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Exhaust Pipe Diameter Calculator

Size header primary tubes using (cylinder CI × peak-torque RPM) ÷ 88,200 and estimate main exhaust CFM and pipe diameter for target gas velocity. Includes common HP rule-of-thumb reference.

What this calculator is for

Header primary tube diameter trades exhaust velocity against flow area. Too small chokes high-RPM power; too large kills scavenging on the street. Common builder formula: primary area = (cylinder CID × peak-torque RPM) ÷ 88,200; ID from area.

The tool also estimates exhaust CFM and main pipe diameter for target gas velocity (about 250–300 ft/s in many references) plus an HP rule-of-thumb band.

A good outcome: a primary ID and collector/main pipe size to compare against header catalogs before you weld custom pipes.

Calculator

How to use this calculator

  1. Enter CID, cylinder count, and RPM where the engine makes peak torque (not peak HP).
  2. Primary tube inside diameter comes from the 88,200 formula used in header design articles.
  3. Compare main-pipe results to velocity-based sizing and the HP rule-of-thumb band.

Use peak-torque RPM, not peak horsepower RPM — torque peak sets the pulse frequency that primary length and diameter target.

Cylinder CID = total CID ÷ cylinder count.

Single vs dual exhaust splits CFM — compare main pipe results to your actual layout (H-pipe, true dual).

The math: do it without a calculator

Primary area = (cylinder CI × peak-torque RPM) ÷ 88,200

Primary ID = √(area × 1.273)

Exhaust CFM ≈ (peak-torque RPM × 0.001 × CID) ÷ 2

Primary ID = √(area × 1.273). Exhaust CFM ≈ (peak-torque RPM × 0.001 × CID) ÷ 2.

Velocity method: area from CFM and target ft/s — different engines and mufflers may tolerate different velocity targets.

Real-world examples

350 CID V8, peak torque 4,000 rpm

Per-cylinder 43.75 CI × 4,000 rpm ÷ 88,200 → primary area → ID about 1.59 inches — typical for a 1⅝″ primary tube header on a small-block street/strip build.

Troubleshooting & fine-tuning your setup

Header Size Math vs Seat-of-Pants Performance

Primary tube math targets torque peak RPM — a street car that never revs past 5,500 needs different tubes than a road-race engine. Main pipe too large kills low-RPM velocity on mild builds.

Muffler restriction is not modeled — aggressive mufflers act like backpressure that helps or hurts depending on combo.

Frequently asked questions

Exhaust Pipe & Header FAQs

Are bigger exhaust pipes always better?

No. Oversized exhaust on mild NA engines can hurt low-end torque by slowing exhaust velocity.

Why use peak torque RPM for primary sizing?

Pulse tuning and scavenging align with cylinder filling at torque peak — not HP peak RPM.

What exhaust velocity should I target?

Many builders target roughly 250–300 ft/s in the main pipe for NA gas engines — the calculator’s velocity mode helps bracket pipe ID.

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