Exhaust port timing mainly dictates the engine’s ability to create RPM and make peak horsepower. In order to do this, the principles of effective pressure and blowdown timing are applied. The effective pressure is from the time the piston reaches TDC (Top Dead Center) and the new charge fires until the exhaust port opens. This is also sometimes referred to as “The Power Stroke”.

The next phase of combustion is called the “Blow Down Period”. Blow down time happens from the time the exhaust port opens until the time the intakes ports opens. This determines how much time the cylinder has to expel the burnt mixture.

After the blowdown period, the intake ports open and begin the fill process. The fill duration lasts from the time the transfer ports open and then close again after going past BDC (Bottom Dead Center). During this time the fresh charge, that is waiting in the lower casing, is introduced into the cylinder through the transfer and rear booster ports.

While this is happening, another process is taking place at the same time and that process is called scavaging. Scavaging is the process of expended gases exiting the combustion chamber through the exhaust port. Not only does the expended gas leave, but also a percentage of the fresh charge that just entered the combustion chamber.