I wouldn't be so quick to disregard the 3 bolt, presently a limited number are available from
www.theturboplace.com (call the number and ask for Sam). These are from the same batch of housings pictured in post #20 of this thread.
At the recent Lorenz Performance and Racing open house two GN's with 3-bolt BorgWarner EFR-9174's were dyno tested:
First car was John Schmidt's, e98, GN-1 stroker with rollers and GN-1 heads and matching intake, to the rolls it made, 822 Ft-Lbs @ 3,800 RPM and 659 HP @ 5,200 RPM on ~29 psi, this car previously ran a 9.97 @ 135 with this set up but pig rich. Leaning the car out a bit while on the dyno bumped the power from 600 to 659 HP.
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Second car was Bryan Lynch's, (phoneguy) stroker, pump gas with alcohol, TAI class legal, to the rolls it made 856 ft-lbs @ 3,700 RPM and 640 HP @ 4,500 RPM
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Bryan's feedback about boost response is that it comes up on boost very fast!
Another recent development (Thanks to GEE-M racing) is true twin scroll headers!! These headers were on display at the 2018 TurboBuick.com nationals along with a BorgWarner EFR 7163-G turbo. These headers should accommodate all EFR twin-scroll turbos and also many of the BorgWarner AirWerks turbos.
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There are now available options for 3-Bolt, single scroll four bolt, and true twin scroll four bolt.