The last time we checked in on our Subaru STI sedan Test & Tune project, we extracted an additional 34.5 WHP by simply changing engine oil, installing new spark plugs, fine-tuning the engine calibration and increasing peak boost pressure by only 1.5psi. This time around, we enlisted the services of several bolt-on parts that are common to the upgrade path for EJ257 owners. In this process, we discovered a few more limitations of the factory system. Understanding these limits allows enthusiasts to make the right choices to safely increase power.

Text by Arnold Eugenio // Photos by Richard Fong // Dyno Testing and Tuning by Crawford Performance

The Parts

test-tune-subaru-sti-122-001On many turbocharged vehicles, simply replacing the factory intake system with a well-designed aftermarket performance intake system results in increased power. On MAF-equipped vehicles, an aftermarket intake system may be purposely designed to alter the MAF signal to report less airflow than actual to the ECU. This can trick the ECU into leaning the air/fuel ratio and advancing the ignition timing. On a bone stock vehicle with no other modifications, this may produce a safe tune. However, when other performance parts are added to the mix, the likelihood of an unsafe tune increases. Fortunately, we’ve already equipped our T&T STI with a Cobb AccessPORT engine management system that allows us to properly calibrate the engine for any modifications that are added to the mix. As such, the installation of AEM Induction’s performance intake should yield measurable gains without jeopardizing the safety level of the tune. In addition, the Cobb AP also allows us to regulate boost presure to prevent “phantom” power gains seen from boost pressure increases as opposed to increased airflow offered by the new intake system.

test-tune-subaru-sti-122-002Love it or hate it, the EJ257 boxer engine produces a distinct exhaust note that’s muted by the factory exhaust system. To realize more performance, we chose the Subaru Performance Tuning (SPT) cat-back exhaust system to increase power while producing adequate sound suppression. The SPT system is constructed from mandrelbent stainless-steel and features dual twin-entry, twin-exit mufflers with a total of four tips extending from the canisters. The natural dull finish lends a stealth aspect to the system as to not attract unnecessary attention from the tax-collectors parading as police.

test-tune-subaru-sti-122-004A downpipe can deliver big power gains and improve boost response. However, a reliable method for boost control should be explored before a downpipe installation. Our Cobb AccessPORT will regulate the factory boost control solenoid to keep pressures in check. For the downpipe upgrade we selected a Crawford Performance one-piece cat-less downpipe. With the main pipe constructed of a single piece of mandrel-bent stainless steel, the Crawford unit steps down from a 3-inch to 2.75-inch diameter to retain compatibility with aftermarket cat-back or factory exhaust systems.

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