Camshaft Duration Calculator

Camshaft duration measures how long, in crankshaft degrees, a valve stays open -- and the higher the RPM at which you want peak power, the more duration you generally need, because at higher engine speeds there is less TIME (even though the crank-angle window may be similar) for the cylinder to fill or empty through the valve. Peak power RPM is accordingly the dominant driver of both intake and exhaust duration in this calculator's simplified linear model, while rocker arm ratio has no effect on duration at all -- it only scales how far the valve physically opens (valve lift = lobe lift times rocker ratio), which is why rocker ratio is instead the dominant factor for valve lift, not duration. Lobe separation angle (LSA) governs valve overlap -- the crank-degree window where both intake and exhaust valves are open together -- and a WIDER LSA reduces overlap (smoother idle, broader torque), while a TIGHTER LSA increases it (peakier power, rougher idle), matching established camshaft-design convention; overlap itself is calculated from ADVERTISED (seat-to-seat) duration, not the @0.050" figures, since a symmetric cam lobe opens and closes around its own centerline at seat-to-seat timing, and LSA enters twice (once for how early the intake side opens, once for how late the exhaust side closes) rather than once.

Cam advance shifts the intake centerline earlier without changing duration or overlap at all, which is the standard way installers fine-tune low-end torque without re-grinding the cam. This calculator's duration-from-RPM relationship is a simplified linear approximation, not a value pulled from a specific camshaft manufacturer's catalog, so treat the numeric duration and overlap outputs as a rough starting point for a conversation with a cam grinder or catalog, not a spec to order a cam from directly -- it is anchored to real reference cam cards at both the low and high ends of the peak-power-RPM range (for example, a duration around 224-230 degrees at 0.050" for a roughly 5,000-5,500 RPM peak-power target, matching a well-known street/strip hydraulic-roller grind), but a single RPM input can never capture every variable (displacement, compression ratio, cam type, valve size, induction) a real cam selection depends on.