Tuning Wiki

Camshafts and valve timing

How duration, lift and overlap move the torque range, and what variable valve timing changes with engine speed.

Updated by llandowtuninguk@gmail.com   Revision 1

Timing is a speed-range choice

The camshaft decides when each valve opens, how far it opens, and how long it stays open. Those three choices move the engine speed at which the cylinder fills best. They do not add a fixed amount of torque at every speed. More overlap and more duration usually help the top of the range, where the inlet and exhaust waves described on the manifold and exhaust pages have less time to act, and they usually hurt idle and low-speed running, where that same overlap lets exhaust stay in the chamber or lets the new charge escape.

Duration, lift and overlap

  • Duration is how long the valve is off its seat. A longer inlet duration gives the cylinder more time to fill at high engine speed, when each stroke is short in time. At low speed the piston can push charge back out of the inlet before the valve closes.
  • Lift is how far the valve opens. It only helps while the curtain area, rather than the port, is the restriction. Past the lift at which the port or the seat is smaller than the curtain, further lift adds spring load and valve-train speed without adding air.
  • Overlap is the period when the inlet and exhaust valves are open together. MIT's gas-exchange material is the context: a low pressure in the exhaust port during that window can scavenge the chamber and start the inlet flow. A high pressure in the exhaust port during the same window does the reverse. Overlap without a matching exhaust, or overlap on an engine that never runs in the speed range it was cut for, is mostly a rough idle.

Lobe separation is the angle between the inlet and exhaust lobe centres. Closing that angle up generally increases overlap for a given duration. It is one way to describe the same compromise, not a separate source of power.

Published duration figures are not comparable until the lift at which they were measured is known. A duration quoted at the valve, at a small lift, is a longer number than the same cam measured at a larger checking clearance. Compare figures taken at the same lift, and compare them with the valve events the engine will actually run, including any rocker ratio.

Variable timing

A fixed camshaft is a compromise cut into metal. Variable valve timing moves the inlet cam, the exhaust cam, or both, so the overlap and the closing point can change with engine speed and load. BMW's VANOS on the MS42 and MS43 engines is one implementation; the calibration side is in MS42 and MS43 fuel and camshaft control. The mechanism does not replace a mismatched camshaft. It moves the events the installed camshaft is capable of, and the ECU has to know which position it is in. A camshaft change on one of those engines without a matching timing map leaves the variable mechanism aiming at the old events.

Systems that also change lift, or that open a second set of lobes, are doing a larger version of the same thing: one valve event for low speed and another for high speed. The switching point is a calibration, and a hydraulic or electric failure that leaves the engine on the wrong event is a fault, not a power mode.

Clearances, springs and the drive

The timing on the cam card is the timing with the intended clearance or with the hydraulic lifter bled down as the maker specifies. A tight clearance opens the valve early and closes it late. A wide clearance does the opposite and adds impact load. Springs have to keep the follower on the lobe at the engine speed you intend to use. A spring that was adequate for the original cam and rev limit is not automatically adequate for more lift, a steeper ramp or a higher rev limit. The belt, chain or gears have to stay in time as well. A performance camshaft on a jumped chain is just a wrong camshaft.

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