Tube Diameter

To decide on the right primary tube diameter, there are numerous formulas used in the design of race headers, but for a street car, Don Lindfors of PerTronix (makers of Doug’s, Patriot, and JBA headers) says the following is a great rule of thumb: 200-325 hp look for 1.50-inch diameter primaries, 275-425 hp use 1.625 inches, 400-500 hp use 1.75 to 1.875 inches, and 500 hp and up look for 2 inches or bigger. If you’d like to get more specific, keeping in mind that smaller diameters set a lower limit on peak torque, a 300hp motor could use 1.5- or 1.75-inch primary depending on whether you want the torque at a lower rpm, or to allow it to keep moving higher at a higher rpm. Just a .25-inch increase in primary diameter will push the torque peak up by about 700-800 rpm.

We know many hot rodders like hard numbers, so if you’d like to get as specific as possible and try to size your primary tubes to work effectively within a specific rpm range, try these formulas out:

Primary Pipe Area (PPA) = (peak torque RPM ÷ 88,200) × 1 cylinder ci

To find the PPA, we use A=πr²
For a few common header sizes, this is:
1⅝ = 2.07
1¾ = 2.19
1⅞ = 2.53

For example, for a 350ci engine with a peak torque goal of 5,000 rpm:
PPA = (5,000 ÷ 88,200) × 43.75 ci
PPA = 2.48

To see what the theoretical peak torque rpm of a certain set of headers is for your particular engine, just rearrange the formula:
Peak torque rpm = (PPA × 88,200) ÷ 1 cylinder ci
1⅝ (2.07) = 4,175
1¾ (2.19) = 4,415
1⅞ (2.53) = 5,100

So theoretically, our fairly high-winding 350 with a peak torque of 5,000 rpm should run a set of 1⅞-inch headers. But if your street-oriented 350 makes peak torque at a more typical 4,000 rpm, a set of 1⅝-inch headers would make more sense.

Primary Tube Length

How long should your primary tubes be? In most cases, this one is going to be mostly dictated by your chassis constraints and emissions considerations on a street car, and what is available in the aftermarket for it. In general, the ideal overall length of the primary header pipe is governed almost exclusively by the target engine’s rpm range. Typically, a lower engine rpm range likes a longer primary pipe, while a high-rpm engine prefers a shorter primary. Also, the longer the primary tube, the greater the emphasis on torque production below the peak-torque rpm of the header. Making the primaries shorter will slide the emphasis above the peak-torque rpm. So, with slightly different length primary tubes you can actually tune the engine’s powerband, and increase torque at a slight cost of peak horsepower.

More esoterically speaking, primary-tube length also works to control the timing of the exhaust pressure wave that is travelling near the speed of sound. Some of that pressure will exit the collector through the exhaust pipes, but the remaining pressure wave will reflect back up the primary tubes with a negative pressure. This effect is called scavenging. Lindfors explains: "The idea is to time this negative pressure wave to coincide with the cam overlap so the low pressure helps draw the incoming intake charge into the cylinder for more complete filling of the cylinder while at the same time pushing out any residual spent exhaust gases. The timing of these events is mostly controlled by the length of the primary pipes in the header design."

Much like length, there are several different formulas to calculate a theoretical ideal length for a header primary tube.

One example is:
Primary Tube Length = [(850 × Exhaust Duration) ÷ RPM] − 3

Exhaust Duration = 180 + the number of degrees before bottom dead center (BDC) that the exhaust valve opens

RPM = the engine speed we are trying to tune the primary length for

For example: the exhaust valve opens at 60 degrees BDC, so the exhaust duration is 60 + 180, or 240

Using the same 350ci engine running in the 5,000-rpm range from before:
Primary Tube Length = 850 × 240 = 204,000, then 204,000 ÷ 5000 = 40.8, then 40.8 − 3 = 37.8 inches

So a good starting point for the length of the primary tube (as measured from the back of the exhaust valve to the collector (not the exhaust port) should be around 37.8 inches.

But it’s really not just that simple in the world of racing. To get the full picture of exactly what length (and diameter, for that matter) that a custom set of headers for a particular car should be, Chris Vandergriff of Hedman’s Husler race header division says he needs to consider every aspect of the combination from camshaft specs, to cylinder heads, to fuel type, power adder, and, of course, rpm range. For example, nitrous cars tend to want more volume than naturally aspirated cars. Formulas can take you so far, but much of what top custom race header builders create is based on experience and testing on the track.

There is also somewhat of a debate over equal versus unequal tube length, since in theory equal-length tubes are preferred. "The thing to keep in mind about equal length is that you can’t just measure the length of each tube and compare. Since bends are necessary in the tubes and airflow is laminar in nature, bends cause a disturbance that change the flow rate. The radius of the bend will also change the effective length of the tube. In reality, there can be some benefit to slightly different length tubes on a street car header," Lindfors explained. That type of esoteric data is exactly what Vandergriff is referring to.