[Walt Fricke]

Chris

If you are building an engine, I think you are behind the curve. Do you have a copy of Wayne's engine rebuilding book? Or Anderson? Haynes, even? Take a look, and you'll understand. You will need to, because I am guessing you are going to have to time the cams on the motor you are building.

Generally, our 911 cams are timed by the amount of lift on an intake at the TDC where the exhaust cycle is ending and the intake cycle is starting. Both intake and exhaust valves are usually open at that point (hence "overlap."), though it doesn't matter practically as we just use intake lift, as the relationship to the exhaust is built into the cam. 3.8mm is the figure the guy who makes them says you should use for his cam. A lot of stock cams are in the ranges discussed above. Race cams can be a lot more - one of mine is 5.8mm.

If you used 4.2 mm, you would be advancing the cam. Normally this helps torque at the expense of high end horsepower. Retarding is the opposite (3.4, for instance). All of this is also constrained within the range where the valves won't be hitting the piston.

Or did I miss something in your question?

Walt