Exhaust Header Length & Diameter Calculator

Primary length comes from the widely-used tuned-length rule of thumb — L (in) = (850 x exhaust duration @ 0.050") / target RPM − 3 — an empirically-calibrated industry starting point for timing the reflected pulse back to the exhaust valve, not something derived here from raw acoustics (that needs the local gas velocity superimposed on the speed of sound, which is beyond a simplified tool). Diameter is derived directly from exhaust mass flow (VE-based intake flow plus fuel mass, expanded to volumetric flow at EGT) sized to your target gas velocity. Treat both as a fabrication starting point to dyno-tune from, not a final spec — and note this tuning approach applies to naturally-aspirated exhaust; turbocharged manifolds are a different design problem, covered separately by the turbo airflow calculator. Primary tube volume (length x cross-sectional area) is included for stepped-header design: when packaging forces unequal primary lengths across cylinders, stepping each tube's diameter to match this volume — rather than holding every primary to the same length — keeps the tuning effect closer to even across cylinders.