Andy --

I've got a dual 3" exhaust now that is perfectly straight thru, loud, and still probably too restrictive for max power (for me). I say that it's restrictive because only at 650 rwhp or so, I'm seeing 1.5 psig back pressure after the turbine. That's acceptable for now, but the back pressure scales with about the square of the horsepower so the system doesn't have much headroom there. I'm left with the dilemma of trying to both up-size the exhaust to dual 3.5" and simultaneously make it quieter, which is a challenge. So I can feel you.

If you can hook up a pressure sensor to the exhaust right after the first cross-over, then you can measure how much restriction there is in both the 2.5" and 3" systems. That pressure number will tell you a lot about what tradeoffs you have there.

We could also figure out something from comparing the torque curves of 2.5" and 3" exhausts. If it's just the pipe being smaller and the exhaust sizing decision is simply trading off flow restriction with sound suppression, then the torque curves should be almost identical until the hp gets high. By my understanding, a zero-loss system will need about .024 square inches of cross sectional area per hp. Straight pipe dual 2.5" system is loss free up to 366 hp and straight pipe dual 3" system is loss free up to 533 hp. If this logic is correct, then you should see the torque curves lining up very close to each other up to that 366hp level and only after that you should see divergence. If they don't line up below 366hp, then there are resonance etc. effects at work, and it's not just the inevitable restriction with smaller pipe. But don't take that as gospel, think it thru yourself and see if you agree or not.

One thing that factories do nowadays in high-end cars is they have active valves in the exhaust where the sound suppression and power tradeoff can be altered. At low load, they are really quiet and then when the load increases they open the bypass valves in the exhaust and make efficient power. There's additional cost and complexity in a system like that, of course.

Another idea is to use half an inche larger inlet and outlet mufflers. Usually, straight pipes flow better than mufflers of the same inlet and outlet size. So do a transition cone from the pipe to say from 2.5 -> 3 or 3 -> 3.5 and then back after the muffler. This will reduce the gas velocity in the muffler thus the pressure loss. Just an idea.

Best, Tuomo

 

thanks Tuomo,

that is the math behind the effects.....good to know... I think the biggest impediment is the automatic transmission......but too expensive to swap transmissions.....

 

Says the guy with the six million dollar ITB'ed stroker 928.

 

Roger will be shattered . .

 

I would think that motor of yours mated to the auto tranny is exceptional and saves you from ripping the comparitively puny manual tranny apart with your torque curve. Had you a manual you might well have had to rebuild the gearbox by now!

On a stock 5 litre motor the auto kills the performance to some extent once third is done and dusted.

Now if only you could graft the speed tiptronic in my Cayenne Turbo S to your power unit!