Modified Mustangs & Fords
Rocker Arms and Adjustable Valvetrain Geometry
Rocker Arm Reaction
Important Valvetrain Terms
Camshaft Degreeing—Measuring the exact position of the camshaft in degrees of rotation relative to the engine's combustion cycle. Doing so allows timing correction to adjust the performance of the engine.
Coil Bind—The point at which a valvespring can't be compressed any further and the coils touch each other. Valvetrain damage can occur when this happens.
Duration—The amount of time the cam lobe is lifting the valve, measured in degrees of camshaft rotation.
Hot vs. Cold Valve Lash Setting—Lash is the setting for a mechanical camshaft that allows for a measurable distance between the rocker arm tip and valve tip. The cold setting is set before the engine has been run and is larger than the hot lash setting, which is completed after the engine has achieved operating temperature.
Intake Centerline—The midpoint of the cam's lobe, however this may or may not be maximum lift as some cams have an asymmetrical design.
Lift—The amount of travel the lifter moves from the base circle of the cam up to the nose of the cam lobe. That measurement, when multiplied by the rocker arm ratio, generates the total valve lift figure.
Lifter Preload—The initial pressure applied to the lifter through the pushrod and rocker arm when adjusted.
Lobe Separation Angle—The number of degrees between the intake and exhaust lobe centerlines.
Piston to Valve Clearance—The distance between the valve and the piston when the two are at their closest point.
Opening Ramp—The portion of the cam lobe between the base circle and the lobe peak.
Open Pressure—Spring pressure that is created when the valve is open. There needs to be enough pressure at this point to maintain control of the valvetrain.
Overlap—The time, measured in crankshaft degrees, when the exhaust valve and intake valve are both open on an engine's cylinders.
Ramp Speed—The ramp angle of a cam lobe. The more aggressive the lobe's ramp, the faster the valve opens and closes.
Rocker Arm Ratio—The difference between the pushrod end and valve tip end of a rocker arm from the pivot point, expressed as a ratio.
Seat Pressure—The pressure exerted on the spring seat, most commonly stated in pounds per square inch.
Shim—A thin, flat disc available in varying thicknesses used to adjust (compress) the valvespring's installed height.
Single and Dual Pattern Camshafts—A single-pattern cam will have a profile with identical intake and exhaust lift and duration. When the intake and exhaust lift and duration are different, it's considered a dual pattern cam.
Symmetrical vs. Asymmetrical lobes—When the opening and closing halves of a camshaft lobe are different, it's considered an asymmetrical lobe.
Split Duration Camshaft—A cam that has different intake and exhaust duration specifications.
Valve Float—The point at which the intake lifter and its cam lobe are not following the same track/path. In extreme conditions, this will cause valve contact with the piston.
Valve Lash—A measurement taken on solid lifter cams between the tip of the valve and tip of the rocker arm. Valve lash is always measured with the lifter positioned on the base circle of the cam.
Valve Overlap—Overlap is a function of both a cam's duration and lobe separation angle. If the lobe separation angle remains the same but duration is increased, the amount of overlap will also increase.
Valvetrain—Valvetrain is an all-encompassing term used to describe the parts of the engine that operate the engine's intake and exhaust valves.