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Registered
Join Date: Oct 2002
Posts: 31
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Ported vaccum and vaccum advance
I see advice on the forum from knowledgeable people that confuses me.
Porsche spec for ignition timing is supposed to be done mostly (not always) with the vacuum advance disabled. Some folks recommend 35 degrees on my Euro SC stock class race car with the vacuum hooked up. Since the advance is hooked to ported vacuum doesn't that cause inconsistent settings? Ported vacuum as I understand it is nil at idle and begins to activate with throttle opening. However, it is not consistent and increases with more throttle opening. 6000 rpm with no road load will be at partial throttle and with who knows what vacuum applied. Would 35 degrees at 6000 rpm and no load equal more like 38-39 degrees at WOT and the car at full acceleration? |
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03-20-2009, 12:47 PM
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Registered
Join Date: Mar 2006
Posts: 1,107
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High speed timing is checked with the vacuum disconnected and plugged. You are right, that with no load, vacuum at 6000 rpm is not consistent and your reading will be incorrect. Under load, vacuum drops to near zero at WOT and vacuum advance is irrelevent. The purpose of setting the high speed timing with vacuum disconnected is to avoid detonation. The factory spec for the late HC SC car is "not more than 25 BTDC." Adding 10 degrees and running on a track would require higher octane fuel than the factory spec.
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Paul |
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03-20-2009, 03:09 PM
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