Calculator Inputs
Example Data Table
| Intake Duration | Exhaust Duration | ICL | ECL | IO | IC | EO | EC | Overlap | LSA |
|---|---|---|---|---|---|---|---|---|---|
| 280° | 286° | 106° ATDC | 114° BTDC | 34° BTDC | 66° ABDC | 29° BBDC | 77° ATDC | 111° | 110° |
Formula Used
1) Intake opening and closing
Intake Opening = (Intake Duration ÷ 2) − Intake Centerline
Intake Closing = (Intake Duration ÷ 2) + Intake Centerline − 180
2) Exhaust opening and closing
Exhaust Opening = (Exhaust Duration ÷ 2) − Exhaust Centerline
Exhaust Closing = (Exhaust Duration ÷ 2) + Exhaust Centerline − 180
3) Lobe separation and overlap
LSA = (Intake Centerline + Exhaust Centerline) ÷ 2
Cam Advance = LSA − Intake Centerline
Overlap = Intake Opening before TDC + Exhaust Closing after TDC
Use timing numbers taken at the same checking lift. Mixing seat timing with duration at 0.050 can distort the result.
How to Use This Calculator
- Enter intake and exhaust advertised durations in crank degrees.
- Enter the installed intake and exhaust centerlines from your cam card.
- Add maximum valve lift values for the visual graph scale.
- Set a target RPM to help classify the cam setup.
- Press Calculate Timing to show events above the form.
- Review overlap, LSA, advance, and the graph before making changes.
- Export the summary as CSV or PDF for workshop records.
Frequently Asked Questions
1) What does this calculator measure?
It calculates intake opening, intake closing, exhaust opening, exhaust closing, valve overlap, lobe separation angle, and cam advance from duration and centerline inputs.
2) Are the timing values crank degrees or cam degrees?
These formulas use crankshaft degrees. Most cam cards publish valve timing in crank degrees, so enter the same style of numbers here.
3) Should I use advertised duration or duration at 0.050?
Use one consistent standard only. Advertised duration pairs best with advertised opening and closing events. Duration at 0.050 should be compared only with 0.050 timing data.
4) Why is overlap important?
Overlap affects scavenging, idle quality, exhaust reversion, and high-rpm breathing. Larger overlap often favors upper-rpm performance but may reduce idle vacuum and low-speed smoothness.
5) What does cam advance mean here?
Positive cam advance means the intake centerline is earlier than the lobe separation angle. That usually shifts torque lower in the rpm range.
6) Is the graph a real valve lift trace?
No. The graph is a smooth visual aid built from event timing and maximum lift. It helps compare timing windows, not measure an exact lobe shape.
7) Can I use millimeters for lift?
Yes. The graph uses lift only for scaling, so inches or millimeters both work. Keep both lift entries in the same unit system.
8) Why do my numbers differ from a manufacturer cam card?
Manufacturers may rate duration at a different checking lift, include asymmetrical lobes, or publish rounded values. Use the same measurement standard for accurate comparison.