What is valve overlap?

Valve overlap is the number of crank degrees during which both the intake and exhaust valves are open at the same time. It occurs around TDC at the end of the exhaust stroke and beginning of the intake stroke. Overlap helps scavenge spent exhaust gases, but too much overlap at low RPM causes rough idle and poor vacuum.

How is overlap calculated?

Overlap (degrees) = IVO (degrees before TDC) + EVC (degrees after TDC). For example, if the intake opens 20° BTDC and the exhaust closes 16° ATDC, overlap = 20 + 16 = 36 crank degrees.

What is lobe separation angle (LSA)?

LSA is the angle in camshaft degrees between the intake and exhaust lobe centerlines. It is calculated as (intake centerline + exhaust centerline) / 2. A tighter LSA (e.g. 106°) yields more overlap and a peakier power curve; a wider LSA (e.g. 114°) reduces overlap, improves idle quality, and broadens the torque curve.

What overlap is good for a street engine?

Mild street cams typically have 20–40° of overlap. Performance street or street/strip cams run 40–70°. Race-only engines can exceed 100° of overlap but will idle poorly and have very low manifold vacuum, which affects power brakes and emissions.

What is intake centerline?

Intake centerline is the crank angle at which the intake lobe reaches peak lift, measured in degrees after TDC. It equals (IVC + 180 - IVO) / 2, or Intake Duration / 2 - IVO, where IVO is degrees before TDC and IVC is degrees after BDC. Advancing the intake centerline (smaller number) moves the power band lower in the RPM range.

Does overlap matter differently for turbocharged engines?

Yes. Under boost, exhaust backpressure can cause reversion through high-overlap cams at low RPM, pushing exhaust gases into the intake manifold. Turbo-specific cams typically use a wider LSA and less overlap to improve low-end response and prevent reversion issues.

Valve Overlap Calculator

Enter your intake opening and exhaust closing points to calculate valve overlap duration in crank degrees.

Cam Timing Events

Enter the four valve events from your cam spec card. Only IVO and EVC are required for overlap.

Intake Valve Opens (IVO) — degrees before TDC

° BTDC

Exhaust Valve Closes (EVC) — degrees after TDC

° ATDC

Optional — for centerline & LSA

Intake Valve Closes (IVC) — degrees after BDC

° ABDC

Exhaust Valve Opens (EVO) — degrees before BDC

° BBDC

Example Cam Profiles

Valve Overlap

Enter IVO and EVC above to calculate overlap.

Summary

Valve overlap is the brief period near top dead center (TDC) when both the intake and exhaust valves are open simultaneously. This tool calculates overlap duration in crank degrees from your camshaft timing events: exhaust valve closing (EVC) after TDC and intake valve opening (IVO) before TDC. It also computes lobe separation angle (LSA), intake centerline, and exhaust centerline — the key metrics used to evaluate cam profiles for idle quality, power band, and engine character.

How it works

Enter the exhaust valve closing point (degrees after TDC on the exhaust stroke).
Enter the intake valve opening point (degrees before TDC on the intake stroke).
Optionally enter intake valve closing (degrees after BDC) and exhaust valve opening (degrees before BDC) for full cam event data.
The tool computes overlap = IVO + EVC. Both values measured from TDC sum to the total overlap window.
Intake and exhaust centerlines and lobe separation angle (LSA) are derived from the full four-event spec.
Results update instantly as you type.

Use cases

Evaluate a camshaft specification sheet before purchasing.
Compare overlap duration between stock and performance cam profiles.
Estimate idle quality and vacuum level from overlap characteristics.
Select a cam grind to match a target power band and intended RPM range.
Verify lobe separation angle when degreeing a cam during engine assembly.
Understand the trade-off between overlap and low-end torque on street engines.
Assess forced-induction cam compatibility — high overlap can cause reversion under boost.

Frequently Asked Questions

Last updated: 2026-06-19 · Reviewed by Nham Vu