Hi

I'm having a header made up for my bridgeport 13b. They are going to make the primaries equal length (or as close as possible) and mandrel bent and im thinking the primaries should be 2" inner diameter collecting into 2.5" collector and then megaphoning up to meet with my 3" exhaust. Does anyone have any suggestions on length of the primaries? I cant afford to go through testing lots of different lengths so I just need a ballpark idea. My current header is crush bent, 2" outer diameter and far from equal length (its a complete piece of crap) so I want to get a new header made up before I've finished breaking in this engine because I plan to get it on the dyno. Also do you think I should just have the primaries collect at 3" or do you think starting from 2.5" and then going up to 3" is better? Any other suggestions would be appreciated. Noise levels are not a problem im going for pure power.

Thanks

 

i'm in a similar boat, i was thinking about making mine with slip fits, so i can vary the length of the primaries a bit.

i like the broader powerband of the long primary exhaust, and it more room for mufflers, so i'm doing mine about 90-92in (230ish cm), like the setup in the factory competition book.

header tubes should match the exhaust port diameter, or slightly larger. if the exit of the exhaust port is 50mm then the ID of the pipe should be 50mm...

anymore than that and i really havent decided either

 

edit.

 

24 inch short equal length got me pretty good results. Except I just made my collector 3 inch instead of megaphoning.

 

you guy might want to try my expansion chamber mid pipe with a equal length header feeding it

 

small carbed cars should collect at 22-25'' or 120-125''. bridges at 10-18'' or 113-118'' and perry ports at 10-18'' or 89-94''... remember you can collect the header at 25'' and make peak torque at 5000 rpm or collect at 18'' and make peak torque at 6250 rpm... Exhaust tuning has a much bigger role then people let off especially on a rotary with huge the amounts of overlap even with stock ports and the importance of pressure waves and collecting.

 

IIRC RotaryGod made at least one post where he explained how to figure out header length. It varies based on porting, where you want the power to peak and so on.

 

This it?
https://www.rx7club.com/showthread.p...ghlight=tuning

 

That would be it, and the only place I've seen anyone actually explain how to figure it out rather than just spewing numbers.

 

I dont know where you planned on sourcing a megaphone, but Burns Stainless has mild steel 2" to 4" short or long m-phones for $24.
Im sure shipping would be a lot to South Africa, but if you're having trouble soucing one, heres the link.
http://www.burnsstainless.com/Megaph...egaphones.html

 

fyi I wasn't just spewing numbers I got those numbers from racing beat. I think they've been in it long enough to be reputable.

 

Out of curiosity, what is the definition of primaries? Is it from flange to flange (exhaust flange to collector flange)? Or is it flange to the point where the 2 tubes meet at the collector?

 

on my bridged 13b, i ran 2 seperate pipes from the exhaust ports all the way back and joined them at a Y just before the diff. running out an a'pexi can. all stainless steel.

 

I didn't mean it like that, I just meant that he actually gives all the background info for one to actually calculate it themselves, rather than just going off a number with no idea where it came from, if it'll be right for their ports, their desired rev range, etc.

 

3" ID or OD for your exhaust?

 

flange to collector

 

Tubing is measured in OD, and pipe in ID.

 

No, pipe has a consistent outer diameter, thicker walls mean smaller ID. Also, a 2" piece of pipe doesn't have a 2" OD or even ID necessarily (depending on the schedule you might get close on the ID). http://www.engineersedge.com/pipe_schedules.htm

Tube is both somewhat, it's based on the OD, but you can get thicker or thinner stuff with the same ID if you choose the wall thickness and OD properly (2"x0.25" has the same ID as 1.75"x0.125" for example).