Exhaust Scavenging Calculator

Scavenging is the art of using the exhaust's own pressure waves to help empty and refill the cylinder. When the exhaust valve snaps open, a sharp positive pressure pulse races down the primary tube at roughly the local speed of sound. At the collector the tube suddenly opens into a larger volume, and that area change reflects the pulse back toward the valve as a negative (low-pressure) wave — the classic quarter-wave behaviour of an open-ended pipe. If the primary length is tuned so that this returning vacuum arrives at the port during valve overlap, it actively sucks the residual exhaust out and helps draw the fresh intake charge in, effectively raising volumetric efficiency without any extra fuel or boost. Because wave travel time is fixed by the speed of sound and the tube length, the length you choose tunes the effect to one particular RPM: shorter primaries return the pulse sooner and favour high RPM, while longer primaries favour low-end torque.

This is fundamentally a resonance phenomenon, so it has harmonics. A primary tuned for, say, 6,500 RPM will also show a weaker secondary scavenging node near a third of that speed, which is why a well-designed header can add a useful bump in the midrange as well as at the peak it was cut for. The unavoidable catch is that tuning sharpens the powerband: a header optimized for one RPM gives away a little everywhere else, and the narrow tuned peak can feel peaky on the street. Stepped primaries — growing the tube diameter in stages along its length, such as 1.625 to 1.75 to 1.875 inch — broaden the effective range by roughly 500 RPM per step at the cost of a touch of absolute peak. Diameter still has to keep gas in the scavenging velocity window, so after picking a primary length here, cross-check the bore with the Exhaust Gas Velocity calculator before ordering tubing.