Engines · Piston · Demo
The cam decides where the engine breathes best — and there’s no cam that breathes everywhere.
A commuter cam is ~250° with 20° of overlap; a race cam ~310° with 70°. Watch the lift curves crowd together around exhaust TDC as overlap grows — and watch what that does to the idle meter.
Qualitative volumetric-efficiency model: duration sets where the breathing bell peaks, overlap adds high-rpm scavenging and low-rpm reversion. The shapes and trade-offs are the real story — a dyno would put numbers on them, but never change their direction.
In plain terms
An engine is an air pump, and the camshaft is its choreography: when the valves open, how long they stay open (duration), and how long intake and exhaust are open together around top dead centre (overlap). Air has inertia — at high rpm it needs a long head start, at low rpm a long-open valve just lets it flow back out. So every cam is a bet on an rpm range: a street cam breathes beautifully at 2 500 rpm and suffocates at 7 000; a race cam is the reverse, and idles like it’s about to die.
At high rpm the intake stroke lasts milliseconds, so the valve must open early and close late to catch the fast-moving air column — long duration. But at low rpm that same late-closing valve lets the rising piston push fresh charge straight back into the intake. Longer duration slides the whole torque curve up the rev range; it never adds torque everywhere.
Around exhaust TDC, holding both valves open lets the departing exhaust column pull fresh charge in — free cylinder filling at high rpm, where the gas moves fast enough for the trick to work. At idle the same open pair lets exhaust wander back into the intake instead: lumpy running, that classic race-cam “chop”. Overlap buys top end with idle quality.
Two identical engines with different camshafts are different engines: one a tractable commuter, one a peaky screamer. This is why variable valve timing was such a big deal — VTEC, VANOS and friends are ways of having two cams in one, switching the bet as the revs rise.