Duration

Duration measures how long the valve remains open, expressed in degrees of crankshaft rotation. For example, a cam duration of 220° means the valve is open for 220° of crank rotation.

In high-RPM engines, longer durations help air continue flowing into the cylinder even at fast piston speeds. In low-RPM engines, too much duration can reduce power due to poor timing with piston movement.

Note: Most engine builders use duration measured at 0.050" lift to avoid advertising tricks based on looser standards like 0.020".

Centerline

The centerline tells you when the camshaft lobe reaches its peak lift in relation to crankshaft rotation. It's critical for calculating valve opening/closing, aka valve events.

Valve timing is key to engine performance — valves must open/close in harmony with piston movement and crank speed.

Finding Intake Lobe Center

• Remove all lash, place dial indicator on retainer or follower.
• Rotate engine forward to 0.050" lift. Note opening (e.g., 10° BTDC).
• Continue rotating until it drops back to 0.050" on the other side (e.g., 39° ABDC).

Note: If intake opens after TDC, subtract it instead. Example:

Finding Exhaust Lobe Center

• Rotate to 0.050" lift (e.g., 44° BBDC).
• Continue to 0.050" off base circle (e.g., 8° ATDC).

Note: If exhaust closes before TDC, adjust accordingly:

Separation

Lobe separation is the number of degrees between the centerlines of the intake and exhaust lobes, measured in camshaft degrees. Example: 108° separation.

Alone, this spec is not very useful unless you also know centerline and duration. It affects valve overlap and cam behavior but is best used in conjunction with full valve event analysis.

Lift

Lift measures how far the valve opens. If lift is too low, it restricts airflow. If it's too high, flow gains plateau.

Analogy: Like a garden hose, opening more increases flow — but only to a point. After that, additional opening does nothing.