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Increase would not be that great, but there would be alot less stress on the turbo due to alot less backpressure. Even if you just run 4" straight out of the turbo for about 2 feet then neck it down to your 3" exhaust it would be a big help for the life of the turbo and a gain in power.
I have always heard and read (the new "turbo" book by Jay K. Miller) that if you have at least 18"-24" of the outlet size (4" in your case) before you neck it down (3" in your case) the difference is negligable(spelling? lol) because the exhaust has already reached diffusion (stopping of the swirling action caused by the turbine). So in other words. If your turbo has a 4" exhaust outlet, just run 2 foot of 4" then neck it down the your 3" and connect it to the exhaust you already have. It will save you alot of money and you will see no noticable diffference by running 4" all the way back anyways. That is if this is a street driven car. If this is a race car. Runa bout 2' of exhaust and take it from the 4" outlet and neck it UP to a 5" as an elarging bell shaped downpipe of about 2 foot is the best possible senario for a turbo.
Due to all the conversations I've seen here and there, I've decided my winter project (VS S480 Clone) will have a 3.0" going to the ubiquitous Borla CatBack, but also a QTP 3.5" electric cut out goin straight out to the passenger fender exit. I bought the Monkey fab 4.0" stainless fender exit so it will look a bit better. Plan is to drive to/from car meetings with the cutout closed but when I go to the track , let it flow out both the 3.0 and the 3.5 cutout.
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Turbo Guys Come inside 
