GrimmSpeed BB450+ and BB550+ STI Turbo Tests: Read the Curve, Not Just the Peak
Editor’s note: The hero is a conceptual turbo-and-powerband illustration, not a photograph or dyno chart of a GrimmSpeed product.
GrimmSpeed has published development testing for two upcoming Subaru STI turbochargers, the BB450+ and BB550+. Its October 30 report compares them with the current JB400 and BB500 on the same test vehicle. For shoppers, the useful lesson is not a headline horsepower number: it is how each setup delivers power across the rev range, and which supporting systems shaped the result.
What GrimmSpeed actually tested
The manufacturer says its test car was a 2020 Subaru STI with a built engine, upgraded rods and pistons, Stage 3 bolt-ons and E85 fuel. The pulls were made on a Mustang Dyno. That is a specific development combination, not a promise that another STI will make the same wheel horsepower. Fuel, calibration, engine condition, drivetrain losses and test procedure all matter.
In that configuration, GrimmSpeed reports approximately 485 wheel horsepower and 460 lb-ft of wheel torque for the BB450+, versus roughly 395 wheel horsepower and 415 lb-ft for the JB400. GrimmSpeed says the BB450+ replaces the JB400’s journal-bearing design with ceramic ball bearings. The manufacturer’s comparison also shows the BB450+ carrying more power near the top of the tested range, which may matter to a driver choosing between response and sustained high-rpm output.
For the larger BB550+, GrimmSpeed reports approximately 530 wheel horsepower versus roughly 460 for the BB500. The company says the test car’s 1300 cc injectors constrained fuel delivery at that point, so it stopped the pull at 27 psi. GrimmSpeed estimates there may be more boost potential, but that estimate is not a measured result on this car and should not be treated as a safe target for another build. Its published curves show the BB550+ reaching its torque peak later than the BB500 and gaining a stronger upper-rpm advantage.
Three checks before planning a turbo change
- Choose the powerband you want. Compare the full curves at the rpm you actually use, rather than selecting solely by peak horsepower. Street response, track use and the rest of the drivetrain can point to different choices.
- Plan fuel and calibration together. An E85 development pull with upgraded injectors does not establish what a stock fuel system can support. Confirm pump, injector, regulator, sensor and tune requirements for your particular combination before ordering hardware.
- Wait for the final application details. GrimmSpeed has not published complete specifications or an order-ready release date in this announcement. Verify the exact model year, engine, mounting and supporting-part requirements, as well as emissions rules for where the vehicle is driven, when final product documentation is available.
GrimmSpeed’s original test report has the manufacturer’s curves and context. If you are planning an STI turbo-and-fueling package now, use those questions to compare currently listed parts with your build goals; do not assume the BB450+ or BB550+ is available to order yet.