Cam Timing Overlap Input Form
Use cam card timing values at the same checking lift for consistent comparisons.
Plotly Graph
This graph compares duration, overlap, and centerline values from the current timing setup.
Example Data Table
| Scenario | IO BTDC | IC ABDC | EO BBDC | EC ATDC | Overlap | General Character |
|---|---|---|---|---|---|---|
| Mild street cam | 8 | 40 | 42 | 6 | 14° | Smooth idle and higher vacuum |
| Balanced performance cam | 12 | 44 | 48 | 10 | 22° | Broader powerband and usable idle |
| Aggressive race cam | 24 | 58 | 62 | 18 | 42° | High-speed scavenging and rough idle |
Formula Used
1) Intake Duration
Intake Duration = Intake Opens + Intake Closes + 180
2) Exhaust Duration
Exhaust Duration = Exhaust Opens + Exhaust Closes + 180
3) Valve Overlap
Overlap = Intake Opens BTDC + Exhaust Closes ATDC
4) Intake Centerline
Intake Centerline ATDC = (Intake Duration ÷ 2) − Intake Opens − Cam Advance
5) Exhaust Centerline
Exhaust Centerline BTDC = (Exhaust Duration ÷ 2) − Exhaust Closes + Cam Advance
6) Lobe Separation Angle
LSA = (Intake Centerline + Exhaust Centerline) ÷ 2
7) Overlap Time at RPM
Cycle Time = 120000 ÷ RPM and Overlap Time = (Overlap ÷ 720) × Cycle Time
These equations assume a four-stroke cycle and consistent checking lift references. Always compare timing figures measured at the same advertised or checking lift standard.
How to Use This Calculator
- Enter intake opening in degrees before top dead center.
- Enter intake closing in degrees after bottom dead center.
- Enter exhaust opening in degrees before bottom dead center.
- Enter exhaust closing in degrees after top dead center.
- Add engine speed if you want overlap time in milliseconds.
- Use cam advance input to study installed timing changes.
- Submit the form and review overlap, duration, centerlines, and LSA.
- Use the graph and export buttons for reporting.
Frequently Asked Questions
1) What is cam timing overlap?
Cam timing overlap is the crankshaft interval when the intake valve is opening while the exhaust valve is still closing near top dead center. It affects scavenging, idle quality, vacuum, emissions, and high-speed breathing.
2) How do I calculate overlap quickly?
Add intake opening before top dead center to exhaust closing after top dead center. If the intake opens 12° BTDC and the exhaust closes 10° ATDC, overlap is 22°.
3) Why does higher overlap change idle quality?
Higher overlap can reduce idle vacuum and increase reversion at low speed. That usually creates a rougher idle, weaker manifold vacuum, and a narrower low-speed torque range.
4) Does overlap always improve power?
No. Overlap must match displacement, head flow, compression, exhaust tuning, and intended RPM. Excessive overlap can hurt drivability and low-speed performance even when top-end flow improves.
5) What is lobe separation angle?
Lobe separation angle is the angular distance between intake and exhaust lobe centerlines. Wider LSA generally softens overlap effects, while tighter LSA usually increases overlap and sharpens the power curve.
6) Why should checking lift stay consistent?
Timing figures change with the lift point used for measurement. Always compare cam cards and measured events at the same checking lift to avoid misleading duration and overlap comparisons.
7) Can cam advance change overlap results?
Installed cam advance mainly shifts centerlines and valve event timing. It can alter how the engine responds, but the basic overlap relationship should be interpreted together with all event changes.
8) Is this calculator useful for diesel engines?
It is most useful for conventional valve-timed four-stroke engines. Diesel applications can still use the math, but combustion behavior and tuning priorities differ from spark-ignition engines.