If the camshaft turns at half the speed of the crankshaft, what is the crankshaft when compared to the camshaft?

• A. The same speed
• B. Twice the speed
• C. Half the speed
• D. Four times the speed

Answer: (B)

In a typical four-stroke engine, the camshaft runs at about half the crankshaft speed so valves open and close at the right times. If the camshaft turns at half the crankshaft speed, the crankshaft must be twice as fast as the camshaft. So, the crankshaft is rotating at double the camshaft speed (for example, if the camshaft is 3,000 rpm, the crankshaft would be 6,000 rpm). The effect of this 2:1 ratio is what allows one valve event per cylinder every two crank revolutions.