In our last STI GVB Test & Tune, we increased the EJ257’s operating efficiency with a bolt-on intake system, a high-flow downpipe, and an exhaust system. These bolt-ons netted a peak horsepower gain of just over 31 horsepower but revealed the limitations of the factory intercooler system as the power curves showed signs of detonation. In this segment we’ll explore solutions that allow us to safely increase the EJ’s power output while eliminating harmful detonation.

Text and Photos by Arnold Eugenio // Dyno Testing and Tuning by Bill Knose/HG Motorsports

The Parts

test-tune-subaru-sti-125-002The STI’s top-mounted intercooler sits above the engine, the turbocharger and exhaust components. It is fed cold air by a small vent in the GVB’s hood. The placement of the intercooler and lack of direct air-flow make the setup less than desirable when increasing engine power output. In fact, we determined the detonation seen on our T&T STI to be caused by the extremely high intake temperatures. The Crawford Performance FMIC locates the intercooler in the front of the vehicle, allowing the core to be both larger and thicker than the factory unit to better exchange heat. Aluminum pipes replace all the factory rubber and plastic charge-air piping and help prevent pipe expansion under extreme boost conditions.

test-tune-subaru-sti-125-003Accessory pulleys driven by engine rotation can reduce potential power output when dealing with performance modified engines. The often-heavy pulleys introduce drag through weight, even without the accessory activated, and can contribute to an engine’s overloaded condition. Go Fast Bits (GFB) offers a three-piece pulley kit for the EJ257 engine that replaces the crank, alternator and power steering pulleys with same-diameter lightweight units. The lighter units are said to reduce parasitic drag on the powertrain without under-driving the accessories.

test-tune-subaru-sti-125-004Because our Test & Tune STI was slated for weekend track duty, we decided to further ensure the setup against detonation by providing a race-gas tune for such excursions. VP Racing Fuels’ MS109 is an oxygenated, unleaded fuel with an R+M/2 octane rating of 105. It is said to provide increased protection from detonation in applications boosting up to 25psi, perfect for our STI’s peak-boost target. The oxygenation additive can help provide more oxygen for combustion, but may not be approved by all sanctioning bodies for competition; check your specific organization’s class rules for confirmation.

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