If you are like most folks you just take the manufacturers word on what you need for headers on your hot rod right? Well why not know exactly what your engine needs? I have listed a formula taken from a book written by a gentleman named A. Graham Bell. His book is called Four-Stroke Performance Tuning.
If you know what camshaft is in your engine and know or can find the timing specifications to it, then you can use this formula to find out how long the primary tubes on your headers should be for you engine. With a little help from your friendly calculator you can figure it all out in no time. I will also write out another formula to figure out primary pipe diameter. The diameter is probably one of the most important parts to correctly sizing headers for your engine. Most of the time due to magazine bullshit we are led to believe that larger tubes are the best. This is not true in most cases, certainly not for most street machines. If the tube diameter gets too large the exhaust gas velocity falls off and a real loss in low-speed torque occurs. For each 1/8 ” in pipe diameter decrease your torque rpm will lower about 500-600 rpm in large engines and 650-800 rpm for small blocks. Each engine is different I know that. But this is a good formula to use to see if the headers you have been dreaming about are in the ball park for you motor. Why bolt on something that’s not gonna work until you spin the motor past 6500 rpm? Most street motor will never see that or only on occasion when impressing the folks at a cruise in.
PL=850 * ED/rpm -3
PL= Primary tube length.
ED =180 ̊+ exhaust opening before BDC
Rpm= desired tuning rpm.
The first thing you need to do to even work this formula is look at the timing figures on your camshaft card. If you don’t have one go online and see if the manufacturer has one for you cam. You need the exhaust opening figure ( use the @.050 lifter rise). Mine is 64 ̊before bottom dead center (BDC). So once you have that add 180 ̊ + 64 ̊=244.
Now multiply this number by 850 * 244=207400.
Ok now divide 207400 by whatever rpm you need to use mine is 5000. So we have 207400/5000=41.48. now subtract 3 from this answer or 41.48-3=38.48” rounding it off to 38 inches.
Hope this helps with your header selection.