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I can give you one sample point to judge return on investment on a similar setup. Currently running a stock S5 13BT with about 50K miles on the block. No porting or internal mods to the engine. I'm running a BNR Stage 1 turbo, with 720cc Bosch FI's in all 4 holes, and a Supra TT fuel pump. Stock ECU with Rtek 2.x mod for control. Intake is stock with a K&N filter, and when I got it dyno tuned last winter, the exhaust was a Bonez DP into the stock main cat, into a Borla catback.
Took it to Dave at Speed1 (Brian's boss) for dyno tuning. Going in with the default Rtek settings (other than checking the box for 720's FIs in all 4 holes), my car put down about 189rwhp, but boost was maxing out at just under 6 psi -- Dave found this was due to a restrictive main cat. After Dave's tuning magic, which cost just under $200, I left with about 210 rwhp. Not bad for boost limited to <6psi!
Over the summer, I managed to get historic plates on the car and yanked the main cat & Borla DP and replaced them with the Racing Beat DP & pre-silencer combo. Still using the stock EBC solenoid as controlled by the ECU which now nets me a very consistent 8~9 psi boost all the way up to redline. Haven't brought it back to Dave yet for dyno tuning, but the butt dyno tells me the improvement is significant -- need to make an appointment to get it dialed in.
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