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Cam timing calculator

Turn the numbers on a cam card into the four valve events, overlap and advance. Then send the intake-closing point straight to the compression calculator.

Enter your numbers
From your cam card
Duration is measured at
Installed intake centerline. Straight up, it equals the lobe separation angle.

Intake closes

–
The event that sets dynamic compression.
Intake opens
–
Exhaust opens
–
Exhaust closes
–
Overlap
–
Exhaust centerline
–
Cam advance
–
Use in compression calculator

Degree your cam

Timing sets, keyways and cam grinds all have tolerances. Checking the installed intake centerline with a degree wheel is the only way to know where your valve events really are.

From cam card to valve events

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

A negative result simply means the event happens on the other side of the dead center, for example the exhaust closing before top dead center. Overlap figures at 0.050 in are much smaller than at seat timing, and small or negative 0.050 overlap is normal.

Advance and retard

Advance is the lobe separation angle minus the intake centerline. Installing a cam advanced moves all the events earlier, which generally shifts power down the rpm range and closes the intake earlier, raising dynamic compression. When you degree a cam, the intake centerline is the number you check with a degree wheel and dial indicator.

Why intake closing matters most

Of the four events, intake closing has the biggest effect on cylinder pressure, which is why the compression calculator asks for it. Use the button to carry it over.

Frequently asked questions

What is "advance" and why does it matter?

Advance describes how many degrees the cam's lobe centerline sits before top dead center relative to a straight-up (zero advance) install — installing a cam with a few degrees of advance is common practice to build a bit more low-end torque.

Why do I need to know the intake-closing point for the compression calculator?

Dynamic compression ratio depends on how long the intake valve stays open after bottom dead center — the later it closes, the more effective compression is reduced — so that single number from the cam timing calculation feeds directly into an accurate dynamic compression figure.

What's the difference between duration and overlap?

Duration is how long one valve (intake or exhaust) stays open through the cam's rotation; overlap is the separate period where both the intake and exhaust valves are open at the same time near top dead center — overlap affects idle quality and low-speed response in particular.

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