Wankel engines generally are able to reach much higher engine revolutions than reciprocating engines of similar power output. This is due partly to the smoothness inherent in circular motion, and the fact that the "engine" rpm is of the output shaft, which is three times faster than that of the oscillating parts. The eccentric shafts do not have the stress-related contours of crankshafts. The maximum revolutions of a rotary engine are limited by tooth load on the synchronizing gears. Hardened steel gears are used for extended operation above 7000 or 8000 rpm. Mazda Wankel engines in auto racing are operated above 10,000 rpm. In aircraft, they are used conservatively, up to 6500 or 7500 rpm, but as gas pressure participates in seal efficiency, racing a Wankel engine at high revolutions under no-load conditions can destroy the engine.