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
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
Most Cam Cards (a paper that lists all the important specs of the cam) 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.
- Intake: Opens at 7° BTDC, closes at 39° ABDC.
- Exhaust: Opens at 51° BBDC, closes at 3° ATDC
Duration = 7° + 39° + 180° = 226°
Duration = 51° + 3° + 180° = 234°
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.
I found this awesome technical and product information on Summit Racing’s website.
I have an article here that you might find useful, Exhaust Headers – A Proven Way to Boost Performance.