Performance Camshafts Explained: Duration, Lift and Overlap
A camshaft decides when the valves open, how far, and for how long, which makes it the part that sets where in the rev range an engine makes its power.
Cams move the powerband rather than simply adding to it. More duration shifts power up the rev range and takes it away lower down, and the engine must be tuned around the new cam to gain anything.
Everything else on a tuned engine feeds air in and lets it out. The camshaft controls the doors. That is why cams change an engine’s character more than almost any other part.
The three numbers
Duration is how long a valve is held open, measured in degrees of crankshaft rotation, and it is the number that most changes engine character. More duration keeps the intake valve open later, which suits high rpm when the incoming charge has momentum, and hurts at low rpm where it lets mixture push back out. Lift is how far the valve opens, and more of it increases flow without directly affecting the timing, though it puts more strain on the valvetrain. Lobe separation angle is the spacing between the intake and exhaust lobe peaks, and it sets how much the two valves are open at once.
Overlap is what you feel
Overlap is the period where the exhaust valve has not shut and the intake has already opened. A big overlap lets exhaust flow help pull the intake charge in at high rpm, which is where the power comes from. At idle it lets exhaust push back into the intake, which is precisely why a heavily cammed engine idles roughly and produces that lumpy sound. It also means less manifold vacuum, and vacuum runs the brake servo and various valves, so a road car with an aggressive cam can have a soft brake pedal and unhappy emissions behaviour. That is the trade, and it is a real one.
What a cam change drags with it
Cams are rarely a standalone part. Higher lift and stiffer profiles usually need matched valve springs to avoid valve float, and possibly retainers. The engine always needs remapping afterwards, because the airflow model the ECU was calibrated with no longer describes the engine. On a modern variable valve timing engine the factory cam profile is already being moved around by the ECU, so the gains from a mild aftermarket cam can be smaller than expected and the calibration work is harder. Emissions and idle quality change with the cam, so check what your market requires before committing.
The conditions that matter
- ✓More duration and overlap raise top end power at the cost of low rpm torque.
- !Valve springs and a remap are normally part of the job, not optional extras.
- !Big overlap reduces manifold vacuum and changes idle quality and emissions behaviour.
Sources
These are the provisions the road-legality rules linked above rest on.
- Modifying your vehicle's emissions (GOV.UK)
- MOT inspection manual: 8. Nuisance (GOV.UK)
- Diesel particulate filters guidance (GOV.UK)
General guidance only. Details vary by exact vehicle and change over time; confirm with a reputable specialist before modifying.