Headers, cats, and mufflers, oh my.
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Joined: May 2014
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From: Madison, MS
Headers, cats, and mufflers, oh my.
Hey guys, so I've been recently delving more and more into my z and wanting to design something myself for it. I decided that it would be kinda cool to design my own headers. I have the math down for it I just need help figuring out the length for the primary tubes.
(Peak Torque RPM) = ((Primary Pipe Area)(88,200)/(1cyl displacement))
I was looking for ~6k peak torque so I decided to plug in some different pipe areas to see which peak torque rpm I liked.
1 3/4" pipe area = 2.41in^2
1 cyl displacement = ((3500/6) * 0.0610237) = 35.597in^3/cylinder
Plug into formula - Peak TQ RPM = ((2.41)(88200)/(35.597))
Peak TQ RPM = 5,971 RPM
This is the closest to what I was looking for.
Now for a street car, a good multiplication factor for collector size is anywhere from 1.6 - 1.8. I want to be a little more aggressive so I'm going with 1.75.
So to find it we take the OD of the primary pipe and multiply it times the 1.75 and we get 3.0625 so 3" pipe is almost perfect. I wouldn't want the collector size to be too long so I feel 5" should be good, depending on the fit with the cats. I just don't know how to calculate the tube length. I've searched far and wide but there's no definite answer. Anyone have any idea? Thanks for your time. -David
EDIT: Ok, so I found a formula from A. Graham Bell's book that says header pipe length is ((850*ED)/RPM) - 3 with ED being 180 + degrees the exhaust valve opens before reaching BDC and RPM is the RPM you're tuning to.
So I looked up stock cam specs and the EO is 52 degrees. So my length should be ((850*(180+52))/RPM) - 3 which would give me 32.86 inches. Can anyone run through these numbers with me? Any engineers confirm? Fluid dynamics gives me headaches.
(Peak Torque RPM) = ((Primary Pipe Area)(88,200)/(1cyl displacement))
I was looking for ~6k peak torque so I decided to plug in some different pipe areas to see which peak torque rpm I liked.
1 3/4" pipe area = 2.41in^2
1 cyl displacement = ((3500/6) * 0.0610237) = 35.597in^3/cylinder
Plug into formula - Peak TQ RPM = ((2.41)(88200)/(35.597))
Peak TQ RPM = 5,971 RPM
This is the closest to what I was looking for.
Now for a street car, a good multiplication factor for collector size is anywhere from 1.6 - 1.8. I want to be a little more aggressive so I'm going with 1.75.
So to find it we take the OD of the primary pipe and multiply it times the 1.75 and we get 3.0625 so 3" pipe is almost perfect. I wouldn't want the collector size to be too long so I feel 5" should be good, depending on the fit with the cats. I just don't know how to calculate the tube length. I've searched far and wide but there's no definite answer. Anyone have any idea? Thanks for your time. -David
EDIT: Ok, so I found a formula from A. Graham Bell's book that says header pipe length is ((850*ED)/RPM) - 3 with ED being 180 + degrees the exhaust valve opens before reaching BDC and RPM is the RPM you're tuning to.
So I looked up stock cam specs and the EO is 52 degrees. So my length should be ((850*(180+52))/RPM) - 3 which would give me 32.86 inches. Can anyone run through these numbers with me? Any engineers confirm? Fluid dynamics gives me headaches.
Last edited by leafjerky; Apr 28, 2015 at 07:21 PM. Reason: sizes
Thread Starter
Registered User
Joined: May 2014
Posts: 29
Likes: 0
From: Madison, MS
You still can since it mostly is, I just missed the section I guess. TH math? Forgive me if I missed something, I've been non stop all day and dead.
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