How To Read A Cam Card

How To Read A Cam Card
Why the heck would you want to read a cam card?! Better yet, how do you read a cam card?! When you purchase a
cam, a cam card is provided to help the installer understand more about the cam set-up. This usually gets thrown out
with the box the cam came in, because most of the time the cam you purchased is a tried-and-true product. Since
knowledge is power, we compiled the following cam card guidelines to help you better understand what it is you
just purchased. This is vital information, because there is a lot more to a camshaft than lift and duration.
A: Intake & Exhaust Duration: This is measured in degrees of how long the valves are open. This number is rated at
the camshaft and end result will be the same at the valve.
B: Intake & Exhaust Lift: This is measured in inches and in most cases this number refers to the valve lift and not
cam lift. Valve lift is the measurement at the valve side of the rocker arm. This number is a calculation of cam lift
with rocker ratio figured into the equation. (See D rocker ratio)
C: Intake & Exhaust Clearances: This is the recommended valve lash for these particular cams. The measurement is
given in inches and the card explains that measurements should be done with the engine cold.
D: Rocker Ratio: The rocker ratio is the proportional relationship between measured lift at the cam and measured lift
at the valve based on a rocker arms fulcrum point.
E: Duration @ .050: This is the U.S. standard of measurement. U.S.-spec cams are measured at .050 thousands of an
F: Lobe Lift: This is the actual lift of the cam without the rocker ratio calculated into the equation. Cam lift is
figured by measuring the cam from the tip of its ramp to the bottom of the base circle. Then measuring the base
circle and subtracting the base circle measurement from the tip-to-base measurement.
G: Lobe Center: This is also referred to as lobe separation angle. Lobe center is measured in degree comparisons
between the intake lobes and the exhaust lobes from the center of the camshaft. Since most imports are DOHC this
number can be tailored with the use of cam sprockets. DOHC engines have two centers because of the use of two
cams. Since changing the degrees on the cams can widen or shorten the lobe separation angle this number is often
ignored. On single-cam engines this number is fixed since the intake and exhaust lobes are on the same shaft.