Exhaust header length (3.8 Race)

[Cookie Porsche]

Two Q's in one day - my new record!

And yes - I have searched and read the board before posting!

I have a 3.8 race engine, factory RSR cams, 6x T/B's. What is the best (recommended) header length?

1) I do not want TOO much torque low down (915 g/box !) and would like bhp btw 4000-7000 rpm.

2) Currently running 44mm OD pipe, 940mm header length!!! It was a test pipe that just worked well but now I need a little extra.

3) Very short 400mm exhaust pipe (about 80 OD) with no silencer - should I fit a reasonator, megaphone or similar for more power or is a 'straight' pipe fine?

Yep - sounds glorius.

Tom

[Bill Verburg]

Short is supposed to move power up but narrow the band, long smears it out, not as high but broder, thats the theory anyway. But it does depend on the cam timing and intake manifolds/tbs
short 19" tube
long 29" tube

[snowman]

Based on your graphs the long tube wins, hands down.

But it dosen't make sense. The short tube should have higer high end hp and torque, but it dosen't. Something else going on here. To many variables maybe??? The intake is also dependant upon the exhaust, so its quite complex.

[Bill Verburg]

No it's not easy or obvious, to throw some more fuel on the fire
small 1 5/8" ID (41.275mm)
large 1 3/4" ID (44.45mm)


factory RSRs were 42mm

everything from air cleaner to exhaust tip is causing something to happen at different points in the rpm range.

I have seen studies that suggest that absolutly equalk length isn't as important as the collector and pipes and transitions down stream of the collctor

[Cookie Porsche]

Bill,

Firstly - is that your car running 500bhp+ ???

So, all other things equal a 'longish' pipe (25"-30") with an ID of 42mm looks like a good starting point for my 3.8 (race only).

I shall report back.

Tom

PS. Is that actual figures or a simulated model?

[Bill Verburg]

Actual but I forget what car they are from

[Bill Verburg]

All the lengths and diameters are from examples I researched for my 3.8 motor