Editing Hot rodding the HEI distributor (section)
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===Vacuum advance using overdrive=== The all in by rpm is often said to ideally about 3000 rpm, or less if the engine will allow that w/o detonation. This is a generic setting for a performance vehicle, and that usually means a rear gear ratio of at least 3.73:1 and w/o OD. In the case of an OD vehicle where the cruise rpm is relatively low, you can supplement the advance curve by allowing the vacuum advance to give more advance than what's usually recommended. This can make up the difference between the mechanical advance you get at your cruise rpm and total amount of mechanical advance. So say the mechanical advance at 2000 rpm is 12 degrees. The max mechanical at 3000 rpm is 18 degrees. The "missing" 6 degrees can come from the vacuum advance. One thing to watch out for by using more vacuum advance is the engine can 'surge' at elevated advance settings and also there's the chance it will have a transient ping when hitting the throttle quickly when the vacuum advance is all in. That said, being as how we're only talking about 6 to maybe 8 degrees added vacuum advance, there's a good chance there will be no problems at all from using more vacuum advance.
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