Exhaust Flow Calculator

Exhaust pipe diameter from engine output and RPM.

Horsepower

Peak torque RPM

Number of cylinders

Exhaust (1=single, 2=dual)

Target gas velocity (ft/min)

Tab to move between fields

Primary tube OD (in)

0.14

Primary length (in)

12.5

Collector ID (in)0.29

Main pipe ID (in)0.3

Primary area (in²)0.02

Collector area (in²)0.06

Exhaust flow (CFM)960

Total flow area (in²)0.13

How to Use This Calculator

How the result changes with Number of cylinders

Horsepower: Flywheel (crank) horsepower.
Peak torque RPM: RPM at peak torque — determines optimal header primary length.
Number of cylinders: Engine cylinder count (one primary tube per cylinder).
Exhaust (1=single, 2=dual): Single or dual exhaust system.
Target gas velocity (ft/min): Target exhaust gas velocity. Street: 4500-6000, race: 6000-8000 ft/min.

Primary tube OD (in): Recommended header primary tube inner diameter.
Primary length (in): Optimal primary tube length for peak torque at the given RPM.
Collector ID (in): Recommended collector inner diameter where primaries merge.
Main pipe ID (in): Recommended exhaust pipe diameter after collector.
Primary area (in²): Cross-sectional area of each primary tube.
Collector area (in²): Cross-sectional area of each collector.
Exhaust flow (CFM): Estimated exhaust gas volume at typical exhaust temperature.
Total flow area (in²): Combined exhaust flow area of all collectors.

Identify Input Parameters
4 parameters
Horsepower = 350, Peak torque RPM = 5500, Number of cylinders = 8, Exhaust (1=single, 2=dual) = 2 = 5 input(s) provided
Calculate Primary tube OD
Primary tube OD = sqrt((horsepower * 2.2) / (exhaustGasVelocityFpm * 0.7854 * numPrimaries))
0.143 = 0.143
Calculate Primary length
Primary length = max(12, (85000 / peakRpm) - 3)
12.5 = 12.5
Calculate Collector ID
Collector ID = primaryId * sqrt(primariesPerCollector)
0.29 = 0.29
Calculate Main pipe ID
Main pipe ID = collectorId * 1.05
0.3 = 0.3