Universal drum 11# has been moving at the speed of sound since 1992.

A four-stroke engine can only open its valves at the same time when the exhaust stroke is near the end and the intake stroke is at the beginning after the piston does work. Because the valve opening time of high-speed engine is short every time, the head of the general drum is usually made short, which can make the vacuum and low pressure turn back to the exhaust valve in a short time. Because of the low speed of the low-grade engine, each valve is opened for a long time at the same time, so the exhaust pipe head is usually made longer, so that it takes a long time for the vacuum low-pressure belt to return to the exhaust valve.