I agree with what has been said, with one exception.
You mention that is has an On Center style T4 turbine housing. IMHO, that is the sorriest example of a turbine housing that you could possibly get. In some cases, the 3 bolt PTE housing will flow better than the On Center housing. If you take the turbo off and look into the inlet of the turbine housing, you will see just how bad a design it really is by not only how small the inlet necks down, but the very sharp 90* angle that the inlet wraps around the scroll of the housing. The On Center housings were designed for applications where installation of exhaust components were very tight, and was not designed for optimal flow. This is why you tend to see this type of housing installed inside very cramped engine bays, or on twin turbo applications where you can simply point the turbo straight on. Inlet towards the front of the car, downpipe straight out the back and on the centerline of the rotating assembly. The T4 Tangential style housings inlet is slightly off set and requires more room for installation.
As far as hp differences are concerned, it's really hard to say. Meaning what hp difference would you expect if going from a T4 On Center to a 3-bolt Buick setup, as it's not really comparing apples to apples. More like comparing a spoiled banana to an apple.
I can tell you that in some applications, I got reports of gaining as much as 70 hp by simply changing from an On Center housing to a T4 Tangential housing. These were typically on high hp V8 applications where low back pressure and huge flow rates are critical.
If you were to switch from the On Center style housing over to a T4 Tangential style housing with a .68 A/R, you might see a very nice increase in power up stairs and not really loose too much on the bottom end. You would loose a lot of back pressure tho that's for sure.
Here are two pictures that show the two types of T4 housings.
The first one is a T-netics On Center T4.
Notice how sharply the exhaust has to turn in order to wrap around the scroll of the turbine housing. The inlet hole that is created by this sudden 90* bend is really a small area. It's called On Center because the turbine housing discharge is on center with the turbine housing inlet hole.
This is a picture of a PTE TH5876A T4 Tangential style housing.
As you can see, the exhaust has more of a straight and smooth path to flow when wrapping around the scroll. This dramatically helps reduce back pressure vs the On Center style housing.
If it were me, and you wanted to make a change, I would either install a .63 THB3-76 turbine housing on it, or a TH6876A T4 Tangential turbine housing on it and go. The Buick 3-bolt housing won't hurt flow or spool up at all. The T4 Tangential might require slightly more stall, but it will free up some hp in the upper rpm range.
Hope some of this helps and sorry so long.
Patrick
