Step Header Formula

We are building a new set of headers 2 1/8" to 2 1/4" with a 4" merge collector. Does anyone have a formula for the distance from the exhaust port to the start of the step. I seem to remember seeing one somewhere but I can't remember where it was. Thanks Mike.

I have one in my literature however, I have never used it myself. It requires some guesstimates.

Ti = Crank degrees from Exhaust Opening to Intake Opening
t - Crank degrees from exhaust gas flow start to T.D.C.
V + exhaust gas wave velocity at exhaust temperature, (1,750 F.P.S. mean).

Total primary length = t x V divided by desired RPM divided 2; + or - 2"
Primary length; valve head to step = Ti x V divided by desired RPM divided 4

"Desired RPM" is usually peak horsepower for total length.
"Desired RPM" is usually peak horsepower or peak torque for the step length.

As you can see, there is a LOT of room for differentiation.

EDIT: the exhaust velocity will change with temperature changes
You've already chosen your sizes however, the entire formula also gives an equasion for tube size.
1. Tube diameter inner diameter to step sized for 300 fps gas speed at peak torque
2. Tube diameter from step to collector sized for 300 fps gas speed at peak horsepower
Gas Speed equals;
Piston Speed in fps times (cylinder diameter squared) divided by (tube I.D. squared)..

That looks like a place to start. I think we guessed on my current set of headers. I'll be interested to see how close they are to this. Thanks Randy.

Randy do they use 1750 FPS for V? Or do I have to try to calculate the velocity? That's going to require some serious thought. I'm going to have to sleep on that one.

512Fairlane wrote:Randy do they use 1750 FPS for V? Or do I have to try to calculate the velocity? That's going to require some serious thought. I'm going to have to sleep on that one.

Depending upon the actual temperature of the exhaust it will change but, that is about the average velocity of the wave which will be seen.

I never used a fancy formula when I built mine, The step headers I had on the tfs deal were 2 1/8" then at about 10" we went 2 1/4", right where it went into the collector there was about 7" of 2 3/8"..........but that was maninly due to the collectors being 2 3/8 instead of 2 1/4, we just made what we had work. This was a 4" collector, tubes were all in the 30" range on length.

I consulted my engine guy here last year w. my engine and he ran it though his program. It said 31-34 primary length w. the step at 16-17 inches from the head.