I don't mean to put myself out there as a cam expert. I'm soaking up cam info at a pontiac site, cuz I'm in the planning stage of a Poncho 406 build for my LeMans street/strip project.
Based on the info in the first post, the first lift and first duration #s are for the intake, and the 2nd lift and 2nd duration #s are likely for the exhaust. So it may be a dual pattern cam.
Various cam manufactures play with all the variables to get differing performance out of similar lift and duration numbers.
A 500 lift would be .500" inches or 1/2 inch. To get lobe lift, divide valve lift by rocker arm ratio. You can use high ratio rockers to increase valve lift without a large increase in duration. That helps power in your desired powerband. But then you've got to pay extra special attention to coil bind and P to V clearance.
The cam events are referenced to 360 crankshaft degrees. Dont forget that the crank goes around twice for every once of cam rotation. So if a cam has 250 degrees of duration at .050" lift, that means that from the time the lifter moves .050" upward from seated (base circle) thru max lift to the time its .050" from seated again, the crankshaft has rotated 250 degrees. Usually when you degree in a cam, its referenced to the Intake Center Line. (easiest way to do it).
When I degreed the M4F cam in my 244 (235 bored .060") I installed it "straight up", with the Intake Centerline at 106 degrees of crankshaft rotation past TDC. (howard's spec for Intake Centerline.) When you advance or retard a cam, you move the IC relative to crankshaft rotation, which moves the powerband up or down in the RPM range. Advancing a cam moves powerband downward in RPM. Retarding a cam moves powerband upward in RPM. I'm looking into a trick of using a slightly hotter cam than needed, then advancing it 2 to 4 degrees to move powerband downward in my Poncho, while still streaching the RPM range past 5500 RPM. I'm trying to get 450 HP at the flywheel (high 12s in 1/4 mile) and still be torquey on the street. But with high ratio rockers I want to use, I've got to make sure a piston and valve don't have a one-night stand.
From what I've heard, you only want to advance the cam in a streeter (if needed) so you don't kill all low-end torque. Some racers retard the cam 'timing' to reduce low-end torque to prevent wheelspin and give them more power at the far end of the track.
You can change cam timing by using a combination of offset crank keys and a timing set with 3 keyways: Advanced, straight up, and retarded.
Still kinda fuzzy on if the resultant IC number goes up or down if you advance a cam.
Quick Quiz: Advancing the cam 4 degrees on a stock Intake Centerline of 106 would give you an IC of 102 or 110????
anybody know??
chip
