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Old 02-26-2001, 11:25 PM   #1
RidinLow
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Question wrx stock exhaust diameter good?

Here I was thinking that I'd upgrade my WRX's puny 2.25" exhaust to a 3" one as is common in the tuning world. However, I just saw this graph in my new copy of Maximum Boost:

<IMG SRC="http://www.erichsachse.com/garrick/turboexhaustsize.jpg" border=0>

This graph shows the proper exhaust diameter to maintain what the author says is an appropriate flow rate thru the pipe (450 ft/sec). What do you guys think? Is Corky Bell being too conservative?

[This message has been edited by RidinLow (edited February 26, 2001).]
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Old 02-27-2001, 06:24 AM   #2
svxtrem
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Although I have never been a fan of the 'sewer pipe' exhaust pipe, I do have a problem with this graph. If you take the twin exhaust, and measure the cross section, then take the corresponding single exhaust size on the same spot, the cross sections will be more than 15% off. Perhaps it is possible that the chart was not reproduced correctly.

IMHO, the real reason people find gains with a 3" exhaust after a turbo comes from the characteristic of the flow from the turbine. The exhaust exits the turbine in a helix- (spiral). To force the spiral to break up into a turbulent flow, you need a sharp diffuser angle after the turbine. I never see this done on turbo installations. I believe the 3" pipe can help in that the helix is able to continue its spiral for some distance. However, if the proper diffuser (or you could use a dual exhaust exit which tangentially splits the turbine), you would not need to use such a large diameter pipe. This would help the velocity and increase the scavenging affect.
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Old 02-27-2001, 07:08 AM   #3
Jon Bogert
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I think the cross section discrepancy allows for the larger boundry layer in two pipes compared to one.

Still, that chart is very interesting, given some people's pre-occupation with huge diameter exhausts.
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Old 02-27-2001, 04:13 PM   #4
RidinLow
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Cool ideas... anybody else have any insight?
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Old 02-28-2001, 07:19 AM   #5
Keith
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ttt
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