Hey guys I'm new here. I'm an ME student and I'm working on a project. I originally posted this on my350z forum and no one replied. Maybe it's too boring I don't know. If anyone has experience can you help me? Thanks. 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.