Cam Calculator

How the camshaft calculator works

A cam card describes the camshaft in two kinds of degrees. Durations, valve events and centerlines are crank degrees, measured against the crankshaft as it turns. The lobe separation angle is in cam degrees: the angle between the peaks of the intake and exhaust lobes on the camshaft itself, which turns once for every two turns of the crank. This camshaft calculator moves between the two, so you can check a card, compare two cams, or see what installing a cam advanced or retarded does to the valve events.

The Valve events tab takes both durations, the lobe separation angle and the installed intake centerline, and returns when each valve opens and closes, the overlap around TDC and how far the cam is advanced. The Lobe separation tab works backwards from the four events printed on a card. The Valve lift and Rocker ratio tabs handle the other half of the card: how far the lobe rises and how much the rocker arm multiplies it.

What to enter and where to find it

• Duration. Use the figure at 0.050 in of lifter rise that every cam card prints. It is the most comparable number between cam grinders. Advertised duration works too and gives seat-to-seat events, but never mix the two bases in one calculation.
• Lobe separation angle. Ground into the camshaft and printed on the card. Changing it means a different cam.
• Intake centerline. The crank angle after TDC at which the intake lobe reaches full lift, as installed. The card gives the recommended figure, and a degree wheel and dial indicator confirm it when the cam goes in. Installed straight up, it equals the lobe separation angle.
• Valve events. Cards list them as degrees before or after TDC or BDC. An event on the other side of the dead center goes in as a negative number, such as an intake valve that only opens after TDC.
• Lobe lift and rocker ratio. Lobe lift is the rise of the lobe above its base circle, also on the card. The rocker ratio is the nominal ratio of the rocker arms.
• Valve lash. Zero for hydraulic lifters. For solid lifters, the lash on the cam card, set between the rocker and the valve tip.

Lobe separation angle calculator: centerlines and advance

The lobe separation angle fixes how far apart the two lobe peaks are, and together with the durations it sets the overlap: overlap = (intake duration + exhaust duration) ÷ 2 − 2 × LSA. Tighten the LSA by one degree and overlap grows by two crank degrees. Tighter angles give more overlap, a rougher idle and a narrower power band; wider angles give less overlap and a smoother, broader one.

The installed intake centerline moves the whole camshaft relative to the crank without changing the LSA. Installed advanced, with the intake centerline below the LSA, the intake opens and closes earlier, which tends to move torque toward lower rpm; retarded does the opposite. In the cam card example below the cam is ground on a 112° LSA and installed at a 108° intake centerline, so it is 4° advanced.

Intake closing is the event that matters most for cylinder pressure, because the piston cannot start compressing the charge until the intake valve shuts. That is why the compression ratio calculator asks for it to work out dynamic compression. Give it the closing point on the basis it asks for: the 0.050 in closing point comes earlier than the seat-to-seat one.

Valve lift calculator and rocker ratio calculator

Lift is simpler than timing. The lobe raises the lifter, and in a pushrod engine the rocker arm multiplies that motion by its ratio before it reaches the valve. The valve lift calculator multiplies lobe lift by rocker ratio to give gross valve lift, the figure cam cards print: in the example below, 0.320 in of lobe lift on 1.50:1 rockers gives 0.480 in.

Solid lifters run with a small gap, the lash, that the lobe has to close before the valve starts to move, so the lift at the valve is gross lift minus lash. Hydraulic lifters run with no lash, so gross and net lift are the same. Real valvetrains also lose a little to pushrod and rocker flex, which no simple calculation captures.

The rocker ratio calculator works the other way round: valve lift divided by lobe lift. It shows what a card's lift becomes with different arms, or checks the real ratio of a rocker when you measure both lifts with dial indicators. Stamped ratios are rounded, and the effective ratio changes slightly through the lift as the rocker swings.

Camshaft formulas

Everything here is geometry, with no empirical constants, so the results are exact for the numbers you enter on the 0.050 in basis that cam cards use for centerlines. With advertised durations the events are close but not exact on asymmetric lobes, whose seat-to-seat points are not centered on the 0.050 in centerline. Events and durations count crank degrees, with TDC to BDC being half a turn, and the camshaft turns once for every two crank revolutions.

1. Exhaust centerline (BTDC) = 2 × LSA − intake centerline (ATDC)
2. IVO (BTDC) = intake duration ÷ 2 − intake centerline
3. IVC (ABDC) = intake duration ÷ 2 + intake centerline − 180
4. EVO (BBDC) = exhaust duration ÷ 2 + exhaust centerline − 180
5. EVC (ATDC) = exhaust duration ÷ 2 − exhaust centerline
6. Overlap = IVO + EVC = (intake duration + exhaust duration) ÷ 2 − 2 × LSA
7. LSA = (intake centerline + exhaust centerline) ÷ 2; advance = LSA − intake centerline
8. Valve lift = lobe lift × rocker ratio (minus lash at the valve on solid lifters); rocker ratio = valve lift ÷ lobe lift

The event formulas follow from the fact that a lobe's centerline sits halfway through its opening period. The intake opens IVO degrees before TDC and closes IVC degrees after BDC, so its duration is IVO plus half a turn plus IVC, and its peak comes half a duration after it opens. The exhaust works the same way, measured from TDC backwards. Overlap, the time both valves are open around TDC, is simply intake opening plus exhaust closing; when the result is negative, there is a gap between the exhaust closing and the intake opening at that lift.

Overlap depends on the duration basis. At 0.050 in a mild cam often shows only a few degrees of overlap, or a negative figure, while the same cam's advertised overlap is far larger. Compare cams on the same basis only.

Worked example

Four everyday questions, worked out with the same functions that run the calculator.

Typical overlap by duration and lobe separation angle

For a single-pattern cam, with the same duration on both lobes, overlap depends only on duration and lobe separation angle, not on where the cam is installed. The table covers durations from mild to race at 0.050 in and the angles most cams are ground on. A split-pattern cam with a longer exhaust adds half the difference between the two durations.

Advertised overlap is larger by roughly the average difference between the advertised and 0.050 in durations, because each lobe's extra duration is shared between its opening and its closing side.

Common mistakes

• Mixing duration bases. Advertised durations with 0.050 in events, or the other way round, give meaningless results. Use one basis for every number.
• Treating the LSA as crank degrees. The lobe separation angle is measured on the camshaft. It is half the sum of the two centerlines, not their difference.
• Confusing ground-in and installed advance. Advance changes the installed intake centerline, never the LSA. A cam ground with advance and installed dot to dot is already advanced: check the card's installed centerline.
• Dropping the sign of an event. Mild cams at 0.050 in can open the intake after TDC or close the exhaust before it. Those events are negative here, and the overlap can be negative too.
• Forgetting lash on solid lifters. The card's valve lift is gross lift; with solid lifters the valve opens less by the lash. The Valve lift tab shows both.
• Taking the stamped rocker ratio as exact. Real arms differ a little from their nominal ratio, and flex takes a little more. Measure lift at the valve when the exact figure matters.