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Old 07-21-2006, 10:19 AM   #16
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Member#: 84105
Join Date: Mar 2005
Chapter/Region: SCIC
Location: I'm a Newbie
2005 Super Slow STi


Originally Posted by Sypher
and going from 4 inch (bellmouth right?) to 3 inch to 2.5 inch to 3 inch is bad exhaust flow mojo. your gonna have so much danm turbulance dude its not even funny. Its best to have either full 2.5 inch or 3 inch all the way from your downpipe back.
Not true at all. If your sticking with the stock turbo which doesnt boost that much efficiently, then that actually makes for a very smooth transition. Gas is hottest as it exits the turbine so the large 4" bellmouth is accommodating for the expanded gases. As they travel further back down the exhaust stream the gases cool which cause the stream to contract. By the time it hits the rear section it has cooled enough to where the 2.5" taper still has plenty of room w/o causing any excessive back pressure or turbulance. If the turbo is upgraded then yes, the gases get hotter and need more room to decrease BP so a straight through design is a big benefit but anything less really doesnt need it.
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