The Difference Between A Two-Stroke And Four-Stroke Engine

There are two engine types available for dirt bikes: two-stroke (also known as a two-cycle motor, smoker, pinger, or 2T) and four-stroke (aka four-cycle motor, thumper, four-banger, or 250F for 250cc race bikes). Let's take an illustrated tour through one complete cycle of each type. Note that with one full cycle, the two-stroke piston makes one trip up and down, and the four-stroke piston makes two trips up and down.

1. When the piston reaches the top of the cylinder, the spark plug ignites the air/fuel mixture, which explodes, driving the piston downward.
2. Expanding gases force the piston down and escape out the exhaust valve as it is exposed. The piston forces the next air/fuel charge below it up through transfer ports to the area opening above it.
3. When the piston is at the bottom of the stroke, the transfer port's top is exposed to let the air/fuel charge into the cylinder above the piston.
4. As the piston heads back up, it creates a vacuum beneath it that sucks the next air/fuel charge in through the one-way reed valve system. The piston also compresses the charge above it to be ready to explode when the spark plug ignites again.

1. When the piston reaches the top of the cylinder the spark plug ignites the air/fuel mixture, driving the piston down on the powerstroke. All valves (intake and exhaust) are closed.
2. The piston reaches the bottom of the cylinder and starts back up on the exhaust stroke. A cam pushes the exhaust valves (usually there are two) slightly open into the cylinder and the piston forces the exhaust gases out.
3. The piston reaches the top (the exhaust valves close to avoid contact) then starts down on the intake stroke. The spark plug does not fire. Now the intake valves (usually there are two) are pushed open by the cam, and the vacuum created by the piston sucks in a fresh air/fuel charge.
4. The piston reaches the bottom and starts back up on the compression stroke. All the valves are closed. At the top, the spark plug will ignite the air/fuel mixture to cause the next powerstroke.

Is your philosophy “There’s no replacement for displacement” or “Power corrupts”? Just as with most aspects of motorcycles, you have to give to get. The displacement choices for full-sized dirt bikes range from 125cc to over 500cc.

Big Engines

Engines with greater displacement create more torque and horsepower, but weigh more and also “feel” heavier. The stronger engine forces affect suspension action more than with smaller engines, so the rear shock and spring rate need to handle those forces as well as bumps and obstacles. Still, there’s nothing like having massive amounts of power on tap with the twist of your wrist.

Small Engines

These produce less horsepower, and even less torque. The bikes generally are and feel lighter, but the power requires more focus, shifting and higher revs to use effectively. These bikes are more nimble than their bigger brothers, and some riders are willing to give up raw power for a bike that is more responsive to body English.