Cam Timing Calculator

The terms on a cam card

• Duration is how many crankshaft degrees a valve is open. This calculator uses duration at .050-inch lift, the figure the Chevrolet Performance spec sheets below list.
• Lobe separation angle (LSA) is the angle between the intake and exhaust lobe centerlines, in camshaft degrees.
• Intake centerline (ICL) is where the intake valve reaches peak lift, in crankshaft degrees after top dead center.
• Advance is how far the cam is installed ahead of straight up. It equals the LSA minus the intake centerline, so a cam with a 112° LSA installed at a 108° intake centerline is 4° advanced.

How to calculate valve timing events

Intake centerline = LSA − advance
Exhaust centerline = LSA + advance
Intake opens (BTDC) = intake duration ÷ 2 − intake centerline
Intake closes (ABDC) = intake duration ÷ 2 + intake centerline − 180
Exhaust opens (BBDC) = exhaust duration ÷ 2 + exhaust centerline − 180
Exhaust closes (ATDC) = exhaust duration ÷ 2 − exhaust centerline
Overlap = intake opens + exhaust closes

Each valve event sits half the duration either side of that valve's centerline. The 180 appears in the closing and opening points near bottom dead center because those are measured from BDC instead of TDC.

Example: Chevrolet Performance 350 HO cam

The 350 HO's cam is listed at 212° intake and 222° exhaust duration at .050-inch lift, with a 109° intake centerline and a 116° exhaust centerline.

1. LSA is (109 + 116) ÷ 2 = 112.5°, and advance is 112.5 − 109 = 3.5°.
2. Intake opens at 212 ÷ 2 − 109 = −3° BTDC, which is 3° after TDC.
3. Intake closes at 212 ÷ 2 + 109 − 180 = 35° ABDC.
4. Exhaust opens at 222 ÷ 2 + 116 − 180 = 47° BBDC.
5. Exhaust closes at 222 ÷ 2 − 116 = −5° ATDC, which is 5° before TDC.
6. Overlap is −3 + −5 = −8°.

Why some results are negative

A negative opening or closing point means the event happens on the other side of the reference point. An intake opening of −3° BTDC means the valve reaches .050-inch lift 3° after top dead center. Negative overlap at .050-inch lift means the intake and exhaust valves are never both above .050 inch at the same time, as it is for five of the six Chevrolet Performance cams below.

Events calculated from .050-inch duration show when each valve passes .050 inch of lift, not when it leaves or returns to its seat. Advertised duration is measured at a lower lift, so it gives earlier openings and later closings. Compare cams using the same basis.

Chevrolet Performance crate engine cams

Valve events at .050-inch lift

Duration and centerlines from the Chevrolet Performance spec sheets for the 350/290 HP, 350 HO, SP350/357, ZZ5, Ramjet 350 and ZZ454/440. LSA, advance and valve events are calculated from those figures with the formulas above.

Advancing or retarding the cam

Advancing the cam lowers the intake centerline, so every valve event moves earlier by the same number of degrees. Retarding it moves them all later. Overlap doesn't change, because it depends only on the durations and the LSA.

The intake closing point is the event to watch when you're matching a cam to compression. The cylinder can't start building pressure until the intake valve closes, so a later closing lowers the effective compression. Use the compression ratio calculator for the static ratio, and record your cam's grind number, duration, lobe center and installed intake centerline on the engine data sheet (PDF) as you degree it in.