Valve Timing Calculator

Valve timing describes exactly when the intake and exhaust valves open and close relative to the position of the piston, measured in crankshaft degrees around top dead center (TDC) and bottom dead center (BDC). On a four-stroke engine the camshaft orchestrates four distinct valve events per cylinder: intake opening, intake closing, exhaust opening, and exhaust closing. Where these events fall determines how well the cylinder fills with air and fuel, how completely it scavenges burnt gases, and where in the rpm band the engine makes its best power.

This valve timing calculator takes the four timing events straight off a camshaft specification card — intake opens before top dead center (BTDC), intake closes after bottom dead center (ABDC), exhaust opens before bottom dead center (BBDC), and exhaust closes after top dead center (ATDC) — and converts them into the numbers engine builders actually use: intake and exhaust duration, valve overlap, intake and exhaust lobe centerlines, lobe separation angle, and the amount of cam advance or retard. From those figures it estimates the powerband, idle quality, and manifold vacuum you can expect.

Because the four events are usually quoted at a fixed checking clearance — commonly 0.050 inch of tappet lift — the calculator lets you record that clearance so your results are comparable from one cam to another. Reading these four numbers correctly and turning them into duration, overlap, and centerlines is the foundation of camshaft selection, and this tool removes the arithmetic guesswork.

The calculator is built directly on the geometry of a four-stroke cycle, where every 180 crankshaft degrees the piston travels between TDC and BDC. Each valve duration is the time the valve is off its seat expressed in crankshaft degrees, and it equals the half-stroke window of 180 degrees plus the degrees the valve is open on either side of it.

Intake duration is the intake-opens figure (BTDC) plus 180 degrees plus the intake-closes figure (ABDC). Exhaust duration works the same way: exhaust opens (BBDC) plus 180 degrees plus exhaust closes (ATDC). Valve overlap is the brief window near TDC where both valves are partly open, and it equals the intake-opens figure plus the exhaust-closes figure, because both of those events straddle top dead center.

The intake centerline is the crankshaft angle ATDC at which the intake lobe reaches peak lift, calculated as half the difference between intake closing and intake opening, plus 90 degrees. The exhaust centerline uses half the difference between exhaust closing and exhaust opening, plus 90 degrees. The lobe separation angle (LSA) is the average of those two centerlines, and the cam advance is the LSA minus the intake centerline: a positive value means the cam is advanced, a negative value means it is retarded, and zero means it is ground straight up. Finally, the tool classifies the powerband by intake duration — under 220 degrees is stock/mild, 220–239 is street performance, 240–259 is performance/race, and 260 or more is full race — and predicts idle quality and vacuum from the overlap figure.

Every reputable camshaft ships with a specification card that lists the four numbers this valve timing calculator needs. They are normally given at 0.050 inch of tappet lift — the industry-standard checking clearance — so two cams can be compared on equal footing. A typical street performance grind might read intake opens 10 BTDC, intake closes 50 ABDC, exhaust opens 50 BBDC, and exhaust closes 10 ATDC.

Plugging those four values into the calculator yields a 240-degree intake duration and a 240-degree exhaust duration (a symmetrical, dual-pattern grind), 20 degrees of valve overlap, a 110-degree ATDC intake centerline, a 70-degree BTDC exhaust centerline, a 90-degree lobe separation angle, and 20 degrees of retard. The intake duration of 240 places this grind in the performance/race powerband window. The table below shows how this tool maps intake duration onto a powerband, exactly matching the thresholds used in the calculation.

Valve overlap is one of the most telling numbers this calculator produces because it drives the personality of the engine. Overlap is the span near top dead center where the exhaust valve has not yet closed while the intake valve has already begun to open. More overlap helps high-rpm cylinder scavenging and peak power, but at idle it lets exhaust gas dilute the incoming charge, lowers manifold vacuum, and creates the lumpy, lopey idle that defines a performance cam.

This tool estimates idle character directly from the overlap figure. Up to 20 degrees of overlap it reports a smooth idle with 17–21 in/Hg of vacuum, the kind of behavior you want for power brakes and a stable idle. From over 20 up to 40 degrees it reports a mild lope with 14–17 in/Hg. From over 40 up to 60 degrees it reports a lopey idle with 10–14 in/Hg. Above 60 degrees of overlap it flags a rough idle with only 6–10 in/Hg of vacuum — enough to demand a vacuum pump for the brakes on a street car.

Because overlap in this calculator is simply the intake-opening figure plus the exhaust-closing figure, two cams with the same duration can produce very different overlap depending on how the events are split around TDC. That is why reading the four events individually, rather than trusting duration alone, matters when you are predicting how a cam will behave on the street.

The calculator's cam advance output tells you whether the camshaft is timed to favor low-end torque or top-end power. It is found by subtracting the intake centerline from the lobe separation angle. When the intake centerline is smaller than the LSA, the intake lobe peaks earlier in the cycle and the result is positive — the cam is advanced, which generally builds low-rpm torque and sharpens throttle response. A larger intake centerline gives a negative result, meaning the cam is retarded and power shifts higher in the rpm band.

With the default street/strip events on this page (intake opens 10, intake closes 50, exhaust opens 50, exhaust closes 10), the intake centerline lands at 110 degrees ATDC and the LSA at 90 degrees, so the advance figure is 90 minus 110, or 20 degrees retarded. Changing any single event shifts the centerlines and therefore the advance figure, which is exactly how installers fine-tune a cam with offset keys or an adjustable timing set.

Use this valve timing calculator as a companion to the cam grinder's recommendations, not a replacement for them. Enter the four events for each cam you are comparing, note the duration, overlap, centerlines, LSA, and advance, and match those outputs to your converter stall, compression ratio, and intended use. Pairing the results with a lobe separation calculator and a cam duration calculator gives you a complete picture of how the camshaft will work in your combination before you ever turn a wrench.