Here’s the formula to figure out your primary header tube diameter. This formula also came from A. Graham Bell’s book entitled Four-Stroke Performance Tuning.

Don’t let the radical square root symbol fool you this is an easy formula to work. You just have to do the steps in the correct order. I always approach formulas like this; I do the indicated function inside the brackets first. In the case of this formula it wants you to do the addition in the brackets first, then multiply that answer by 25. Then do the indicated division of that answer into the cc’s. after that, find the square root then multiply that answer by 2.1. I explained it in the text below.

Tube diameter is really important. On engines that have long strokes that don’t get revved much past 5000 rpm you don’t need a huge primary pipe. A primary that’s too large can kill low-speed torque and throttle response. This also applies to exhaust valve sizes in the heads too.

ID=primary header tube diameter

CC= cylinder displacement in cubic centimeters

P= Primary tube length.

First find the displacement of one cylinder by dividing your engines total displacement by however many cylinders it has. Mine is a 461 cuin. V8 so 461/8= 57.625 cubic inches. Converting it to CC’s rounding up and multiply by 16.3871 gives us 944.3 cc’s.

Primary tube length for my engine is 36”. So in the denominator do the addition in parenthesis first. P=36+3=39. Then multiply that answer by 25 =975. Divide this number into the numerator which is 944. So 944/975=.9682. So we can round that off to .968. Now take the square root of .968 which is .9838 so we’ll round this off to .984. Now multiply this by 2.1. .984*2.1=2.0664 inches. We can round that to 2.00 inches.

Most good header companies will have this all done for you, but they also have difference sizes available too. Just because you think your hot rod needs race car headers don’t make it so. Do the math and see what you really need. That’s why I put this here.

Good luck!

Dave