Valve Timing Calculator

Calculator

Engine type Four-stroke Two-stroke

Cycle degrees shown for context only.

Engine speed (RPM)

Used to convert crank degrees into milliseconds.

Cam correction (crank °)

Positive advances all events; negative retards.

Intake opens (BTDC °)

Intake closes (ABDC °)

Exhaust opens (BBDC °)

Exhaust closes (ATDC °)

Valve lash (mm)

Shown as setup context for your notes.

Result emphasis Degrees and milliseconds Milliseconds focus

Does not change the core calculations.

Submit Reset

Example data table

These rows show typical seat timing style inputs and the resulting key outputs.

Formula used

• Intake duration: ID = 180° + IO + IC
• Exhaust duration: ED = 180° + EO + EC
• Valve overlap: OV = IO + EC
• Intake centerline (ATDC): ICL = (180° + IC − IO) / 2
• Exhaust centerline (BTDC): ECL = (180° + EO − EC) / 2
• Lobe separation angle: LSA = (ICL + ECL) / 2
• Advance / retard estimate: Shift = LSA − ICL
• Milliseconds per angle: ms = 1000 × deg / (RPM × 6)

Inputs assume crankshaft degrees referenced to TDC/BDC events. For precision work, always use the same lift reference when comparing cams.

How to use this calculator

1. Enter IO, IC, EO, and EC from your cam card or measurements.
2. Set RPM to convert degrees into time for your operating point.
3. Use cam correction to model advancing or retarding the cam.
4. Press Submit and review durations, overlap, and centerlines.
5. Export results as CSV or PDF for build notes and sharing.

FAQs

1) What do BTDC and ABDC mean?

BTDC is degrees before top dead center. ABDC is degrees after bottom dead center. They describe when a valve starts or finishes its motion relative to piston position.

2) Why is duration computed with 180 degrees?

A piston travels from TDC to BDC in 180 crank degrees. Duration adds the early opening and late closing angles to this base stroke span.

3) What is overlap and why does it matter?

Overlap is when intake and exhaust are both open near TDC. More overlap can improve scavenging at high speed but may reduce idle quality and low-speed torque.

4) What is lobe separation angle (LSA)?

LSA is the crank-angle separation between intake and exhaust lobe centers. Smaller LSA usually increases overlap; larger LSA tends to smooth idle and broaden the torque curve.

5) How does cam advance or retard affect events?

Advancing shifts events earlier: IO and EO increase, IC and EC decrease. Retarding shifts them later. The calculator applies this as a simple crank-degree correction.

6) Are these results valid for any lift reference?

The math works for any consistent reference, but numbers change with the lift point used for measurement. Compare cams only when measured at the same lift specification.

7) Why include RPM and milliseconds?

Degrees are easier for geometry, but time helps visualize how quickly events occur at speed. At higher RPM, the same degrees happen in fewer milliseconds.

8) Can this replace a full dyno or simulation?

No. It summarizes timing geometry and shifts, which supports decisions and documentation. Final performance depends on airflow, compression, ignition, exhaust design, and many other variables.