Calculator Inputs
Example Data Table
| Intake Duration | Exhaust Duration | ICL | ECL | RPM | IVO | IVC | EVO | EVC | Overlap |
|---|---|---|---|---|---|---|---|---|---|
| 260° | 268° | 106° | 114° | 3500 | 24.00° BTDC | 56.00° ABDC | 68.00° BBDC | 20.00° ATDC | 44.00° |
Formula Used
This calculator uses a common symmetric-lobe timing model for a four-stroke engine.
- IVO = (Intake Duration / 2) − Intake Centerline
- IVC = (Intake Duration / 2) + Intake Centerline − 180
- EVO = (Exhaust Duration / 2) + Exhaust Centerline − 180
- EVC = (Exhaust Duration / 2) − Exhaust Centerline
- Overlap = max(0, IVO) + max(0, EVC)
- LSA = (Intake Centerline + Exhaust Centerline) / 2
- Time per crank degree = 60000 / (RPM × 360)
- Valve open time = Duration × Time per crank degree
Positive IVO values are shown as BTDC. Negative IVO values are shown as ATDC. Positive IVC values are ABDC. Positive EVO values are BBDC. Positive EVC values are ATDC.
How to Use This Calculator
- Enter intake and exhaust duration values in crankshaft degrees.
- Enter the intake centerline in degrees after top dead center.
- Enter the exhaust centerline in degrees before top dead center.
- Provide the engine speed to estimate open time in milliseconds.
- Submit the form to see opening, closing, overlap, and inferred separation values.
- Use the chart to review where each event sits inside the 720-degree cycle.
- Export the result as CSV for spreadsheets or PDF for shop reports.
About Valve Timing Analysis
Valve timing controls when the intake and exhaust valves open and close during a four-stroke engine cycle. These events strongly affect idle behavior, cylinder filling, scavenging, dynamic compression, and high-speed breathing. A timing calculator helps engineers, tuners, students, and builders compare camshaft choices without redrawing the cycle by hand.
Intake duration and intake centerline determine how early the intake valve opens and how late it closes. Exhaust duration and exhaust centerline do the same for the exhaust side. When these values change, overlap also changes. More overlap can help high-speed airflow in some engines, but it may also reduce idle quality and vacuum.
This page uses a symmetric timing approach that is useful for quick planning, education, and baseline checks. It also converts duration into milliseconds at the chosen RPM, which is useful when relating crank angle to real operating time. The plotted cycle gives a clear view of event placement over 720 crankshaft degrees.
FAQs
1. What does valve timing mean?
Valve timing describes when the intake and exhaust valves open and close relative to piston position. It affects torque, idle quality, emissions, and high-speed breathing.
2. What are IVO, IVC, EVO, and EVC?
They are the four main valve events: intake valve opening, intake valve closing, exhaust valve opening, and exhaust valve closing. These events define the breathing window.
3. Why is overlap important?
Overlap is the period near top dead center when both valves are open together. It can improve scavenging, but too much overlap may hurt idle stability and vacuum.
4. Why does the calculator ask for RPM?
RPM lets the tool convert crankshaft degrees into milliseconds. That helps you understand how much real time each valve event occupies at a chosen engine speed.
5. What is the lobe separation angle shown here?
The displayed separation is inferred from the entered intake and exhaust centerlines. It is useful for comparison, but final camshaft specs should always be checked against the manufacturer sheet.
6. Can negative event values occur?
Yes. A negative result means the event moved across the nearest dead-center or bottom-center reference. The calculator relabels the event so the timing still reads clearly.
7. Is this useful for custom cam comparisons?
Yes. You can enter different duration and centerline sets to compare event placement, overlap, and open time before selecting or degreeing a camshaft.
8. Does this replace measured cam checking?
No. This is a planning and estimation tool. Final verification should come from actual cam degreeing, indicator measurements, and the engine builder’s confirmed specification sheet.