Cam Duration Calculator

Calculator Inputs

Enter common valve timing points. This calculator uses standard 4-stroke crankshaft degree relationships and then converts duration into camshaft degrees.

Intake Opening Angle

Intake Opening Reference

Intake Closing Angle

Intake Closing Reference

Exhaust Opening Angle

Exhaust Opening Reference

Exhaust Closing Angle

Exhaust Closing Reference

Rocker Ratio

Intake Lobe Lift (mm)

Exhaust Lobe Lift (mm)

Checking Lift Reference

Engine RPM

4-stroke timing basis Crank and cam degrees Overlap and centerline results Export ready

Example Data Table

Use this example to verify the calculator and compare your own cam card numbers.

IVO	IVC	EVO	EVC	Intake Duration	Exhaust Duration	Overlap	ICL	ECL
10° BTDC	40° ABDC	45° BBDC	8° ATDC	230°	233°	18°	105° ATDC	108.5° BTDC

Formula Used

This page uses standard cam timing arithmetic for a 4-stroke cycle. Angles are handled in crank degrees first, then converted into cam degrees.

Intake Duration = 180 + IVO + IVC
Exhaust Duration = 180 + EVO + EVC
Valve Overlap = max(0, IVO + EVC)
Cam Degrees = Crank Degrees ÷ 2
Intake Centerline = (Intake Duration ÷ 2) − IVO
Exhaust Centerline = (Exhaust Duration ÷ 2) − EVC
Lobe Separation Angle = (ICL + ECL) ÷ 2
Valve Lift = Lobe Lift × Rocker Ratio
Open Time in ms = (Duration ÷ 360) × (60000 ÷ RPM)

For sign handling, early opening or late closing values that add duration are treated as positive. After-reference values that reduce duration are treated as negative.

How to Use This Calculator

Enter the intake opening value and choose whether it occurs before or after top dead center.
Enter the intake closing value and choose whether it occurs after or before bottom dead center.
Enter exhaust opening and closing values with their correct references from the cam card.
Add rocker ratio and lobe lift values to estimate real valve lift.
Enter the checking lift value you used, such as 0.050 or another reference point.
Enter engine RPM to estimate how long the valve stays open in milliseconds.
Press the calculate button to view durations, overlap, centerlines, lobe separation angle, and the timing graph.
Use the CSV and PDF buttons to export the result summary.

FAQs

1) What is cam duration?

Cam duration is the number of crankshaft degrees that a valve remains open. It is measured between the selected opening and closing points on the cam card.

2) Why does this calculator use crank degrees first?

Most cam cards publish opening and closing events in crankshaft degrees. Converting from that basis keeps the calculation consistent and lets you also see camshaft degrees afterward.

3) What does overlap mean?

Overlap is the period around top dead center when intake and exhaust valves are open at the same time. It affects idle quality, scavenging, and high-speed breathing.

4) Why is the checking lift important?

Duration depends on the lift point used for measurement. Advertised duration and duration at 0.050 inch are different, so always compare numbers measured at the same checking lift.

5) What is the difference between lobe lift and valve lift?

Lobe lift is the cam lobe’s actual rise. Valve lift is the amplified motion seen at the valve after multiplying by the rocker ratio.

6) What does lobe separation angle show?

Lobe separation angle is the angle between the intake and exhaust lobe centerlines. It helps describe how tightly or widely the timing events are spaced.

7) Can I use this for intake-only or exhaust-only checks?

Yes. The intake and exhaust results are computed separately. If you only care about one side, focus on that duration, centerline, and timing window in the result area.

8) Why does the chart sometimes extend beyond 720 degrees?

An event that begins before top dead center and finishes after the next stroke boundary wraps across the cycle. Extending the graph keeps the timing window visually continuous.