Someone explain cam grinding

[Lothar]

Could someone explain how grinding stock cams works. I understand the concept of lift, duration and overlap, but how do these corespond to the shape of the lobes on the cam, i.e. top of lobe is valve open or closed? Where do you take metal out of a cam to enhance performance (no, I'm not thinking of a D.I.Y. project).

I have 964 cams in my SC with SSI's. I have read about the scavenging effects of this combination. I don't think the cams were ground from stock cams, but I'm noy sure.

[A Quiet Boom]

Top of the lobe would be maximum valve lift. The distance in degrees that the lobe covers the base circle is duration. Lobe separation is the distance in degrees between intake and exhaust lobes for one cylinder. Since the valve is closed when the cam follower is on the base circle it is possible to regrind a cam using a smaller base circle thereby leaving "room" to grind the original lobe to the desired shape. A perfect cam would have a square lobe, but this is impossible for several reasons such as the inability for a cam follower to follow a square lobe and the valves opening and closing to fast for the springs to control. Lift and Duration can be looked at as the area under the curve in which the lobe rises above the base circle. Increasing the area under the curve means the valve is open farther longer thereby allowing more airflow into or out of the cylinder. I could get really specific about cam design but it's such a large subject I can't get in to it here. If you have some specific questions about cam design I'll be happy to address them.

[ChrisBennet]

I thought they built up he cam and then ground it to the desired profile.
-Chris

[Zeke]

Originally posted by ChrisBennet
I thought they built up he cam and then ground it to the desired profile.
-Chris

No, not necessarily. The crank, yes on a stroker, because it has to be the origianal diameter at the rod journal. However there is at least one exception to that that I know of from VW. They made a 71mm crank out of a 68mm by grinding the difference off center. Uses smaller rods.

Additionally, a smaller base circle with higher lift requires less reconfiguration of the rocker geometry on a push rod engine. I don't know how this works on the 911 engine (yet), but I would think that it would have a similar effect.

[ChrisBennet]

I did a little research. Web Cam hard welds them and regrinds them.
Web Cam
"The part numbers listed below are factory profiles that WEB-CAM hardweld and grinds on your cams. Any 911/930/964/993 up to 1995 camshaft can be used. Cost is per set. "
-Chris