How do you measure cam duration?

[2002Targa]

Greets all,

I just recieved a motor thats had some performance work done on it long ago and is supposed to have a reground cam.

How do you figure out what the regrind was at?

[edgefinder]

If its a simple regrind they just cut about .02" off the lowest part of the lobe. The area where you adjust clearance. You can measure across the lobe with a caliper from the lowest point to highest and compare with a stock cam. If you see any welding on the high part of the lobe then someones done some serious work to it. If you want to measure duration you'll need a travel dial indicator and degree wheel.

[TobyB]

there are several ways. BmW seems to measure duration with zero lash-

that is, lash everything down tight, put a degree wheel on the crank, and then measure how long (how many degrees) the valve is open for... if you put a dial indicator on the rocker, it's pretty obvious when it starts to open, and how much it opens. But unless you have very accurate measuring tools and methods, you'll vary a few degrees and a few .001"...

However, many (most?) cam grinders measure duration at .050"- that is, how long the cam keeps the valve open for more than .050". It gives you a significantly different (lower) number than the BmW way.

The first time I had a head done for the streeet car, I said I wanted a bit more cam, "maybe a bit more than a 292." The machinist looked at me and said, "Ok, what's it got in there now?" "Oh, I think the stock duration might be 260- ish". He looked at me for a minute and said, "Not a 260 in MY book". Then we checked it both ways, and I think at .050 it came out looking like a 230 or something like that...

Surprised me!

We put in a '260' duration cam and it had a noticable bump in power- but still idled ok... I suspect it's really a 292 in the BmW world!

t