In the past several articles we have been dealing with air flow. It is now necessary to go into the more fundamental area of combustion in order to put airflow into its correct context. First, the combustion process is NOT an explosion, but a relatively slow burning. An explosion is considered an instantaneous burning, usually at a rate of 80 mph and above. Combustion burning rarely exceeds 35 mph. Second, the by-products of combustion, except for heat, would not generate enough pressure to turn the engine over. What causes the pressure and the power is the heating of the gases in the combustion chamber by burning fuel in it. Here is where Boyle’s Law gets into the act. One element of this Law, pressure = volume x temperature, is why the engine runs to double the pressure in a fixed chamber at double the temperature. Combustion represents a tenfold increase in temperature and thus a tenfold increase in pressure. So far, so good. However as the piston begins to descend, the chamber gets bigger and the pressure does not increase as fast. In fact, most racing engineers consider that virtually all the work is done by the time the piston is at 70 after top dead center (TDC), and on very radical racing engines they prepare to open the exhaust valve at that point to get a head start on scavenging. It is important to get as much air and fuel into into the chamber prior to the close of the intake valve in order to get the best pressure rise. However, if the pressure rise is too quick it will produce detonation, and that’s the subject of another installment.