Why Engine Timing Is Measured in Degrees, Not Seconds

Ask when an engine's inlet valve opens and the answer comes back in degrees: about fifteen before top dead centre. Not milliseconds, not "just before the piston reaches the top" — degrees of crankshaft rotation. That choice of unit is not a convention engineers inherited and never questioned. It is the only unit that stays true when the engine changes speed.

The clock that speeds up

A four-stroke engine takes two full turns of the crankshaft to complete one cycle: 720 degrees, 180 for each of intake, compression, power and exhaust. At 1,000 rpm those 720 degrees take 120 milliseconds. At 6,000 rpm they take 20. The mechanical sequence has not changed at all — the same valve opens at the same point in the same stroke — but every interval in seconds has shrunk by a factor of six.

So a valve timing quoted in milliseconds would be wrong everywhere except one engine speed. Quoted in crank degrees it is right at every speed, because the camshaft is geared to the crankshaft and turns at exactly half its rate. The cam does not know what time it is. It knows where the crank is pointing, and that is the only thing the valves need to know too.

Which is why valve overlap exists

Read the timing of a real engine and something looks like a mistake. The exhaust valve is still open when the inlet valve starts to open. For a window of perhaps twenty to forty degrees either side of top dead centre, both are off their seats at once. That is valve overlap, and it is deliberate.

Exhaust gas leaving the cylinder has inertia. A column of hot gas moving down the exhaust port keeps moving after the piston has stopped pushing it, and as it goes it leaves a slight depression behind. Open the inlet valve into that depression and the incoming charge is drawn in rather than merely allowed in. The cylinder scavenges itself: the last of the burnt gas is dragged out and fresh mixture follows it in.

How wide the overlap should be depends entirely on what the engine is for. Gas inertia scales with gas speed, so a wide overlap that scavenges beautifully at 7,000 rpm does nothing useful at 1,200 rpm — at low speed the exhaust column is moving too slowly to pull anything, and the open inlet valve simply lets burnt gas wander back up the inlet tract. That is why a race engine idles badly and a van engine does not, and it is one setting on one shaft.

The spark has the same problem, and a different answer

Ignition is quoted in degrees too — so many before top dead centre — but for the opposite reason. Burning is a chemical process, and it takes roughly the same number of milliseconds regardless of how fast the crank is turning. Light the mixture and it needs its couple of milliseconds to release its heat whether the engine is idling or screaming.

So as revs rise, the spark must be lit progressively earlier in crank degrees just to have the pressure peak land in the same place — a little after top dead centre, where the crank has enough leverage to turn the push into torque. Fire too late and the piston is already running away from the pressure. Fire too early and peak pressure arrives while the piston is still coming up, which is both wasted work and the road to knock. That sliding relationship between engine speed and spark timing is the ignition advance curve, and it is the one place in the engine where the clock genuinely matters.

Both effects are easier to believe once you can move them. The valve timing demo lets you drag the overlap window and watch cylinder filling respond, and the four-stroke demo runs the whole 720-degree cycle at a speed you choose.