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2008 Subaru Impreza WRX STI - Power Pages

The dyno doesn't lie

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Dynomite Dynamometer
Accessport Tuning: Jon Drenas


The '08 model Subaru Impreza STI, powered by a 2.5L boxer engine is rated at a factory 305 hp and 290 lb-ft of torque. The Impreza is an extremely capable and fun-to-drive machine thanks in part to its symmetrical all-wheel-drive system and turbocharged powerplant.

The '07-08 Subarus (and some '06 WRXs) were tuned from the factory to pass strict emissions testing often sacrificing the true potential of the EJ25 engine. They run stoichiometric air/fuel ratios (14.7:1) through peak boost to about 4,000 rpm, which is great for fuel economy but considered bad for power and driveability. The '07-08 ECUs also have multiple timing tables that upon certain situations add, subtract, or even multiply the timing values the motor runs. There are tables within the factory ECU that multiply and add to the total amount of timing (bad). There are also timing floors where the total timing value cannot go below a certain amount, which are also considered bad. What does this all mean to a Subaru owner looking to bolt on a few go-fast parts? If the vehicle is not properly tuned, the ECU can take the tuner's intended timing value (let's say) 11 degrees at peak torque and (using knock feedback and other algorithms) add an additional 8-10 degrees, effectively running 19-20 degrees of timing. The problem is the ECU is not quite fast enough to pull out that timing. If it knocks hard, it will cause damage to the engine.

Simply bolting parts on the '07-08 STI and neglecting to run the proper ECU tune can spell disaster for your engine. Subaru tuners recommend a reflash or stand-alone fuel management system for the ECU, even for a stock Subaru, without any prior modifications. Using a Cobb Tuning AccessPort, we optimized the EJ25 engine's air/fuel ratio and ignition timing, and rewarded from a stock run of 230 hp and 249 lb-ft of torque to the tune of 259 hp and 279 lb-ft of torque, a gain 29 hp and 30 lb-ft of torque.

AEM Cold-Air Intake (Tuned)

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

10, 12, and 14mm socket; ratchet; extension; screwdriver; needle-nose pliers; and crescent wrench

Installation time
40 minutes

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.

Mating the inlet pipe to the tight silicone adapter hose can become a frustrating process during your intake installation. Remove the factory snorkel inlet running across the top of your original intake box and use a screwdriver to gently coax the hose onto the inlet pipe. Be careful not to damage the filter cone in the process.

The AEM intake with tuning didn't disappoint as it picked up both horsepower and torque from its initial pull till redline. The dyno graph revealed a gain of 20 lb-ft of torque at 3,200 rpm. Promising numbers were recorded at 3,750 rpm as a pickup of 15 hp and 20 lb-ft of torque were recorded over our previous tuned baseline run. A solid gain of 15-20 hp and 10-17 lb-ft of torque were obtained from 4,350 rpm to 6,600 rpm.

Tanabe Medallion Touring Exhaust (tuned)

Exhaust, midpipe, bolts, stickers, instructions, and mounting hardware

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

Installation time
90 minutes

The Tanabe Medallion Touring exhaust was developed as a free-flowing exhaust system with 70-60mm SUS304 stainless steel piping uses 1.2mm thin wall piping, as well as hollow exhaust hangers for weight reduction. The mufflers are polished to a mirror finish highlighting the robotic TIG welds throughout, and the larger-than-OE quad tip design fills the large exhaust openings in the bumper. Constructed with Advantex material used within the internal exhaust packaging, increases exhaust longevity and reduces DB decibels, while preventing rust and heat causing deterioration often found with competitors using standard stainless steel wool and composite cube material. Along with its aggressive appearance, the exhaust has been designed to emit a legal 92dBA rating, which ensures cops wont harass you on the streets.

As with any exhaust swap, we recommend asking for assistance for both removal and installation of the exhaust to help speed up the process.

Adding the Tanabe cat-back exhaust system allowed the factory VF48 turbo to breathe more easily as the dyno charted more boost in the midrange as well as allowed us to add more timing and turn up the boost ever so slightly from its factory 14.8 setting to 18 psi using the AccessPort. A quick reference to the boost log showed at 18 psi the boost level was dropping off between 15 to 16 psi. With additional tuning, the EJ25 powerplant came alive from 2,350 to 5,350 rpm. A significant gain of 23 hp and 43 lb-ft of torque were recorded from 2,700 to 3,100 rpm. At 4,000 rpm we saw an increase of 15 hp and 22 lb-ft of torque over our previous run.

Cobb Tuning Catted Downpipe (tuned)


10, 12, 14, and 17mm sockets; 10, 12, 14, 17, and 22mm open ends; ratchet; extension; screwdriver; and WD-40

Installation time
90 minutes

Using a cast bell-mouth housing, the 3-inch downpipe terminates into a metallic core (200 cell count) high-flow catalyst with improved efficiency. We could have easily opted for a non-catted downpipe in search of additional horsepower but the thought of killing the ozone layer was too heart wrenching for us to bear. Living under the strict emission laws of Cali, we kept to the idea of maintaining at least one catalyst to ward off the toxic fumes the Subaru generates when sitting in traffic or at a stoplight.

Removal of the factory downpipe can become a time-consuming affair. Take your time to properly install the turbo-back exhaust system to prevent horsepower-robbing exhaust leaks or dreaded stripped bolts. When removing the downpipe, be careful not to sever or loosen the wastegate lines that regulate your factory boost. The last thing you want on your hand is an overboosting EJ motor that punches a rod out the side of your block.

With the newly installed Cobb downpipe in place, we made use of the AccessPort and remapped the ECU with a more aggressive AVCS and timing map, while turning up the boost to 20 psi. The high-flowing Cobb downpipe allowed us to tune more aggressively whereas using the factory downpipe would limit any tuner's ability to properly extract horsepower. The Cobb downpipe along with tuning offered a significant horsepower and torque increase over our exhaust run. At lower rpm the dyno recorded a gain of 60 lb-ft of torque from 2,500 to 2,850 rpm. A consistent gain of 25 hp was documented from 2,500 to 3,600 rpm while the largest margin of gain was 27 hp and 45 lb-ft of torque at 3,250 rpm.


What began as a baseline dyno run of 259 hp and 279 lb-ft of torque was quickly revamped with our final run of 288 hp and 354 lb-ft of torque, a gain of 29 hp and 75 lb-ft of torque. These were impressive power gains that were made possible with the proper selection of parts and tuning. We can't stress enough how important tuning is when looking to create horsepower as our STI is living proof that Subaru's have definite power potential.

Conclusion HP +HP TQ +TQ
Baseline (AccessPort Tuned) 259 279
AEM cold-air intake 271 12 296 17
Tanabe Medallion Touring exhaust 276 5 315 19
Cobb Tuning SS 3-inch downpipe 288 12 354 39
Final 288 29 354 75

Cobb Tuning AccessPort $595
AEM Cold-Air Intake $300
Tanabe Medallion Touring Exhaust $1,340
Cobb Tuning SS 3-inch Catted Downpipe $595
TOTAL $2,830

Cobb Tuning
2311 W. Rundberg Ln Suite 500
TX  78758
Tanabe USA
1849 Western Way
CA  90501
AEM Induction
PO Box 1312
CA  90250
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