Originally Posted by IlChengis
Someone (sorry, don't remember whom) posted this
link a couple of weeks ago and I found it extremely interesting.
Here's a quote:
Based on the above, wouldn't you say that 3" catless DPs tapering down at the midpipe would be optimal as opposed to earlier tapering or plain 2.5" DPs? I understand that dual 2.5" DPs could easily handle 450-500hp, but wouldn't 3" DPs drop backpressure enough to yield quicker spool at Force-Inducted power levels below that range?
If I recall correctly, someone on this Forum had custom 3" midpipes done that tapered down to 2.5" at the mufflers. I suppose such setup could have easily handled over 500whp given the reduced backpressure caused by moving the taper so far down the exhaust.
LOL.... smalll world. That post about exhaust design was made by a buddy of mine (Jay Kavanaugh) at my request several years ago to clear up some myths about "bigger-is-better" downpipe design on the Subaru forum. Back then, some people had the notion that a single 4" dump tube coming off the tiny WRX turbocharger is better than the more convention 3". The manufacturer quoted the Ideal Gas Law to defend an exhaust design that tapered down from 4" to 3". So Jay laid down some facts.
But it's true that you do want to push ID transitions as far downstream as possible since that is where the exhaust is coldest and has the least velocity. But in the case of the n54's dual exhaust system where each downpipe is only flowing half of the total exhaust flow, even 2.5" is sufficient. Running dual 3" exhausts (downpipe+midpipes) wont help or hurt you anymore than running dual 5" exhausts. It's all overkill above a certain point.
I think the most important point that Jay made is that you want to make the transition between the turbine output and the downpipe inlet as smooth as possible and then gradually flare it out from there. This would suggest that the idea exhaust system would start off at ~2.25" coming off the turbo, flaring out to 2.5" (or 3") and staying that diameter all the way to the dual tips.