What do H/C really do??

Ok, as far as the cams go, duration is how long the cam holds the valves open and is measured in degrees of rotation. Lobe lift is how tall the lobes on the cam are. Valve lift is how far the valves open at max lift. Valve lift is increased over the lobe lift due to the ratio of the rocker arms. Heads allow more air to get in, better combustions, and more air to get out. Compression is how far and hard your A/F mixture is compressed. I don't know exactly what the ratios mean, but I know what compression is and how it works. lol

A/F ratios depend on the car, induction system, spark tables, etc. Most N/A motors like somewhere between 13.0-13.5.

Camshaft specs:

Duration @ 0.050" is the measurement of how long the valve stays open from 0.050" after complete close to 0.050" before complete close. It is measured in degrees of revolution of the crankshaft. I tend to look at the duration numbers @ 0.050 first, then look at the advertised duration figures to get an idea of how fast the ramps are.

There are two different ways to measure lift; lobe lift and valve lift. Lobe lift is simply that, the measurement of the lobe. Valve lift is measured after the rocker arm multiplication of the lobe lift. The industry standard is to present the camshaft with the valve lift figures on it. With a SBF, it's understood that the standard rocker arm ratio is 1.6:1.

For example: If you had a .320" lobe lift and a 1.6 rocker, you would have .320 x 1.6 = .512" valve lift.

If you saw a cam card that said that the valve lift w/ 1.6 rockers was .589", but you wanted to see what the lift would be with a 1.7:1 rocker ratio, you would follow this process: (.589 / 1.6) = .368" lobe lift x 1.7 rocker ratio = .626" valve lift w/ 1.7 rockers.

Compression is also a ratio, which means it's a measurement of two things. 1) The total volume of the cylinder when the piston is the furthest down the bore (BDC), and 2) the total volume of the cylinder when the piston is furthest up the bore (TDC).

If you had a huge cylinder that had 250 cubic inches of volume when the piston was at BDC and it had 20 cubic inches of volume when the piston was at TDC, that means you would have a compression ratio of (250/20) = 12.5:1.

Joe

95Vert said:

thanx man, i'll make sure to refer to this thread if i ever need to calculate that. but still, the lobe and duration depend on what you want to do with the car? you just can't set up certain heads to a certain cam, right?

Click to expand...

Sure, you could. I know I wouldn't though....

Without getting too technical, yes, lobe and primarily duration depend on what your goals are for the car.

Joe