From the mid-1970s through to 2001, the top class of GP racing allowed 500 cc displacement with a maximum of four cylinders, regardless of whether the engine was a two-stroke or four-stroke. This is unlike TT Formula or motocross, where two and four strokes had different engine size limits in the same class to provide similar performance. Consequently, all machines were two-strokes, since they produce power with every rotation of the crank, whereas four-stroke engines produce power only every second rotation. Some two- and three-cylinder two-stroke 500s were seen, but though they had a minimum-weight advantage under the rules, typically attained higher corner speed and could qualify well, they lacked the power of the four-cylinder machines.