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However, it seems most SC's have "Vac Retard". This should be hooked to a vac line that only lets it work at idle. Thus, even if it is left hooked up it should not effect in the driveway timing.
The engine in question, an 83SC, has both vacuum advance and vacuum retard. There is no "should" or "thus" about it, both need to be disconnected and plugged to accurately measure the timing against the factory spec. The retard is hooked up to manifold vacuum. Try it yourself and you will see your error.
Pre ignition is almost never an issue on a N/A road car engine and is usually a red herring in these discussions. Everyone I know that can speak intelligently on this topic uses the term "knock". The Bosch chart shows the ignition point before the pressure spike, so it is obviously not pre ignition. The best way to understand this issue is to start with the indicated pressure diagram of an SI engine and get an understanding of peak cylinder pressures. An engine with a cranking compression of 180 psi, will show a peak cylinder pressure of +600 psi running, and this can jump to +2000 psi under detonation. The magnitude of the pressure spike is one issue, when it occurs and how fast it rises is another.
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Paul
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