When the exhaust valve opens, a high-pressure pulse travels down the primary tube. When it reaches the collector, it reflects back as a low-pressure (negative) wave. Get the tube length right and that negative wave arrives back at the exhaust port during overlap, helping pull residual exhaust out of the cylinder. This calculator uses A. Graham Bell's formulas from Four-Stroke Performance Tuning for length and diameter, and compares the result with the sizes header makers actually sell.
The length formula
Primary length (inches) = 850 × (180 + EVO) ÷ RPM − 3
EVO is the exhaust valve opening in degrees before bottom dead center, from the cam card (seat timing). 180 + EVO is the time from the valve opening to top dead center, when the returning wave is useful. Bell's 850 corresponds to a pressure wave speed of about 1,700 ft/s. A small-block 350 with EVO 70° tuned at 4,500 rpm comes out at about 44 inches — typical long-tube territory.
If you only know the duration, EVO ≈ advertised exhaust duration ÷ 2 + exhaust centerline − 180. With a 270° exhaust lobe on a 110° LSA (exhaust centerline about 114°), EVO is about 69°. From duration at .050", add about 22° to reach seat timing (makers' cam cards put it 14–31° outside the .050" point).
Long, mid-length and shorty tubes
- Long tube: Bell's length. The wave makes one round trip in the time available.
- Mid-length: about 2/3 of the acoustic length (the wave makes more trips, each one weaker).
- Shorty / block-hugger: about half the acoustic length. Used when space is tight; some tuning benefit but much less than long tubes.
Header makers don't publish primary length — it is set by the chassis — so treat the length as a target, not a catalog spec.
Tube diameter and the torque curve
Primary ID (inches) = √(cc per cylinder ÷ ((length + 3) × 25)) × 2.1 (Bell). Tuning for a higher rpm gives a shorter tube and a bigger diameter.
- Smaller tube keeps velocity high at low RPM — better low-end and mid-range torque. Ideal for street and towing.
- Larger tube flows more at high RPM but softens response down low.
What the makers sell: Hooker and Patriot use 1-5/8" for street small-block Chevys and 1-3/4" for street-strip; Kooks, ARH, Stainless Works and Hooker use 1-7/8" for street-strip LS engines and 2" for race; Hedman and Doug's ship 1-3/4" for street-strip big-blocks. Almost all of them use a 3" collector through 650+ hp (2.5" on Fox-body 5.0s).
Collector size
The calculator sizes the collector at 0.85 × √(number of primaries) × primary ID — about 1.7 × the primary ID for a 4-into-1. That factor is our estimate, chosen so the common presets land on the makers' catalog collectors listed below.