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      11-18-2010, 08:52 PM   #57
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Drives: 2012 E92 M3
Join Date: Aug 2008
Location: Miami and Atlanta

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Originally Posted by shiv@vishnu View Post
Besides, the 3.0" downpipes taper back down to 2.5" where the meet up with the midpipes.
Someone (sorry, don't remember whom) posted this link a couple of weeks ago and I found it extremely interesting.

Here's a quote:
Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.
Necking the exhaust down to a suboptimal diameter is never a good idea, but if it is necessary, doing it further downstream is better than doing it close to the turbine discharge since it will minimize the exhaust's contribution to backpressure. Better yet: don't neck down the exhaust at all.
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.

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