Because four strokes take two full crankshaft revolutions, the camshaft that opens each valve exactly once per cycle must turn at exactly half the crankshaft’s speed — never the same speed.
Intake, compression, power and exhaust each take one piston pass (a half-revolution); four halves make two revolutions, but each valve still only needs to open once, so the cam driving it is geared or chained down 2:1 from the crank.
Only on a four-stroke. A two-stroke fires every revolution, so any camshaft it has runs at crank speed. And on a variable-valve-timing engine the ratio holds while the phase does not — the cam is still at half speed but no longer at the timing you assumed.
Estimate with the rule, then check it against the calculator that models it properly.
Open Engine Cycle Simulator →