This gives the average piston velocity throughout its stroke. (NOTE: Calculating the instantaneous velocity as the crank swings in its arc of rotation and accelerates the piston from rest requires differential calculus, which is inappropriate for this level of discussion.) For demonstration purposes, let’s plug in a 3.750-inch stroke and an engine speed of 1,200 rpm into the equation. This would yield a mean piston speed of 750 fpm. Assuming a flame speed of 15 m/s and converting the piston speed to m/s, the piston is travelling substantially faster at 41 m/s. By varying the amount of spark lead, we can choose where peak cylinder pressure will occur in the crankshaft’s arc of rotation. Ideally, this would occur immediately after the piston attains TDC.

    When the timing is extremely retarded, exhaust-gas temperature rises, and the headers actually glow--the late initiation of the spark, and the fuel mixture still burning during blowdown and out into the manifold or header, cause the increase in exhaust-gas temperatures. When timing is advanced, idle speed increases because the engine is using more of the available energy to push the piston down as the flame expands. Retarding the timing kills power--the piston is already partially down on its stroke when the flame front starts to expand and raise the cylinder pressure. As the piston sweeps toward BDC, the total area of the cylinder increases, effectively reducing the power from the expansion of the flame front.

    Many tuners overlook the importance of defining the proper advance curve, thus ignoring the potential for increased performance. Over time, the basis for this has been created with a fixed spark advance. This works for a drag-race engine, with its limited rpm range and operating conditions. Whenever a protrusion (such as a piston dome) is introduced into the combustion chamber, flame speed decreases, requiring additional spark-lead time.