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Old 02-10-2005, 10:29 AM
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bracketeer bracketeer is offline
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Vacuum advance

It took me almost an hour but I read every posting and article that you suggested. The distributor can go up to 51 deg but not on a running engine. The vacuum loss retards the timing back to mechanical total.
14 initial + 21 centrifugal + 16 Crane adjustable vacuum advance with EGR = 51 total.

I understand that you set your distributor by it's maximum advance but I don't know why. According to these postings. If we were running at 38 deg total and both open up our throttles, you would drop to 35 deg and I would stay at 38 deg. I would be producing more HP.
Combining the 12 degrees of initial advance with about 8 degrees from the centrifugal advance and then adding 20 degrees from the vacuum can when operating at low throttle settings is just too much.. 40 degrees of total advance will probably cause pinging or even detonation.. That would be bad.

At 46 deg you would be on the next cylinder. That would be real bad.
Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
I disagree with the statement of seeing 50 advance if you were going down the highway, (10 initial, 20 - 25 mechanical and 15 from the vacuum) you stated elsewhere when the mechanical advance kicks in the vacuum is not adding anything.

I also disagree with seeing 50 deg of advance.
Under high load, WOT, performance conditions there is almost no manifold vacuum so the vacuum advance does nothing.
So if at say 2500 RPM, your total initial and centrifical advance is 32 degrees,and you are pulling a total of say, 36 with vacuum advance, when you start stepping down on the throttle, the vacuum advance will retard in relation to the loss of vacuum signal, to the point where, the 4 degrees of vacuum advance disappears, if enough vacuum signal is lost.

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Last edited by bracketeer; 02-10-2005 at 03:18 PM.
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