Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to...
My truck has a 3" exhaust and the outlet side of the turbo is 3" diameter. By outlet size I mean as the exhaust passes through the turbo. The boost side that goes to the intake on my application is also 3 ".
What turbo manufactures have done to improve the peak efficiency around wide rpm is .
1)variable geometry exhaust housing, this not used much in gas as temps to high but idea is a variable vanes/deflectors in the housing that can keep efficiency up over engine rpm band .
2) twin scroll housing...
http://www.barber-nichols.com/products/specialty_products/turbo_alternators/default.asp
They can build it for you. Apparently Turbo Altenators have been around since the 80's
What I meant is if your vehicle has 1.5" exhaust the turbo outlet will be 1.5" if you have 3" exhaust your turbo outlet will 3".
I received a 25HP increase by eliminating my belt driven fan to an electric one. And throwing out the smog pump gave me a 10hp increase. Under-drive pulleys also...
Having an extensive automotive background I thought of this as well. The battery powers the vehicle the alternator charges the battery as well as supplying some power to the car.It would work but would have to go under the vehicle where it is very wet so it would need to be put into a water...