Advertised Cam Duration Q?

**89LX** · 07-13-2007, 04:12 PM

What are Advertised Intake Duration /Advertised Exhaust Duration numbers as opposed to Intake Duration at 050 / Exhaust Duration at 050? The advertized duration numbers are always larger. I understand the 0.050 sizeing
but they never mention that figure when talking "advertized" numbers.

**Mr. Luos** · 07-13-2007, 06:26 PM

Intake:
Degrees BTDC intake valve begins to open + 180 degrees of crank rotation + degrees ABDC intake valve begins to close = advertised duration.

Example: 33 degrees BTDC + 180 degrees + 67 degrees ABDC = 280 degrees.

Exhaust:
Degrees BBDC exhaust valve begins to open + 180 degrees of crank rotation + degrees ATDC exhaust valve begins to close = advertised duration.

Example: 69 degrees BBDC + 180 degrees + 39 degrees ATDC = 288 degrees.

**mrr23** · 07-13-2007, 06:42 PM

back in the 'day', each manufacturer 'advertised' their cam specs at different points of lift. some .006" some .004". there was no common ground to compare cams from one manufacturer to another. it was harvey crane who came up with the idea of having everyone put up the specs of .050" lift.

basically it's the amount of time (duration) the valves are open from beginning to ending of the event.

**vettenuts** · 07-17-2007, 02:14 AM

There is a SAE standard that provides guidelines on cam measurement. The standard is for the measurement to be taken at 0.006" lift, with a what was standard 1.5 rocker ratio, this equates to 0.004" at the cam lobe. Isky and Comp do not follow the standard. These numbers are used to represent the valve seat timing. David Vizard's valve train book discusses this and is a very good read if you don't have it.