Exhaust diameter:Fact or fiction?
 

If your turbo has a 2" outlet, and you have a 2" exhaust with no muffler, would you see gains by adding a 3" turbo-back exhaust?

What if you have increased the boost pressure to 25psi with fuel to match?



ps-I got my post-turbo alcohol-port-injection manifold welded yesterday;)

In theory, yes. The less restriction after the turbo the better.

Yep. Not by much though.

Once you get into the 20psi range, you will probably start to see a difference. That's a lot of air to try and flow through the pipe.

Yep! Take a straw from your favorite fast food restaurant, and blow through it. Now cut it to be two inches long. Blow again.... should be a noticeable difference. Whether it makes a difference for our specific application depends on how much back pressure we actually have, etc.

Isnt the turbo housing the most restrictive thing on the exhaust?

It is(especially mine-2.25" outlet)
But the straw comparison makes sense.

How loud do you guys think a straight pipe exiting in front of the rear wheel(passenger side) would be?

The most restrictive thing in the exhaust is the turbine itself. The really short section of the turbo exhaust housing is going to contribute to the back pressure, but, there is no reason to limit the exhaust size to the turbocharger outlet... depending on the HP demands you could very well see a benefit.

The turbine is the biggest restriction. The less restriction after the turbine, the bigger the pressure drop will be and the more efficient it will work.

I ended up going with 2.25" (Recycled alot of the existing exhaust)
Tomorrow I've got to weld in the pipe going back and out in front of the passenger wheel.
Factory exhaust: 4x90* bends, 2x15* bends, and a muffler.
My exhaust: 2x90* bends, 1x45* bend, 1x15* bend. :)

Here's my fancy welded downpipe :)
http://img24.imageshack.us/img24/7341/downpipew.th.jpg