Combustion takes roughly a fixed time — call it 2 ms — so the advance needed to put peak pressure just after TDC grows in direct proportion to rpm: about 12° at 1,000 rpm and 72° at 6,000.
The flame does not know what the crank is doing. Burn time is set by chamber shape, mixture and turbulence, and the crank simply turns further through it as revs rise — 6° per millisecond per 1,000 rpm, exactly. That is the entire job of a centrifugal advance weight, and later of an ignition map: not to optimise anything clever, but to keep a roughly constant burn pointed at the same part of the stroke.
Burn time is not actually constant, so the straight line over-predicts at the top end. Turbulence rises with engine speed and shortens the burn, which is why real advance curves flatten off rather than climbing forever, and why they are mapped rather than calculated. A leaner mixture pulls the other way by burning slower, and knock sets a hard ceiling that has nothing to do with the arithmetic — past it you retard timing and accept the loss.
Estimate with the rule, then check it against the calculator that models it properly.
Open 2 & 4-Stroke Engine Simulator →Combustion takes roughly a fixed time — call it 2 ms — so the advance needed to put peak pressure just after TDC grows in direct proportion to rpm: about 12° at 1,000 rpm and 72° at 6,000. The flame does not know what the crank is doing.