Duration is how long the valve is off its seat. It is listed in degrees of crankshaft rotation. When someone refers to a "big cam," they mean it has longer duration, not higher Lift.
Manufacturers often list 2 different duration values:
- Advertised Duration is the degrees of crankshaft rotation that the lifter is raised more than a predetermined amount. This predetermined amount varies between manufacturers.
- Duration at 0.050" is the degrees of crankshaft rotation between when the lifter is raised 0.050" and when it is 0.050" from its resting position. This is standard among all manufacturers. You should use this value to compare camshafts.
How is it measured?
Most Cam Cards will list duration. However, if you want to find the duration for the camshaft in your engine, you can calculate it.
- Using a Dial Indicator and Degree Wheel, find the opening and closing points of the valves at 0.050 in. of lift.
- If the intake valve opens AFTER TDC, use a negative value.
- If the exhaust valve closes BEFORE TDC, use a negative value.
- Add the numbers together.
- Add 180° to find duration.
For example:
- Intake: Opens at 7° BTDC, closes at 39° ABDC.
- Exhaust: Opens at 51° BBDC, closes at 3° ATDC
Intake Duration = 7° + 39° + 180° = 226°
Exhaust Duration = 51° + 3° + 180° = 234°
How does it affect performance?
At high rpm, longer intake duration fills the cylinder. It also allows more exhaust to escape. This creates more power. (It is why a camshaft's RPM Range is based on its duration.)
However, at low rpm, the open valves reduce the pumping pressure of the piston. This results in lower cylinder pressure and less low-end power.
Long duration camshafts also create more Valve Overlap. At high rpm, this helps promote the Scavenging Effect. But at low rpm, it also contributes to Exhaust Reversion.