Exhaust Diameter Calculator
The right exhaust pipe size (inches & mm) for your power — no bigger.
Get the exhaust matched to your build?
How it works
Exhaust sizing is a Goldilocks problem: too small and the engine chokes on its own backpressure up top; too big and gas velocity drops so low you lose scavenging, low-end torque and throttle response. This calculator sizes the pipe from your target power, whether it’s a single or dual system, and whether the engine is naturally aspirated or boosted.
The rule of thumb is diameter ≈ √HP ÷ 6.32 per pipe, which lines up with real-world builds — roughly 2.5" for 250hp, 3" for 350hp, 3.5" for ~450hp on a single pipe. A dual system splits the flow, so each pipe carries half the power. Turbo engines can go a touch larger (×1.1) because the turbine already kills the exhaust pulses that NA engines rely on for scavenging.
This is a starting diameter, not a spec — header/manifold primaries, cat and muffler restriction, and how the car is used (street torque vs top-end track) all shift the ideal. On a DMS build we match the exhaust to the whole combo and prove it on the dyno, so you don’t trade bottom-end for a big-bore sound.
These figures are engineering guidelines to help you plan. The reliable number always comes off the dyno — that's how every DMS build is proven.
FAQ
Is a bigger exhaust always more power?
No — past a point a too-large pipe drops exhaust gas velocity, which hurts scavenging and low-end torque without adding top-end. The goal is the smallest diameter that doesn’t restrict your target power, not the biggest that fits.
Single 3" or twin 2.5"?
Roughly equivalent flow, but they behave differently: a single is lighter, cheaper and keeps more velocity; duals flow a little more at the very top and sound different. This calculator gives the per-pipe size for either — pick on packaging, weight and sound.
Does a turbo car need a bigger exhaust?
Generally yes — the turbine drops exhaust energy and there’s no pulse-scavenging to preserve, so a slightly larger, free-flowing turbo-back reduces backpressure and helps spool and power. We nudge the size up ~10% for boosted engines.