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The ignition system is one of the most important yet least understood of your engine’s main systems. It’s tough enough trying to determine which system to use. Then when you get the distributor set on the right tooth so that the engine will start and run, everyone gets energized, but ignition timing curves are completely dependent on the combination of factors that comprise the engine. Timing advance curves are totally dependent on how much cylinder pressure the engine is going to make at each portion of the driving cycle. Ignition timing is directly relative to fuel burn time, and fuel burn time varies relative to air fuel ratio, manifold pressure, heat, and static compression to name a few. If you don’t have a dyno handy to get the power curves right then you need to use known good information to get your engine to run properly
Most aftermarket distributors and carburetors are shipped to run rich. A rich engine, and retarded spark, will allow an engine to run without damage (although overly rich and overly retarded are not good for your engine either), and that reduces liability to the manufacturer, and that only makes sense for them. But you need to take those parts and make tune them to your application, and ignition components and ignition timing are extremely important.
Tech notes:
1) As stated above, fuel burn time is relative to several factors, some of which are:
A) Mixture - rich burns slowly, lean burns fast
B) Static compression - higher compression burns faster
C) Fuel - the higher the octane, the slower the burn
D) Manifold pressure. Higher manifold pressure (lower manifold vacuum) means a faster fuel burn. So if you are running a turbocharger or supercharger then you are really going to shorten the fuel burn time as the boost rises. At high boost you may find you may only need about 20 degrees of spark advance to get where you want to be.
E) Engine load. The higher the load the less advance you’ll need, because cylinder pressure is going to go up. You’ll never get a lot of engine load in neutral, so adjusting timing properly takes a dynamic situation such as a dyno, or data collected during actually vehicle driving conditions.
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