Exhaust port

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The exhaust port is a passage in the cylinder which transports spent and burned mixture from the cylinder into the exhaust pipe. The exhaust port window is the actual opening in the cylinder bore, which is opened and closed by the movement of the piston.

Exhaust ports and performance

In piston ported two-stroke engines, moving the top edge of the exhaust port window towards the cylinder head is called "raising the exhaust port." This advances the port timing by making the exhaust port open earlier in the downward stroke of the piston which helps the exhaust gas to escape more quickly. If you raise the exhaust port too much, the gases will escape too early to give the piston the maximum amount of push.

If the sides of the exhaust port window are made wider, this is called "widening the exhaust port." Doing this makes the exhaust port window larger, and the larger the exhaust port window is, the faster the exhaust can escape the cylinder. At higher RPMs, it is desirable to be able to evacuate the exhaust gases from the cylinder as quickly as possible. However, if the exhaust port is made too wide, the piston rings will bulge into the port and get caught while moving rapidly, often breaking the piston ring, and causing catastrophic damage to the cylinder and piston.

The exhaust port extends from the port window down to the bottom of the cylinder where the flange of the exhaust pipe header bolts down. For optimum performance, the opening where the exhaust port meets the exhaust pipe should be the same size as the exhaust pipe. The port should then taper to the actual port window if the port window is smaller than the exhaust header hole.

Modifying your exhaust port to meet these guidelines is called port matching the exhaust.