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.
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.
I was on Facebook this evening and I came across a woman with four names. Why do you need four names? I can understand it to post your maiden name for reasons of identification but what’s the fourth name for? I don’t get it. Is it like painting airplane kills on the side of a WWII fighter showing how many husbands you’ve been thru? What’s the deal here? I’m glad I don’t have to write it all out. Imagine trying to fill in the name space on some small data field? That would be too much fun. By the time you actually got it all in there one would have to put it under an electron scanning microscope to see all of it.
Here’s what I’m getting at: What if I were married okay and I was divorced twice. If men were required to change their last names instead of the women. Let’s say further that I’m one of these people who has to use their middle names. My first wife’s last name was Smith. My current wife’s name is Jones. So my name would be David William Proffitt Smith Jones. Woops now we have five! What if I were married three times? Hey I have friends who were married more than that! Ok lets add another name say Brown. David William Proffitt Smith Jones Brown. Whew!
If I were hiring for a job position and one of the four name deals came crossed my desk I’d put the application on the bottom of the pile. Why would I want to hire someone who I had to address with four names? It’s not a thing of aristocracy either. I don’t know how this got off the ground in this country but it has. Like I said before I can understand three names just fine but you don’t need four, come on you ain’t that important, not to me.