How the numbers are derived
Everything flows from four card values. The exhaust centerline is set by the lobe separation and where the intake is installed:
exhaust centerline = 2 × LSA − intake centerline
Then the valve events split each duration around its centerline:
intake opens = duration ÷ 2 − centerline (°BTDC) · intake closes = duration ÷ 2 + centerline − 180 (°ABDC)
overlap = intake opens + exhaust closes = (ID + ED) ÷ 2 − 2 × LSA
What overlap does
Overlap is the window around TDC where both valves are open. A little lets the exhaust's suction start pulling fresh mixture in (scavenging — free cylinder filling at high RPM). A lot means raw mixture and exhaust fight each other at low RPM: the idle lopes, vacuum drops, and the powerband moves up the rev range. Rough guide:
- < 20° — smooth stock-style idle, strong vacuum for accessories.
- 20–45° — noticeable but friendly; classic street-performance territory.
- 45–70° — lopey idle, weak low-end, wakes up past ~3,000 rpm.
- 70°+ — race cam manners: idle is a suggestion, power lives high.
These bands are for advertised overlap, which is much wider than overlap measured at .050″ — a 268°/110 LSA street cam shows ~48° advertised. Compare like with like.
Duration @ .050″ — the honest comparison number
Advertised duration is measured at whatever tiny lift the manufacturer chose, so it inflates. Duration at .050″ lift is the industry yardstick: under ~200° is a torque/stock grind, 200–215° street performance, 215–230° street/strip, and beyond that you're building for the track.
FAQ
What's the difference between LSA and intake centerline?
LSA is the angle between the intake and exhaust lobe peaks — ground into the cam, unchangeable. The intake centerline is where the intake lobe peak sits relative to TDC as installed; degreeing the cam advances or retards it. Installing "straight up" puts the intake centerline equal to the LSA.
Does advancing the cam change overlap?
No — overlap depends only on the two durations and the LSA. Advancing moves all four events earlier together (more low-end, earlier intake closing), retarding moves them later (more top-end).
Why does a bigger cam need more compression?
A late-closing intake valve bleeds off cylinder pressure at low RPM, dropping dynamic compression. Builders raise static compression to compensate — run your numbers through the Compression Ratio calculator.