Exhaust Header Size Calculator

Exhaust headers are the pipes that channel burned gases out of each engine cylinder — their diameter and length directly affect how much power your engine makes. Enter your engine's bore, stroke, number of cylinders, peak power RPM, volumetric efficiency, valve timing, and exhaust port specs into the Exhaust Header Size Calculator to get the optimal primary tube diameter and primary tube length. Secondary outputs include calculated engine displacement and exhaust duration.

in

Engine cylinder bore diameter in inches

in

Engine piston stroke length in inches

RPM

Peak horsepower RPM for race engines, peak torque RPM for street engines

%

Engine volumetric efficiency percentage

°

Exhaust valve opening in degrees before bottom dead center

°

Exhaust valve closing in degrees after top dead center

in

Exhaust valve head diameter

in

Average distance from valve seat to port exit

Results

Primary Tube Diameter

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Primary Tube Length

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Engine Displacement

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Exhaust Duration

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Frequently Asked Questions

What is the difference between primary tube diameter and length?

Primary tube diameter affects gas velocity and scavenging efficiency, while primary tube length determines the RPM range where the header provides optimal scavenging effect through tuned pressure waves.

Should I use peak horsepower RPM or peak torque RPM for the calculation?

Use peak torque RPM for street engines to optimize mid-range performance, and peak horsepower RPM for race engines to maximize top-end power output.

How do I measure exhaust port length accurately?

Measure from the exhaust valve seat to the port exit at the head surface, taking the average of the roof (long side) and floor (short side) distances for an accurate port length.

What happens if my primary tubes are too short or too long?

Tubes that are too short will tune for higher RPM than intended, while tubes that are too long will tune for lower RPM, reducing scavenging efficiency at your target operating range.

How does volumetric efficiency affect header sizing?

Higher volumetric efficiency means more exhaust gas flow, requiring larger diameter tubes to maintain optimal gas velocity and prevent restriction.

Can I use these calculations for turbo or supercharged engines?

These calculations are primarily for naturally aspirated engines. Forced induction engines have different exhaust flow characteristics and may require different header design approaches.

What wall thickness should I use for the primary tubes?

Standard header tubes typically use 0.050" wall thickness. The calculated diameter represents the outer diameter (OD) assuming this wall thickness.