I think the "horsepower gain" story came from since there isnt hot coolant flowing thru the TB it doesnt heat the IM or the intake air as much so you might gain some hp. Ive seen datalogs of IAT's dropping up to ~10*F just from bypassing the TB coolant, but as for the HP gain ive seen no evidence. Works on the same principle as those IM gasket spacers to keep the head from heating then IM. I doubt it matters on a turbo engine anyways.
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