Camshafts and measuring duration?

After finally reading the July issue of Chevy High Performance's
article of camshaft basics, I'm still confused on advertised duration and duration at 0.050 lift.
My understanding is that Harvey Crane came up with duration
being started at 0.050 tappet lift, to eliminate the confusing
advertised duration, by different companies. So, they go on to talk about how duration can be specified two ways, either advertised duration or duration at 0.050 tappet lift.

Is advertise duration measured at 0.050 tappet lift or some other
point? Can someone clear this up for me?

Thanks

Originally posted by ZYALATER
After finally reading the July issue of Chevy High Performance's
article of camshaft basics, I'm still confused on advertised duration and duration at 0.050 lift.
My understanding is that Harvey Crane came up with duration
being started at 0.050 tappet lift, to eliminate the confusing
advertised duration, by different companies. So, they go on to talk about how duration can be specified two ways, either advertised duration or duration at 0.050 tappet lift.

Is advertise duration measured at 0.050 tappet lift or some other
point? Can someone clear this up for me?

Thanks

Advertised duration is measured at .006 (or thereabouts) tappet lift. In order to take up the lash or 'slack' in the lifter/pushrod/rocker combination the cam initially lifts very gently. This is the "ramp", not to be confused with the "flank" or the part of the lobe where the majority of the lift occurs.

Because the ramp gives only a little lift for quite a few degrees, the .050 tappet (or lifter) spec is used to compare cams. On a solid lifter cam, the ramp slowly takes up the "valve lash" so the parts don't smash together. The same for the closing ramp.

The reason it's called "advertised" duration is probably because it's a bigger number and bigger numbers sell better than smaller numbers. If you are comparing similar cams, like hydraulic flat lifter cams, the advertised durations are just about proportional to the .050 durations. If you were to compare a hydraulic flat lifter to a very aggressive solid roller, that might not hold true.

They said it pretty well in the CHP article, quoted here:

"So let's take a typical cam and look at how duration is expressed. The point at which lobe lift first begins is often difficult to identify since the profile is very gradual at this point. A long time ago, the Society of Automotive Engineers (SAE) decided that 0.006 inch of valve lift was a good place to start, but not all cam manufacturers adhered to that standard. They chose instead to use different heights of >> tappet lift, usually between 0.004 and 0.006 inch. Using 0.004 inch as an example, once lobe lift achieves 0.004 inch, you start recording the number of crankshaft degrees it takes for the lobe to run all the way through max lift and back to 0.004-inch lobe lift on the closing side. Let's say this is 270 degrees. This is the advertised duration of the lobe because this is the number that most cam manufacturers use when referring to their camshaft duration numbers in advertising."

To pick it apart, the SAE .006 valve lift number is about .004 tappet lift with 1.5 rocker arm ratio, so .004 or .006 at the lifter gives pretty much the same duration.

Sometimes I think we ought to go back to "3/4 Race" and "Full Race" to describe cams. It was easier then, unless you choose an Isky "5 cycle" cam! :)

Page 8 of the online Comp Cams catalog is a good read with a diagram:

http://www.compcams.com/catalog/008.html