GT3071R vs. GT3076R:
71 is good for roughly 470WHP and will perform better at higher boosts(~21+ PSI).
76 is good for roughly 520WHP and will perform better at lower boosts.
This is about all you need to know, spool differences will be in the range of 200~400 RPM different if I had to guess given same style exhaust housings and A/Rs.
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