pot is a vacuum retard (at idle) and while the second pot is actually a vacuum advance, but the performance function does not allow advance due to the signal being positive pressure. What you end up with is vac retard at idle, vac/mechnical advance at cruise, and mechanical advance only on boost.
You can test all of this with a vac pump, an air compressor, vac/boost gage and a timing light while your engine is running from 1000rpm - 4000rpm.
I respectfully disagree that the dual diaphragm distributor has no “boost retard.”
In the Sun test, we held the distributor at a constant speed of 800 rpm. Applying +5 psi to the distributor’s “boost port” produced a negative -8 degrees of timing. Yes there is a mechanical advance component in these distributors but only with a positive change in distributor shaft rotation speed. Given the constant mechanical rotation speed during test, the reduction of -8 degrees of timing can only be attributed to boost pressure.
Rhetorically, if there was no “boost pressure retard,” what is the purpose of the dual diaphragm?
If the ‘second pot’ was indeed a "vacuum advance," plumbing the two ports in parallel, which I have done, would result in no advance or retard for the two diaphragms would act against each other. As evidenced by test on the Sun, application of positive pressure to the “vacuum advance” port has no effect on timing.
We did not apply vacuum to both ports at the same time during the Sun test but in actual use on the engine, with the two ports plumbed in parallel and -12.5 inHg vacuum applied, timing retards -20 degrees evidencing that vacuum on the "boost port" has no effect.
Like I said, I respectfully disagree. Can't tell you how much I have learned reading your posts. What am I missing here?
Old 04-06-2008, 07:44 PM
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