I guess it depends on how much hp your anticipating, but as a rule of thumb; turbos hate backpressure... the "bigger is better" mentality can be easy to get wrapped up in, so I would pick a size proportional to the HP you want to make. I think for most applications 2.5" to 3" inch will suffice. I am running 2.5" from the turbo back on my t25.
-dave
FWIW, I gained about 25 wheel HP by doing a full 3" exhaust system with a stock BB DET in my NX2000. And that was at stock boost too, 6.5 psi. I say 2.5" is too small, even with a stock T25 turbo set up.
I don't know if the t25's for the sr20s are different than the ones for eclipses, and although eclipses run higher boost (10 psi stock,) most people i talked to recommended a 3" exhaust, and being a former eclipse person (95 GST) I have talked to many, including guys at Road Race Engineering (pretty much the premier SoCal Eclipse shop.) Considering that they are both 2.0L engines with t25 turbos, much of part diameter advice for the eclipses should apply to the DET's, of course corresponding larger gains in part size with larger CFM #s from the turbo.
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"I swear Boris, if you didnt have bad luck you sure as hell wouldnt have any at all"-Nostrodomas, story of my life
Lewis Black quote of the Week:
"Baseball should go back to its roots and bring back performance hindering substances. If not for me, do it for the children."
You should remember an exhaust system should maintain a certain exhaust gas velocity for optimal performance, This velocity is approximately 250Ft/Sec. To size up the exhaust pipe you can either adhere to the exhaust gas velocity of approx 250 Ft/Sec or more simply by selecting a tube diameter approximately 10% larger than the turbine outlet diameter. Someone correct me if IM wrong.
3" is good for turbo, either custom fab. or the HKS dragger is a good system to get, 95mm diameter piping with a single 130mm diameter stainless steel tip
but keep the cat, the stock cat is very free flowing, and I'm not a big environmentalist or anything, but you alone driving around with no cat puts out more pollution than over 100 cars with a cat.
I've seen dynos with and without a cat, and with it, youre loosing like 1 hp.
3in exhaust for a street car is dame near perfect. It depends on what you are looking for. Are you looking for more torque or Hp? If you want the torque you can put a 2 1/2in down pipe then fade in to a 3in pipe.
yeah, I really need a 3" exhaust, I am running a 2.5" downpipe that goes down to 2.25" then to the stock b-pipe(1.75" or so) that then goes into the catalytic which is hollowed out, and ends up in a Greddy 2.25" exhaust, let's just say that there is plenty of back pressure even at 4psi on a GA16DE with T3 turbo,
JCC
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waiting on more parts and money for a turbo B15...
Turbo's don't like back pressure. It works against the turbo and can ruin the thrust bearing. I had identical results as Zak91SE-R...like 25whp from switching from 2.5" to 3" exhaust.
I've heard similiar results from switching to 3" downpipe (mine is currently 2.5") but FMAX 3" dp's ain't cheap
the greater the pressure differential before the turbine(exhaust manifold) and after the turbine (downpipe) the better the performance.
yes, 3 is better than 2.5, but ive seen a few cars make upwards of 400+whp with a 2.5 downpipe. most of the time its just because there isnt that much room in the engine bay to work with in a FF turbo street car. sometimes its better to use a smooth transistioned smaller downpipe than a larger "cheated" bend downpipe.
it can be done with compromises though. for instance, this 3 inch DP is in a civic street car without PS or AC
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