Editing Camshaft (section)

==== Duration ====
The camshaft's duration determines how long the intake/exhaust valve is open for, therefore it is a key factor in the amount of power that an engine produces. A longer duration can increase [[Horsepower#Engine power test standards|power]] at high engine speeds (RPM), however this can come with the trade-off of less [[torque]] being produced at low RPM.<ref name="hotrod.com">{{cite web |title=Secrets Of Camshaft Power |url=https://www.hotrod.com/articles/ccrp-9812-secrets-of-camshaft-power/ |website=www.hotrod.com |access-date=18 July 2020 |language=en |date=1 December 1998}}</ref><ref>{{cite web |title=Camshaft RPM Range |url=https://help.summitracing.com/app/answers/detail/a_id/4704/~/camshaft-rpm-range |website=www.summitracing.com |access-date=18 July 2020}}</ref><ref name="jegs.com">{{cite web |title=Understanding Camshaft Fundamentals |url=https://www.jegs.com/tech-articles/camshaft-specification-basics.html |website=www.jegs.com |access-date=18 July 2020}}</ref>

The duration measurement for a camshaft is affected by the amount of lift that is chosen as the start and finish point of the measurement. A lift value of {{convert|0.050|in|mm|1|abbr=on}} is often used as a standard measurement procedure, since this is considered most representative of the lift range that defines the RPM range in which the engine produces peak power.<ref name="hotrod.com"/><ref name="jegs.com"/> The power and idle characteristics of a camshaft with the same duration rating that has been determined using different lift points (for example 0.006 or 0.002 inches) could be much different to a camshaft with a duration rated using lift points of 0.05 inches.

A secondary effect of increased duration can be increased ''overlap'', which determines the length of time that both the intake and exhaust valves are open. It is overlap which most affects idle quality, in as much as the "blow-through" of the intake charge immediately back out through the exhaust valve which occurs during overlap reduces engine efficiency, and is greatest during low RPM operation.<ref name="hotrod.com"/><ref name="jegs.com"/> In general, increasing a camshaft's duration typically increases the overlap, unless the Lobe Separation Angle is increased to compensate.

A lay person can readily spot a long duration camshaft by observing the broad surface of the lobe where the cam pushes the valve open for a large number of degrees of crankshaft rotation.  This will be visibly greater than the more pointed camshaft lobe bump that is observed on lower duration camshafts.