header primary tube diameter?

[ericjon262]

I notice all of the header supplies on WOT tech are for 1.625" pipe, but to me, that sounds kinda large... most SBC's only go up to 1.75"...

I crunched some numbers...

a 3500 has approx, 35.5 cubic inches per cylinder

a 3900 has approx, 39.5 cubic inches per cylinder

a 305 has approx, 38.125 cubic inches per cylinder

a 350 has approx, 43.75 cubic inches per cylinder

alot of 350's run 1.75" primaries, or 25 cubic inches per inch of primary diameter.

the same math on a 3500 equals 21.85 cubic inches,


am I the only one that thinks the pipe might be a little big?

[bob442]

so lets specially make collectors and mandural bent pipe for one motor? how much back presure is going to be lost from 1/4" and is it worth it? if it were to come down to it, mathimaticly, you loose .9 HP?

[geoffinbc]

I have seen lots of small block headers up to 2"

It just depends on the RPM range you want to operate in. If you want you could sleeve the first inch or two of pipe and use 1.5"

[ericjon262]

so lets specially make collectors and mandural bent pipe for one motor? how much back presure is going to be lost from 1/4" and is it worth it? if it were to come down to it, mathimaticly, you loose .9 HP?

specially made? stahl has pipe and collectors all the way down to 1.375"

http://www.stahlheaders.com/Frame%20Collectors.htm

http://www.stahlheaders.com/Frame%20Tubing.htm

[ericjon262]

I have seen lots of small block headers up to 2"

It just depends on the RPM range you want to operate in. If you want you could sleeve the first inch or two of pipe and use 1.5"

as have I, but I would hardly call them common on a 350, 1.75 seems to be the right size for a HCI 350, so to me, it seems as though 1.625 might be too big for a 3500.

[Superdave]

header tube diameter has more to do with horsepower per cylinder than engine displacement.

[3400-95-Modified]

I have the 1 5/8 SS on my car and I also have a set of TOG 1 1/2 primaries, I'm planning on fitting those to my car this year OR this winter, whenever I get time to do it and I will be doing testing on both sets... but this is where I agree... A lot of V8 guys will size their headers on the size of the exhaust valve, never mind the displacement or hp per cylinder.

Either way I hope to have some answers when I get my sets installed... I'm hoping for a bit more low end torque from it and hopefully not losing anything up top, but we'll see.

I have seen lots of small block headers up to 2"

It just depends on the RPM range you want to operate in. If you want you could sleeve the first inch or two of pipe and use 1.5"

The headers we just put on the 590hp race car are not even 2" IIRC. I'll measure them tonight but I don't think they are that large.

[geoffinbc]

1.875 is very common for a small block too.

Like Superdave said it has to do with horsepower per CI or Cylinder versus the actual displacement or exhaust valve size. V8's are a little different because to the intake and exhaust system it actually looks like a V6 with two small cylinders and one big one on each bank. Every cross plane V8 fires odd, even, even, odd, even, odd, odd, even. So every time you have those 2 cylinders firing back to back on the same bank it changes the rules. V6's are always bank to bank firing. I am not a fluid dynamics expert but I do know you should not compare V8 intakes, exhausts and camshafts to V6 ones.

[ericjon262]

header tube diameter has more to do with horsepower per cylinder than engine displacement.

Very good point, I should have thought of that!

I have the 1 5/8 SS on my car and I also have a set of TOG 1 1/2 primaries, I'm planning on fitting those to my car this year OR this winter, whenever I get time to do it and I will be doing testing on both sets... but this is where I agree... A lot of V8 guys will size their headers on the size of the exhaust valve, never mind the displacement or hp per cylinder.

Either way I hope to have some answers when I get my sets installed... I'm hoping for a bit more low end torque from it and hopefully not losing anything up top, but we'll see.



The headers we just put on the 590hp race car are not even 2" IIRC. I'll measure them tonight but I don't think they are that large.

Keep me posted!

1.875 is very common for a small block too.

Like Superdave said it has to do with horsepower per CI or Cylinder versus the actual displacement or exhaust valve size. V8's are a little different because to the intake and exhaust system it actually looks like a V6 with two small cylinders and one big one on each bank. Every cross plane V8 fires odd, even, even, odd, even, odd, odd, even. So every time you have those 2 cylinders firing back to back on the same bank it changes the rules. V6's are always bank to bank firing. I am not a fluid dynamics expert but I do know you should not compare V8 intakes, exhausts and camshafts to V6 ones.

most of what you are describing only becomes a problem once you get to the collectors, that's why 180* headers exist, to make each collector have a sequential pattern, 1357 in one collector and 2468 int he other. on our V6's, we don't have to cross the banks because we have an even firing pattern. it's also why V8's have larger collectors.