you want the taper right at the turbine outlet.

but unless you run a super-low-mount turbo, you cant' fit it. this is why I put mine in the middle of the downpipe. not ideal, but I fit it in a street car with PS and AC and long toobz.

 

That's pretty. Can't believe you sold it.

 

Uh huh. I saw a friend who HAD to have 3" exhaust, ended up taking a 1.5" deep pie slice out of it because mandrel bends couldn't be found which would fit in 3".

Seriously, make it smooth and big as soon as reasonable.

 

Some observations from the closing paragraphs:

This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.

As you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would.

1) Best explanation of why big turbos make same power at lower boost. I think we all came to the agreement that the exhaust manifold back pressure was the key, but this takes the understanding to a new level of reason why there is lower pressure in the exhaust. The previous, "bigger wheels", had not satisfied me. This is a bit of the why.

2) Last paragraph shows why it is even beneficial to use a 3" exhaust even at low power (small turbo). Get the most bang for buck from the work done. True that max flow is max flow, but you get it with the least amount of stress on the system.

Anecdotal case: I'm using that pea sized turbo, but segregated downpipe including a baffle welded into the turbine outlet, such that it is nearly like a EWG (on the outlet). Then I taper and taper such that am at 3" at the side of the bell housing. Then a high flow cat, all 3" long radius, and the big perforated Magnaflow muffler. And the car seems to be approaching 200HP {(Vlad's guestimate) and makes sense based on my fueling requirements} at 9psi manifold pressure. (Sorry no hard dyno numbers and a sample set of 1, but I did say anecdotal.