Calculate header primary tube length and diameter from exhaust valve timing and target RPM, and find the peak RPM any primary tube size is tuned for.
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Header Primary Tube Length Formula
The main job of this tool is to size the primary tubes of a set of exhaust headers. It works in three directions. To find the primary tube length, it uses the exhaust valve opening point and the RPM you are tuning for:
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
To work backward and find the RPM a header you already own is tuned for, it rearranges the same two relationships:
RPM = 850 * (360 - EVO) / (PTL + 3) RPM = A * 88200 / (CID / N)
Variables:
- PTL is the primary tube length in inches, measured from the back of the exhaust valve
- EVO is the exhaust valve opening point in degrees before bottom dead center, taken from your cam card
- RPM is the engine speed you are tuning for: peak torque RPM for a street engine, peak horsepower RPM for a race engine
- 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, 1.128, and 3 are fixed constants from the published header design formulas
Pick what you want to find at the top of the calculator. In length mode you enter the exhaust valve opening and the target RPM, and the tool returns the tuned primary length plus an estimated collector length. In diameter mode you enter displacement, cylinder count, and the peak torque RPM, and the tool returns a target inside diameter and the nearest common tube size sold by outside diameter. In RPM mode you enter either a known primary length or a known tube size and the tool tells you the peak torque RPM that header favors, which is useful for judging an off-the-shelf set before you buy it.
The length result is measured from the valve, so it includes the exhaust port. Enter your exhaust port length in the optional field and the calculator subtracts it to give the tube you actually build.
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 the approximate peak torque RPM that each common primary size favors on four popular engine sizes, based on the cross-section area formula above and a 0.049 inch tube wall. It is a quick way to read what an existing header is built for, which most header calculators do not show.
| 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 see how moving up one tube size raises the RPM the header favors by roughly 700 to 900 RPM. A tube that is too large for the engine pushes that peak so high that low-end and mid-range response go soft, which is why a mild 350 street engine likes a 1-5/8 or 1-3/4 in tube rather than a 2 in tube.
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 | Typical use |
|---|---|---|
| 1-1/2 in | 1.40 in | Small-block street, low-end torque, up to about 350 hp |
| 1-5/8 in | 1.53 in | Mild small-block street and strip, about 350 to 450 hp |
| 1-3/4 in | 1.65 in | Hot small-block and mild big-block, about 450 to 600 hp |
| 1-7/8 in | 1.78 in | Big-block and high-RPM small-block, about 600 to 700 hp |
| 2 in | 1.90 in | Big-block race engines run at high RPM |
| 2-1/8 in | 2.03 in | Large-displacement race engines |
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 = 53.3 inches from the valve. If your exhaust port is 4 inches long, the tube you build is about 49.3 inches, and a matching collector is roughly half the primary, near 26.6 inches.
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. Those headers favor peak torque near 4,300 RPM, matching the 350 column in the table above.
Frequently Asked Questions
Where do you measure the primary tube length from?
The formula gives the length from the back of the exhaust valve, so the result includes the exhaust port that is already cast into the cylinder head. Measure your exhaust port from the valve seat to the port face, usually 3 to 4 inches, and subtract it from the calculated length to get the tube you actually cut and bend. The calculator does this for you when you fill in the optional port length field.
Should you use peak torque RPM or peak horsepower RPM?
Use the RPM where you spend the most time and want the strongest response. For a street engine that is the peak torque RPM, which gives longer primaries and better low-end and mid-range pull. For a race engine that is the peak horsepower RPM, which gives shorter primaries tuned for the top of the range. Peak power usually lands about 1,500 to 1,700 RPM above peak torque, so the two choices can produce noticeably different lengths.
Why is the calculated length longer than headers I can buy?
The tuned length measured from the valve is often 45 to 55 inches for a street engine, but production long-tube headers are usually 28 to 40 inches because that is what fits in a car. A shorter tube moves the tuning peak to a higher RPM, and many headers are designed around a shorter section of the full wave rather than the entire length. If you want the low-RPM torque that a long primary provides and cannot fit it, a smaller tube diameter recovers some of the same low-end effect.
