Hot rodding the HEI distributor
m (→Distributor height adjustment) |
(→Example of a "typical" performance ignition advance curve: Add text, link) |
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A typical advance curve for an engine built with a mild camshaft and having a compression ratio correctly matched to the cam will look something like this: | A typical advance curve for an engine built with a mild camshaft and having a compression ratio correctly matched to the cam will look something like this: | ||
| − | *14 degrees initial advance | + | *14-18 degrees initial advance |
| − | * | + | *22-18 degrees centrifugal |
| − | * | + | *10-12 degrees vacuum advance |
| − | The above | + | *Mechanical advance all in by =/< 3000 RPM |
| + | The above gives 46-48 degrees of advance (including vacuum advance) under light throttle cruse/high vacuum conditions. You want the mechanical advance in as soon as the combination will allow, without causing pinging. Using an adjustable vacuum advance unit allows the vacuum advance to be adjusted for what vacuum the vacuum advance falls out and tips in. Set it so the vacuum advance starts to drop out at about the same point that the carb power enrichment circuit (Holley power valve, Edelbrock step up spring, or Q-jet power piston) starts to come in. | ||
| − | If you are in the 45-55 | + | If you are in the 45-55 degree range (about 50 degrees is fine in most cases) of advance including 10-12 degrees from the vacuum advance, you’re in the ballpark. Each engine is different and what works for one engine might be a little different than what works for another engine. Generally, the bigger the cam (more duration/overlap, tighter LSA, later closing intake valve), the more initial timing the engine will need. '''Total''' timing is not affected as much by the cam timing; that's more a function of the compression ratio, fuel octane, [[quench]], engine temperature, air/fuel mixture, cylinder head design and material, etc. |
| − | As has been already stated, most performance engines will work well with around 10- | + | As has been already stated, most performance engines will work well with around 10-12 degrees of vacuum advance. Generally the vacuum advance can be tailored to suit the conditions after the initial and mechanical advance is worked out. That said, there are a few isolated cases where the vacuum advance plays a bigger part in the overall advance curve, like when the vacuum advance is relied on to provide advance at idle in order for the primary throttle blades to be closed down enough to keep the carb from idling on the transition circuit. |
==What vacuum source should I use- manifold or ported?== | ==What vacuum source should I use- manifold or ported?== | ||
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