Rob Reaser
September 1, 2001

Valve lift: The measure of the lift of the valve. Specifically, it's the lift at the cam multiplied by the rocker arm ratio. For example, a lift at cam height of 0.320 inch using 1.6:1 rocker arms would have a valve lift of 0.512 inch (0.320 x 1.6 = 0.512). Use this same cam with the popular 1.7:1 rockers many 5.0 Mustang owners install and the lift increases to 0.544 inch. As you can see, a combination of a hotter cam and higher ratio lifters can significantly increase valve lift over stock. You must be extremely careful, though, because this combination further reduces critical piston-to-valve clearances.

Lift at cam: The amount of lift of the tappet.

Intake timing (open): The crankshaft degree at which the intake valve opens.

Intake timing (close): The crankshaft degree at which the intake valve closes.

Exhaust timing (opening): The crankshaft degree at which the exhaust valve opens.

Exhaust timing (close): The crankshaft degree at which the exhaust valve closes.

Overlap: The point between the exhaust and the intake stroke where both intake and exhaust valves are slightly open (occurs around top dead center). A siphoning effect takes place here, where the outflow of exhaust gases assists the inflow of fresh air/fuel mixture. This is a critical timing event that dictates how rich or lean an engine will run and directly affects fuel economy, throttle response, emissions, and vacuum.

Lobe separation: Refers to the degrees of rotation separating the peak of the intake valve lift (at the cam lobe) from the peak exhaust valve lift. The closer the lobe separation, the sooner peak torque will build in the basic rpm range. The wider the separation, the more the power will get spread through the basic rpm range, with better power on the upper end of this range.

Intake centerline: The point where the intake valve is opened to its maximum (after top dead center). It's also the reference point at which the cam is installed in relation to the crankshaft (also called degreeing). All cams come with a recommended intake centerline installation point. Advancing or retarding this centerline changes when the valves open and close in relation to the crankshaft rotation or, more specifically, the timing of the piston stroke. Advancing the centerline-say, from a recommended 108 to 104 degrees centerline-keeps the intake valves open longer before the piston goes down on its compression stroke. By adjusting the centerline installation point, you can affect changes to a small degree to maximize power at a specific area on the powerband (low end or upper end). In essence, advancing the cam will shift the basic rpm down the band. Retarding the cam will put the power up at a higher rpm range. Because Mustangs experience traction when low-end torque and horsepower are high, you must make sure you don't give the engine too much advance timing. Advancing the timing too much can also affect piston-to-valve clearance beyond the clearance issues associated with higher valve lift and the use of higher-than-stock ratio rocker arms.

Single pattern: The duration of the intake and exhaust lobes is the same.

Dual pattern: The duration of the intake and exhaust lobes is different.

Compression ratio: Many cam spec charts also give a recommended compression ratio range in which a cam will work. The stock compression ratio (using stock heads, pistons, and rod length) as well as cam duration and centerline position (advance or retard), will affect cylinder pressure. Cams are designed to work with a specific compression ratio, so make sure the cam you buy will work with your setup. Too little compression will reduce engine output while too much compression will cause internal damage, preignition, and detonation.