Valve Overlap Calculator
Overlap is the window where both valves are off the seat around TDC. It sets idle quality, vacuum and where the engine comes alive.
| Valve overlap | 21.0 | ° |
- overlap = (intake duration + exhaust duration) ÷ 2 − (2 × LSA)
- Uses seat-to-seat duration. Advertised @ 0.006" and 0.050" figures are not interchangeable.
When you need it
When predicting how a cam will idle before you buy it, and when deciding between lobe separation angles for the same duration.
What overlap actually does
Overlap is the window around top dead center when both valves are off their seats. At high RPM the exhaust leaving at speed pulls fresh charge in behind it, which is free cylinder filling and part of why big cams make top-end power.
At idle there is no velocity to do that work. Instead exhaust gas washes back into the intake and fresh mixture goes straight out of the pipe. That is the lope, the low manifold vacuum, and the reason a big cam will not hold a steady idle or run power brakes.
Lobe separation is the lever
For a given duration, a tighter LSA gives more overlap: lopier idle, stronger mid-range, narrower usable band. A wider LSA gives less overlap: smoother idle, better vacuum, broader and flatter power.
Below about thirty degrees of overlap the engine stays civil and keeps enough vacuum for brakes. Past about fifty-five you are committing to a converter and gear to live with it.
Boost and nitrous want less
On a boosted or nitrous engine, overlap means blowing charge — or bottle — straight out of the exhaust while the intake pressure is higher than the exhaust. Cams for those applications are ground on wider lobe separation angles for exactly that reason, and a naturally aspirated cam moved onto a boosted engine usually idles and drinks badly.