I looked long and hard at a GT28rs before going for IHI.
I have held both these turbos in my hand side by side, and can tell you that the gt28RS is physicaly much larger than the IHIVF34. I can't see any way it's going to spool up as early as the VF34.
Also on a small port head, the VF34 will maintain 22psi to 7500rpm. So unless you're going to rev over that, or run much higher boost pressures, there is no point going for a larger turbo.
If you run higher boost pressures, your timings going to be so retarded, that the EGT's will go through the roof, and the ecu will drag the boost down. So unless you're going to stop the ecu from controlling boost (manual boost controler), you're ********, and iff you do, you're risking your exhaust valves.
Now on a large port head car the ihi is going to struggle to hold that kind of boost pressure even with the larger 20 housing, so the gt28rs will move a bit more air, and make a bit more power.
I've seen logs of gt28rs' moving 250g/sec peak, and the IHI maxes out at about 235-240g/s from my own measurements. This means the rs will make a few % more power than the VF34 on a head that can flow enough air at low enough boost pressure to maintain decent timing advance, an low enough EGT's. Ie not an s3, and not an ibiza unless the heads have had some work done.
This argument about any turbo making more power than another at the same boost pressure is quite frankly rollox.
The pressure is generated by backpressure from the air entering the combustion chambers through the head. The only eficiency gain from one turbo to the next is in pumping losses ie how much restriction they place on the exhaust. These acount for a few % max diferences between turbos. Same pressure at the same rpm = same torque and thus the same power.
Bigger turbos can maintain higher pressures higher in the rev range and make more torque at higher revs. But ultimately if you run too much boost, ignition advance is reduced to such a point that combustion is still occuring when the exhaust valves open. This means the exhaust runs hotter and eventually your manifold, turbo, and valves melt. Also as combustion is occuring after the exhaust valves open, that energy is wasted and not used to push the piston down.
You may well see some dynos from the US with an RS making silly powers, but they're either using race gas or happy dynos.
250g/s is going to give you a max of about 345hp on standard fuels and its going to get very hot very quickly, so it won't maintain it for long. To do this on a small port head, you'll need to run about 23psi at 7Krpm. You may gain 20-30hp on race fuels.
The IHI at 235g/sec will make 325hp on regular fuels. This is with the standard smaller housing. Bills may make a few HP more.
So to my conclusions. A gt28rs is a waste of time over an IHI vf34 on a small port head 1.8t. On a large port head you may gain 20Hp over a VF34, but you'll lose 500rpm of your power band. I wouldn't want to drive a car that didn't make full boost until 4500rpm. Consequently I bought an IHIvf34 and have 22psi at 3500rpm to 7500rpm. Nice power band I can tel you.
