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Exhaust

Header Primary Tube Length & Diameter

A starting point for primary tube length and diameter. Header design is empirical — treat these as the middle of a range, not a single right answer.

Input
Seat-to-seat figure from the cam card
Result
Primary length32.4in
Usable range
to 35.7 in
29.2in
Primary inside diameter1.559in
Suggested tube1-3/4" OD
That tube’s inside diameter1.620in
Step down / step up
or 1-7/8"
1-5/8"
Displacement per cylinder43.75cu in
Longer and smaller favors low and mid-range; shorter and larger favors top end. Length is measured from the valve seat to the collector entry, following the tube.
Formula
  1. primary length (in) = (850 × exhaust duration ÷ peak RPM) − 3
  2. primary inside diameter (in) = √( displacement per cylinder × peak RPM ÷ 108,000 )
  3. The diameter rule is fitted to accepted real-world tube sizes rather than derived from gas dynamics.
Notes

When you need it

When choosing headers off the shelf, or specifying a custom set.

What length and diameter each do

Primary length tunes the returning pressure wave. A longer primary puts the useful reflection lower in the RPM range; a shorter one moves it up. This is why a set of headers can move the torque peak without changing anything else.

Diameter sets gas velocity. Too small and the engine chokes at the top. Too large and velocity collapses, which costs low-end scavenging and throttle response — a common outcome when someone fits race headers to a street engine.

Treat these as the middle of a range

Header design is closer to craft than to arithmetic you can trust to three decimal places. These formulas are fitted to combinations that are known to work, and the diameter rule here is empirical rather than derived from gas dynamics. Being an inch or two off the calculated length will not ruin the engine.

Packaging usually wins anyway

On anything with a chassis around it, what clears the steering shaft, starter and floor decides the design far more than the ideal length does. Genuine equal-length primaries are worth real power and most engine bays will not accept them.

Measure length from the valve seat, following the center of the tube, to the collector entry — not the straight-line distance, and not from the flange.