Estimate nominal header primary length or diameter using published empirical sizing rules, or invert either rule to find its implied RPM. These results are planning estimates, not engine performance predictions.
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Header Primary Tube Length Formula
The length calculation retains the published 360 − EVO(BBDC) heuristic below, including its fixed 3-inch correction. It is not algebraically interchangeable with a 180 + EVO(BBDC) timing convention. No additional port subtraction or valve-to-collector fabrication measurement is inferred:
PTL = 850 * (360 - EVO) / RPM - 3
To find the primary tube diameter, it first finds the cross-section area the engine wants at that RPM, then converts the area to an inside diameter:
A = (CID / N) * RPM / 88200 ID = sqrt(A) * 1.128
The inverse modes rearrange the same relationships to find a nominal RPM implied by the selected formula. They do not predict an engine’s actual torque or power peak:
RPM = 850 * (360 - EVO) / (PTL + 3) RPM = A * 88200 / (CID / N)
Variables:
- PTL is the nominal length output in inches after the formula’s fixed 3-inch correction; verify the physical measurement convention with the header designer
- EVO is the exhaust valve opening point in degrees before bottom dead center, taken from your cam card
- RPM is the selected target speed for the length heuristic; the area-sizing heuristic specifically uses anticipated peak-torque RPM
- CID is total engine displacement in cubic inches and N is the number of cylinders, so CID / N is the displacement of one cylinder
- A is the primary pipe cross-section area in square inches
- ID is the primary tube inside diameter in inches
- 850, 88200 and 3 belong to empirical sizing rules; the circular area conversion is ID = √(4A/π), approximately 1.128379 × √A
Select a nominal length, inside diameter, or one of the two inverse RPM tasks. Enter the visible inputs, select Calculate, and open Calculation details for assumptions and arithmetic. Diameter tasks offer an optional wall allowance: blank means 0.049 inch. The nearest listed OD is a reference only; values outside the listed range are not clamped to an endpoint.
The fixed 3-inch length correction is already present. The earlier extra port subtraction was removed because its physical measurement convention was not established. Do not subtract an exhaust-port length a second time or use this nominal formula output as a fabrication drawing.
Primary Tube Size and the RPM It Favors
Header primaries are sold by outside diameter, and each size holds high exhaust velocity best over a certain engine speed. This table shows rounded RPM values implied by the area-sizing formula for four engine displacements, assuming eight cylinders and a 0.049 inch tube wall. It is a comparison of this heuristic, not measured engine data or a guaranteed torque peak.
| Primary tube (OD) | 302 ci | 350 ci | 383 ci | 454 ci |
|---|---|---|---|---|
| 1-1/2 in | 3,600 | 3,100 | 2,850 | 2,400 |
| 1-5/8 in | 4,300 | 3,700 | 3,350 | 2,850 |
| 1-3/4 in | 5,000 | 4,300 | 3,950 | 3,350 |
| 1-7/8 in | 5,800 | 5,000 | 4,550 | 3,850 |
| 2 in | 6,650 | 5,750 | 5,250 | 4,400 |
| 2-1/8 in | 7,550 | 6,500 | 5,950 | 5,000 |
| 2-1/4 in | 8,500 | 7,350 | 6,700 | 5,650 |
Read down a column to compare the formula’s implied RPM as tube area changes. Actual engine response also depends on cam and port details, firing order, temperature, collector configuration, boost and the remaining exhaust. Validate the final design for the specific engine.
The next table is a plain size reference. Inside diameter is the outside diameter minus twice the wall thickness, shown here for a 0.049 inch wall.
| Tube size (OD) | Inside diameter | Application note |
|---|---|---|
| 1-1/2 in | 1.40 in | Size reference only |
| 1-5/8 in | 1.53 in | Verify fit |
| 1-3/4 in | 1.65 in | Verify fit |
| 1-7/8 in | 1.78 in | Verify fit |
| 2 in | 1.90 in | Verify fit |
| 2-1/8 in | 2.03 in | Verify fit |
Example Problems
Example 1: Find the primary tube length.
Your cam card lists an exhaust valve opening of 82 degrees before bottom dead center, and you want peak torque near 4,200 RPM. Put the numbers into the length formula:
PTL = 850 × (360 − 82) / 4200 − 3 ≈ 53.2619 inches of nominal formula length. The fixed 3-inch correction is already included; no further port length is subtracted. The rough half-length collector rule gives about 26.631 inches, subject to engine-specific development.
Example 2: Find the RPM an existing header favors.
You have a set of 1-3/4 inch headers with a 0.049 inch wall, so the inside diameter is 1.65 inches, and they are going on a 350 with 8 cylinders. The tube area is 0.7854 times 1.65 squared, which is about 2.14 square inches. One cylinder is 350 divided by 8, or 43.75 cubic inches. So RPM = 2.14 * 88200 / 43.75, which is about 4,300 RPM. Using the unrounded ID of 1.652 inches gives approximately 4,321 RPM under this area heuristic, consistent with the rounded 4,300 entry in the 350 column; this is not a measured torque peak.
Frequently Asked Questions
Where do you measure the primary tube length from?
The displayed length is the nominal output after the published formula’s fixed 3-inch correction. A universal valve-seat, port-face or collector measurement convention was not established for this heuristic. Confirm the measurement and any port allowance with the header designer; the calculator does not subtract an additional port length.
Should you use peak torque RPM or peak horsepower RPM?
Use a target RPM appropriate to the engine’s intended operating range for the nominal length estimate. The area-sizing rule uses anticipated peak-torque RPM. The separation between torque and power peaks is engine-specific, so the calculator does not generate a peak-power RPM from a fixed offset.
Why is the calculated length longer than headers I can buy?
A nominal formula can give a length that does not fit the vehicle. Packaging, wave harmonics, port and collector geometry and the rest of the exhaust may require a different design. Treat this estimate as a starting comparison and use engine-specific testing or specialist design before fabrication.
