Recurving Your Distributor for More Power
In a perfect world there would be no need for ignition advance curves. As soon as the piston in your vintage Vette reached top dead center and the engine built maximum compression, you'd be able to light the fire. Kaboom. Job done. But this isn't a perfect world, and neither is the case of ignition advance curves.
Why Spark Advance Is Necessary
Let's look at why advance curves are required in the first place. Spark is most often introduced into the cylinder prior to the piston reaching TDC. This simply gives the spark sufficient time to light the air-fuel mixture. As engine speed increases, then the time required to bring in the advance increases. Everything else being equal, bringing in the spark sooner creates more cylinder pressure, and, consequently, increases low-rpm torque. There's a trade-off though, as the engine speed increases (particularly in high gear), there is a loss of some top end power. That's why high-gear spark retard systems show performance increases. There is a point, however, where you can dial-in too much advance into the ignition system. Too much advance and the engine will detonate. This is caused when the "explosion" in the engine is timed too early.
Initial Advance
Initial advance is the base timing that is dialed into the engine before the centrifugal advance begins. How much initial do you need? It depends. For example, it's interesting to note that as the altitude increases so does the need for additional spark advance. Why? More advance helps to compensate for the lack of oxygen. There are other factors as well. Check out the chart from MSD:
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MSD's Timing Chart FACTOR
ADVANCE TIMING
RETARD TIMING