Compression

Compression is the stage of the engine cycle where the piston moves up with the valves closed (or ports covered in a two-stroke) and squeezes the air–fuel mixture into the small combustion chamber. Compressing the charge makes it burn faster and more efficiently, so good compression is essential for power and easy starting.

Compression is lost when piston rings, cylinder walls or valves are worn, a valve is not seating properly, valve clearances are too tight or the head gasket leaks. A compression test with a gauge in the spark plug hole, and on four-strokes a leak-down test, shows the condition of the top end.

In rally roadbooks, a compression is also a sharp dip or hollow in the track that compresses the suspension hard and can bottom out the bike; it is ridden standing, with weight back and steady throttle.