On a side note, the TO4E originally came from a big diesel truck motor and is a mid eighties design. The common TO4B was designed in the late 60’s. Thus the T04E has the benefit of being designed after at least ten years of fluid dynamic research which shows in its better performance.
On the exhaust side is a T-3 turbine, but it is the biggest T-3 turbine available, known as the stage III (Or TA32) by Turbonetics. It is used on their all out Buick Grand National turbo It was originally OEM for a Navistar Diesel engine! This is a pretty free flowing turbine selected in our case mostly to eliminate backpressure.
These parts were also chosen in part to reduce lag! The T04E wheel makes a good amount of boost at 80,000 rpm, down quite a bit from the 250,000 the T-28 spins at. So even though the T04E is quite a bit bigger and heavier, it does not need to be spun up to such a high rpm to make big boost numbers. The big wheel also starts to move air at a much lower shaft speed so the boost onset rpm is kept low. Thus the lag will be kept reasonable even with big honkin reciprocating parts.
The lower shaft speed of the TO4E results in a more gentile handling of the air and thus less charge heating. A more technical explanation is that the super fast spinning T-28 has the compressor wheel tips traveling at near sonic speed. When the tips reach the speed of sound the air forms shock waves inside the compressor and the turbo stops pumping. Operating at conditions near this cavitation point are not so hot for efficiency which creates more charge heating.
An exotic ball bearing center section was selected to eliminate the traditional sleeve bearing’s oil-induced viscous friction in the center housing. The ball bearings should help the turbo spool 5-600 rpm sooner.
I feel that the big gun turbo will start to produce boost by 2500 rpm, be impressive at 3500 and get scary at 4500 rpm! This is about 1000 to 1500 rpm later than the spool up point that is enjoyed by the ultra responsive turbos run by the T28 crew but is still quite streetable. The power of the big gun will climb strongly until redline instead of surging hard to 5000 to 6000 rpm then falling off. The smaller turbos start to peter out in the 5000 to 6000 rpm zone because the backpressure starts to rise, leading to power robbing.