Cam terms
Duration:
Degrees of CRANKSHAFT rotation that the valve is open. This is usually expressed in two values Advertised and @.050 TAPPET lift.
Advertised Duration:
Normally measured at .006 TAPPET lift but may be measured at any lift the cam grinder deems appropriate. Consequently this figure is not very useful in comparing cam specs unless it is know what tappet lift each manufacturer uses.
Duration at .050:
More useful measurement for comparison as this figure is measured at the same lift (.050) regardless of the cam manufacturer.
Formula:
Open event+180+Close event.
Lift:
Distance the VALVE is opened. This figure will accompany a rocker ratio. The rocker ratio always has to be considered as this is lift at the valve. For example a cam with a lift rating of .550 with a 1.6 rocker will produce .584 lift with a 1.7 rocker and .515 with a 1.5 rocker.
Intake Centerline (ICL):
The midpoint between the open and close valve events. This is expressed in degrees After Top Dead Center. This changes depending on how the cam is installed (advanced or retarded).
Formula:
Duration/2-Intake Open event.
Exhaust Centerline (ECL):
The midpoint between the open and close valve events. This is expressed in degrees Before Top Dead Center. This changes depending on how the cam is installed (advanced or retarded).
Formula:
Duration/2-Exhaust Close event.
LSA:
Lobe Separation Angle. The distance between the intake centerline and the exhaust centerline. This is ground into the cam and does not change if the cam is advanced or retarded.
Formula
ICL+ECL/2
Overlap:
The time the intake and exhaust valves are both open. Expressed in crankshaft degrees.
Formula
Intake open+Exhaust close
Basecircle:
Area of no lift. The size of the basecircle will vary from cam to cam. Although this is not a hard fast rule, generally the basecircle will be smaller as the lift of the cam increases. This is because most cams are ground from the lobe centerline down to the base circle. So a .281 lobe lift cam will have a larger basecircle than a .343 lobe lift cam. Thus the reason for needing longer pushrods as the lift increases.
Advanced or retarded :
This is often confused with advancing or retarding the cam. This spec is the relationship between the Intake Centerline and the LSA. A cam with a 110 ICL, when installed "dot to dot", and a 112 LSA is said to have 2 degrees of advance ground in.
Advancing or retarding:
This is where the cam is installed advanced or retarded from the cam sheet. Typically the cam sheet specs are accurate when the cam is installed "dot to dot". Most aftermarket timing sets have multiple keyways that allow you to install the cam 2,4 or more degrees advanced or retarded. This moves the ICL and ECL that number of degrees form the cam manufacturers specs. Generally speaking advancing the cam will create more low end torque and drop the max RPM while retarding the cam will have the opposite affect.
For information on how these terms relate to choosing a camshaft click here.
Degrees of CRANKSHAFT rotation that the valve is open. This is usually expressed in two values Advertised and @.050 TAPPET lift.
Advertised Duration:
Normally measured at .006 TAPPET lift but may be measured at any lift the cam grinder deems appropriate. Consequently this figure is not very useful in comparing cam specs unless it is know what tappet lift each manufacturer uses.
Duration at .050:
More useful measurement for comparison as this figure is measured at the same lift (.050) regardless of the cam manufacturer.
Formula:
Open event+180+Close event.
Lift:
Distance the VALVE is opened. This figure will accompany a rocker ratio. The rocker ratio always has to be considered as this is lift at the valve. For example a cam with a lift rating of .550 with a 1.6 rocker will produce .584 lift with a 1.7 rocker and .515 with a 1.5 rocker.
Intake Centerline (ICL):
The midpoint between the open and close valve events. This is expressed in degrees After Top Dead Center. This changes depending on how the cam is installed (advanced or retarded).
Formula:
Duration/2-Intake Open event.
Exhaust Centerline (ECL):
The midpoint between the open and close valve events. This is expressed in degrees Before Top Dead Center. This changes depending on how the cam is installed (advanced or retarded).
Formula:
Duration/2-Exhaust Close event.
LSA:
Lobe Separation Angle. The distance between the intake centerline and the exhaust centerline. This is ground into the cam and does not change if the cam is advanced or retarded.
Formula
ICL+ECL/2
Overlap:
The time the intake and exhaust valves are both open. Expressed in crankshaft degrees.
Formula
Intake open+Exhaust close
Basecircle:
Area of no lift. The size of the basecircle will vary from cam to cam. Although this is not a hard fast rule, generally the basecircle will be smaller as the lift of the cam increases. This is because most cams are ground from the lobe centerline down to the base circle. So a .281 lobe lift cam will have a larger basecircle than a .343 lobe lift cam. Thus the reason for needing longer pushrods as the lift increases.
Advanced or retarded :
This is often confused with advancing or retarding the cam. This spec is the relationship between the Intake Centerline and the LSA. A cam with a 110 ICL, when installed "dot to dot", and a 112 LSA is said to have 2 degrees of advance ground in.
Advancing or retarding:
This is where the cam is installed advanced or retarded from the cam sheet. Typically the cam sheet specs are accurate when the cam is installed "dot to dot". Most aftermarket timing sets have multiple keyways that allow you to install the cam 2,4 or more degrees advanced or retarded. This moves the ICL and ECL that number of degrees form the cam manufacturers specs. Generally speaking advancing the cam will create more low end torque and drop the max RPM while retarding the cam will have the opposite affect.
For information on how these terms relate to choosing a camshaft click here.
