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2011 Subaru Impreza STI - Power Pages

Text By , Photography by Augustin Leung, , Manufacturers

Dynamometer: Mustang MD500SE
Testing Facility: Cobb Tuning SoCal
Cobb Accessport Tuning: Jon Drenas


With the popularity of the '11 Subaru Impreza STI and its ever-growing presence on the streets and local racetracks, it was only logical to develop a Power Pages suited for those looking for more than just the typical exhaust, downpipe, and header package. We organized this month's to follow the same three-step format Import Tuner has followed for the past 12 years, but decided to add a slight twist; rather than starting with an unmodified car, we tracked down a street-tuned STI, equipped with an HKS bell-mouth downpipe; DC Sports dual 3-inch catback exhaust, header, up-pipe, and tuned on a Cobb AccessPort

The '11 STI's EJ25 engine doesn't take kindly to major bolt-on modifications as the factory ECU has been characterized as becoming more self-aware to any changes outside of the factory specs. Simply slapping on aftermarket parts without tuning will cause a loss in horsepower, which includes issues such as running erratic, over boosting, running too rich and/or sending incorrect timing values that affects overall horsepower. With many of today's newer cars, including our STI, aftermarket tuning has become the key component to making more power at safe levels.

Prior to our baseline run, a set of DeatschWerks 1,000cc injectors and a DW65C fuel pump was installed to the engine to help feed our STI's fuel demands. DeatschWerks has established themselves as a trusted name within the automotive performance community. Both flow rates and set balancing of every injector is performed in-house by DeatschWerks while featuring drop-in fitment for easy plug-and-play installation. The DW65C is a direct factory replacement fuel pump for the '08-and-newer WRX and STI. The pump's compact design eliminates the need to grind, retrofit, or modify your stock fuel pump housing to fit a fuel pump. The DW65C's high flow rate of 255 lph is great for someone doing a medium-sized build or needing an OEM replacement pump. Cobb Tuning SoCal in Fountain Valley, CA, provided an AccessPort reflash and tune of the factory ECU following each product installed to maximize the potential of each part added. Jon Drenas, lead tuner at Cobb, created a custom map to un-tap power, resulting in a baseline of 299 hp and 347 lb-ft of torque at the STI's wheel. To showcase our initial power gains, we overlaid the tuned Stage 2 baseline with a stock baseline of the same STI for comparative numbers.

AEM Cold Air Intake (Cobb AccessPort Tuned)

Intake, filter, hose clamps, hoses, mounting hardware, license plate, and sticker

10, 12, and 14mm socket; 10, 12, and 14mm open end ratchet; extension; screwdriver; and pliers

Installation time
45 minutes

The AEM cold air intake uses a mandrel-bent 6063 aluminum inlet pipe, finished in a special wrinkle red finish made specifically for the Subaru STI models. The kit also includes a heavy-duty steel heat shield with a powdercoated black finish to prevent the intake from ingesting horsepower-robbing engine heat. AEM engineered the inlet pipe with a built-in air straightener to increase airflow while maintaining proper fuel trims inline with the factory ECU setting to eliminate leaning the engine out and/or throwing a check engine light.

The AEM intake was designed to work with the factory front mount intercooler. Using an aftermarket FMIC requires some modifications to both the heat shield and intake pipes to allow proper clearance of the charge pipes.

With the newly installed intake in place, the AEM cold air system delivered an impressive increase in low to midrange power with a peak gain of 17 hp and 24 lb-ft of torque. At 3,200 rpm the dyno showed an increase of 30 hp and 49 lb-ft of torque with tuning.

ATP GTX3076R Turbo (Cobb AccessPort Tuned)

Turbo, silicone elbow, gaskets, oil/coolant lines, fittings, and mounting hardware

10, 12, 14, and 17mm sockets; 10, 12, 14, and 17mm open-end wrench ratchet and long extension; flat- and Phillips-head screwdrivers; pliers; coolant; torque wrench; and WD-40

Installation time
120 minutes

Garrett has stepped up its line of GT turbos with the release of its newest GTX series turbos. The internally wastegated ATP GTX3076R turbo is designed as a factory bolt-on upgrade for the WRX/STI, offering increased horsepower with minimal lag. The turbo consists of a Garrett factory assembled and high-speed balanced dual ball bearing CHRA, which is then assembled into a set of CNC machine-ported housings and outfitted onto an ATP exhaust housing that have been optimized for maximum flow and efficiency. Typically, swapping out the factory turbocharger for a larger unit delivers an increase in horsepower at higher rpm, but comes with the trade-off of diminished boost response-not true states ATP representatives. The GTX uses a forged, fully machined compressor wheel featuring a next-generation, patented ultrahigh flow 11-blade compressor wheel that enables it to outperform, outflow, and make more power than its standard GT counterparts. The new aerodynamic aluminum alloy wheel delivers 10 to 20 percent more flow and pressure over previous models' turbos of similar size for more power.

Installing the ATP GTX3076R turbo was a straightforward process, but we strongly suggest having an experienced mechanic install and retune your engine following its installation. As with any aftermarket turbocharger, we recommend replacing the restrictive stock 2.25-inch inlet with a Perrin Performance 3-inch silicone turbo inlet hose to promote as much inlet flow as possible. The silicone's smooth inner surface, along with a significantly larger internal diameter than the OEM part, helps to decrease turbo spool time and improve acceleration.

The ATP turbo delivered exactly what it claimed, and proof could be seen in the 30 hp and 27 lb-ft of torque increase from 5,200 to 6,500 rpm. Full boost was achieved only 400 to 500 rpm later than the factory VF48 but maintains boost and top end power where the VF48 starts to run out after 4,500 rpm. From 4,800 rpm to redline, the ATP GTX3076R outflowed the factory turbo, delivering a maximum gain of 32 hp, while maintaining excellent boost pressure at redline, and the potential for even more boost and power than the stocker can produce. Our factory intercooler was showing signs of heat soak after three dyno runs. The top-mount intercooler's size and inefficiency was limiting our horsepower potential; it would need upgrading in order to make additional power.

Perrin Front mount Intercooler (Cobb AccessPort Tuned)

Front mount intercooler, charge pipes, bumper beam, coolant overflow tank, T-bolt kit, silicone couplers, mounting hardware, license plate frames, instructions, stickers, and warranty info

10, 12, 14, and 17mm sockets; 10, 12, 14, and 17mm open-end wrench ratchet; long extensions; flat- and Phillips-head screwdrivers; pliers; and razor blade

Installation time
160 minutes

The factory Subaru top-mount tube and fin intercooler was designed to be easy to manufacture and cost effective, but not ideal for performance. Since heat rises, the factory TMIC heat soaks, which leads to loss in power and engine detonation. Perrin's front mount intercooler setup combats these issues by designing its core with a 3?8-inch bar and plate design. The bar and plate core design is more durable than tube and fin and is powdercoated to further protect your intercooler from the elements. The charge pipes are CNC mandrel bent from 2.25-inch aluminum tubing and powdercoated with a wrinkle red finish for an OEM appearance. Perrin intercoolers come in two finish options, bright silver or stealth black powdercoat.

Installing the FMIC requires modifying your front bumper. If you're not mechanically inclined to install the kit, we recommend spending those few extra dollars to have a professional service the job for you.

With laptop and Cobb AccessPort in hand, Drenas was able to take advantage of the more efficient Perrin FMIC. He optimized the fuel/ignition maps and AVCS timing while raising the factory boost from 20 psi to 21.5 psi. With a few keystrokes and dyno pulls, the STI was rewarded with a peak gain of 25 hp and 31 lb-ft of torque. Upon analyzing the dyno charts, the STI saw its largest gain of 35 hp and 40 lb-ft of torque at 4,300 rpm.


What began as a comparison between products of old and new finished with promising results for our daily driven STI. Using three bolt-on parts along with a Cobb AccessPort tune saw peak output improve to 373 hp and 386 lb-ft of torque-an increase of 74 hp and 39 lb-ft of torque. This is especially impressive considering this engine's internals were still stock. With nearly 400 hp to the wheels, we found this STI a blast to drive on the street, as our new torque output and horsepower gains were a night and day difference.

Baseline (Stage 2 Tuned)299.0 347.0
AEM Cold Air Intake316.017.0371.024.0
ATP GTX3076R Turbo348.032.0355.0-16.0
Perrin Front mount Intercooler373.025.0386.031.0

DeatschWerks 1,000cc injectors$549.00
DeatschWerks 65C fuel pump$149.00
AEM Cold Air Intake$365.00
ATP GTX3076R Turbo$1,392.93
Optional polished compressor($65.00)
Perrin Performance 3-inch inlet hose$230.00
Perrin Front mount Intercooler$1,599.99

DeatschWerks, LLC
1745 W. Sheridan
Oklahoma City
OK  73106
Cobb Tuning
2311 W. Rundberg Ln Suite 500
TX  78758
AEM Power
P.O. Box 1312
CA  90250
Perrin Performance
3111 N.E Shute Rd
OR  97124
ATP Turbo
28988 Hopkins St.
CA  94545
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