Based off my Internet knowledge I'd say that it's loosing out on some top end and it's got more back pressure which if I remember correctly should increase low end torque.
There was a thread years ago on rx7 club discussing all this stuff.
However, if it works it works
it would be interesting to see the difference in EGTs of a T3 versus a T4
I'm going to end up with prety much the same turbo that James ran on his 8 second rx7, and that ran a T6 1.32 rea housing on a s480 turbo and he said he didn't really see the car struggle to spool the turbo even with it being so huge back housing
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