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2011 Subaru Impreza WRX STI - Power Pages

The dyno doesn't lie.

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Dynamometer Model: Dynapack Evolution 4000
Testing Facility: Crawford Performance
Ecutek Tuning: William Knose

Baseline

Pros
The '11 Subaru Impreza STI debuts with two new looks for 2010, along with a much improved suspension package that uses 16 percent stiffer front and 53 percent stiffer rear springs compared to the '10 model. Pillowball bushings on all four corners and sway bars help improve handling while eliminating understeer. Among the many changes for the new model is a revised front fascia, the return of the oversized STI rear wing and a coupe trim for the STI, and a BBS wheel package.

Cons
Similar to the previous models, the '11 EJ25 engine doesn't take kindly to bolt-on modification. The '07 to '10 STIs are more temperamental than the '04 to '06 STIs in terms of ECU tuning, but are nowhere in comparison to the '11. "It seems that the ECUs are becoming more self-aware and simply slapping on aftermarket parts without tuning will cause a loss in power," states William Knose, lead programmer of I-Speed USA and our dyno operator for this installment of Power Pages. Even in baseline testing, the STI never ran consistently, sometimes making 195 whp while at other times up to 236 whp. Knose eventually came to a conclusion that the '11 STI requires a 10- to 20-second battery disconnect/ECU reset between runs to ensure consistency. "There was a learning period of 5 to 10 dyno pulls to get the horsepower numbers to stabilize and run consistent back-to-back numbers," stated Knose.

Notes
Prior to our baseline run, a Crawford Air/Oil Separator was installed on the engine as a method to eliminate oil blow-by, which is notoriously common among Subaru engines. The Air/Oil Separator solves a simple problem by transferring blown-by engine oil back into the pan via the block and not the intake-keeping oil from coating both the intake tract and the inside of the intercooler, improving cooling efficiency and warding off detonation. In the end, after numerous battery/ECU resets and 12 dyno pulls, the STI laid down consistent figures on the average of 234 hp and 262.3 lb-ft of torque to the wheels.

SPT Performance Exhaust

Parts
Exhaust, mid-pipe, nuts, and bolts

Tools
12-, 14-, and 17mm sockets; 12-, 14-, and 17mm open-end wrenches; ratchet; extension; crescent wrench; and WD-40

Installation Time
60 minutes

Pros
The SPT Performance exhaust system features three-inch, full stainless steel construction with quad tips that are polished and etched with the SPT logo.

Cons
The new exhaust was easy to install, requiring a simple spray of WD-40 to the exhaust bushings to aid in removal. Due to the bulkiness of the exhaust, we suggest asking for assistance to help speed up the installation process.

Notes
The SPT quad-tip exhaust system is slightly louder than stock, but not obnoxious, thanks to its resonated center mid-pipe that eliminates monotonous droning when driving on highways. The SPT Performance exhaust made 4 hp and 6 lb-ft of torque at 3,600 rpm over our previous run, with plenty of gains in the midrange.

AEM Cold-Air Intake

Parts
Intake, filter, hose clamps, hoses, mounting hardware, license plate frame, sticker

Tools
10-, 12- and 14mm sockets; 10-, 12- and 14mm open-end wrenches; ratchet; extension; screwdriver; pliers

Installation Time
40 minutes

Pros
The AEM cold-air intake uses a mandrel-bent 6063 aluminum inlet pipe, finished in wrinkle-red made specifically for Subaru STI models. AEM engineered the inlet pipe with a built-in air straightener to maintain laminar airflow delivery to the mass airflow sensor (MAFS), to eliminate improper air/fuel ratios (AFRs) and/or check-engine lights.

Cons
Due to the '11 STI's very recent debut, CARB approval for the AEM intake is currently pending.

Notes
With the newly installed intake in place, the AEM cold-air system delivered an impressive increase in midrange and top-end power, with a gain of 23 hp and 45 lb-ft of torque at 3,900 rpm. An amazing feat, considering these numbers were achieved without ECU tuning.

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