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My take on 3.5 and 4 inches turbine outlet difference.
Back pressure will be a bit different and thereâre a lot of advantages to take from less back pressure with turbocharged motor. As a matter of fact, BMW even uses 4.5 on B58 turbo. While catback pipe that is downstream of downpipe has always been 3 inches. It makes sense as exhaust cools down and shrink as it travels and needs a smaller diameter to keep velocity.
Here is the thing: Far too many PS2 on the road are being pushed far beyond what it is designed for. But there is only so much PS2 can flow. In squeezing last bit power of it, meth, ethanol and a bunch of other dirty tricks play more important roles than half an inch outlet difference.
Probably less back pressure out of 4 inches will net you 15-25whp depends on boost, but PS2 power varies from 400 to 550whp depends on setup, making that difference look insignificant.
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Lemania 2320
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