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2006 Subaru Impreza WRX STI - Unpacking Potential

One Commuter Sti's Bolt-On Experience

Adam S. Geczi
Nov 1, 2007 SHARE
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When Subaru introduced the WRX Impreza it was deemed as one of the most performance-oriented four-door sedans with great aftermarket potential. The ensuing years saw the introduction of the WRX STi - Subaru's answer to the constant threat posed by the evolution of Mitsubishi's Lancer. As the market matures and opinions polarize, a clear champion has yet to emerge in this rivalry. Because of this competition, Subaru is constantly pushing the limits of both performance and regulatory compliance right off the assembly line. Car buffs, in turn, push these limits even higher. Indeed, the beauty of the STi is the engine's well-built bottom end. What's more, the bolt-on aftermarket options available for Subis make the STi an ideal daily driver and project car.

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These are the shared sentiments of Philip Petracca, the owner of this '06 Subaru WRX STi. Petracca was looking for a daily driver and had to choose between the Mitsubishi and the Subaru. Seeing that his friends and acquaintances all arrived at the same conclusion, he decided against the EVO. This choice destined him with the most marvelous platform, perfected by Dr. Ferdinand Porsche and later Subaru: the boxer engine. This impressive flat design permits any set of two pistons on opposing crank journals to reach TDC simultaneously, resulting in increased torque and that distinguishable, throaty, Porsche-like sound that only a boxer engine can make. Add turbo boost to this already potent arrangement and the force is even greater. Increase the size of the turbo and yielding 460 hp at the wheels isn't so unreasonable. So goes the basic formula used to make this street-friendly brute a "bit" more than the average daily driver.

Petracca says, the development could be broken down into two distinct stages: bolting on aftermarket components and begging Geoff Racier at Full-Race to engineer a manifold and up-pipe that would provide sufficient exhaust flow and support the bigger turbo without cracking under the added weight. The first 100 whp on top of the factory supplied 240 whp came at a relatively small price and little down time. Finding the rest of the power was more difficult and lasted roughly 10 months, several of them spent pleading.

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With the exception of the manifold and up-pipe, all of the modifications were done using bolt-on performance products from various manufacturers sourced by Petracca, installed by Straightlinetuning.com and tuned by Agile Automotive Performance of Fallston, MD. This made a fun project car because most of the amendments were made in increments, making it easier on the bank account and relationships. As a veteran of motorsports, Petracca found the use of bolt-on components refreshing following the challenges associated with his recent 800hp twin-turbocharged '03 Infinity G35 coupe development. Besides, the goal of this project was to build an exciting daily driver.

From its modest beginning of a simple turbo-back exhaust, this project mushroomed virtually overnight. While maintaining its driveability and reliability for daily operation with components from Samco and Snow Performance, the products from industry leaders like HKS, Forced Performance, Full-Race, APS Engineering and Cobb Tuning improved the weight-to-power ratio from 11.1 lb/hp to a stunning 7.5 lb/hp, as measured by factory supplied marketing information and dyno data.

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The initial installation of Cobb Tuning's direct fit replacement 3-inch diameter stainless steel exhaust system screamed for more attention and power. The early response included an HKS Twin Power DLI II unit, which delivered optimum spark duration and maximum voltage output using the capacity discharge and transistor ignition methods combined to provide crisper throttle response and a smoother engine powerband. This dual method addresses both the low-rpm need for long spark duration and the high-rpm need for strong rapid spark.

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Understanding that total control of the turbocharger is a must for all owners, the Subi was outfitted with a Turbosmart e-Boost2 controller. This state-of-the-art electronic boost controller gives several boost mapping options to the user. Boost levels can be mapped based on gear, time or engine speed. Equipped with an auxiliary output control, it can also manage water, methanol or nitrous injection against boost or rpm.

Next, the air intake and charge air systems were prepared for higher boost pressure and increased charge air temperature by the installation of products from APS Engineering of Australia. These included a 70mm internal bore diameter cold air intake, a turbo inlet hose and a front-mount intercooler. The APS smooth-bore cast-aluminum cold air intake and air filter is engineered to deliver large volumes of air that is cooler and more consistent in temperature by drawing it from between the inner guard and the fender. The APS turbocharger inlet pipe is a thin-wall, stainless steel tubing that eliminates the need for the traditional silicone hose that could collapse under suction at high-airflow rates. This product is an industry first that doesn't disregard the complex physical environment under the intake manifold. The factory top-mount intercooler was replaced with a front-mount intercooler and necessary ducting that can facilitate the application of large boost pressure, enough to make up to 525 hp. With 314 square inches of surface area and 1,020 cubic inches of cooling volume, the APS intercooler is capable of dropping the charged air temperature as much as 305F at 26.1 psi of boost while reducing the boost pressure across the core and ducting by just a mere 1.5 psi.

Furthermore, chemical intercooling was added with the installation of a Snow Performance Stage 2 water/methanol injection kit. Both water and methanol act as a coolant inside the combustion chamber that suppresses pre-ignition. Methanol has the added benefit of acting like high-octane fuel.

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Petracca's STi was now ready for an ECU re-flash, more fuel and more air. The ECU was reprogrammed using a Cobb AccessPORT programmer. The proprietary AccessPORT comes with preloaded maps, but can also be programmed by an authorized ProTuner center or StreetTuner software package. These custom maps can then be programmed into the vehicle's ECU. Pumping enough fuel from the tank to supply the Helix 860cc injectors is a 255lph Walbro fuel pump. However, exclusive of the high output Forced Performance Green Turbo, the bolt-on parts reached their limit of contribution at approximately 394 whp. However, no aftermarket manifold could support the weight of the turbocharger without cracking. The stock manifold, which the turbocharger was designed for, was the only part that could handle the load, but it couldn't produce the sought after horsepower. And so, the begging began. Petracca successfully persuaded Full-Race to develop and sell the only aftermarket manifold that could step up to the big turbo challenge.

In the end, the final trip to the dyno returned extraordinary results: 27 psi of boost by 4,100 rpm, 484 lb-ft of maximum torque and 460 whp on a completely stock long-block. Not too shabby for a daily driver.

Subaru Impreza Wrx Sti
Power Output: 460 Whp / 484 lb-ft @ 27 psi
Engine
Full-Race Turbo Manifold
Full-Race Up-Pipe
Cobb Turboback Exhaust
Forced Performance Green Turbo
HKS Twin Power DLI II
HKS Super Sequential Blow-Off Valve
APS Turbo Inlet Hose
APS 70MM Cold Air Intake
Beatrush Aluminum Cooling Panel
Beatrush Pulley Cover
Cusco WRX STI Radiator Cap 1.3 Bar
Samco Radiator Coolant Hose Kit
Samco Ancillary Hose Kit
Go Fast Bits Hybrid Blow Off Valve
Cobb AccessPORT
AgileAuto.com ProTuned
Cusco Engine Mount
Helix 860cc Injectors
Walbro 255lph Fuel Pump
APS DR525 Subaru Front-MountIntercooler
NGK One Step Colder Spark Plugs
Snow Performance Stage 2
Water/Meth Injection Kit
Snow Performance Safe-InjectionModule
Spec C 12L Intercooler Tank Used for Water/Meth Injection

Drivetrain
Exedy STI Twin Plate Clutch
Stainless Steel Braided Clutch Line
Motul Gear 300V 75W-90 100% Synthetic
Cobb Double Adjustable BilletShort Shifter
Cobb Six Speed Shifter Bushings
Cusco Transmission Mount

Suspension
RobiSpec.com Full Race Preparation
KW Clubsport Coilovers
Whiteline 26mm Adjustable
Front Sway Bar
Whiteline 27mm Adjustable Rear Sway Bar
Whiteline Rear Toe Lock Kit
Whiteline Front & Rear Endlinks
Whiteline Race Anti-Lift Kit
Whiteline Steering Rack Bushings
Whiteline Lateral Links
Cobb Tuning Billet Adjustable Rear
Sway Bar Mounts
Cusco Rear Strut Tower Bar
Cusco Rear Camber Plates
DMS Front Camber Plates
M-1 Tubular Race Under Body Brace

Brakes
StraightLineTuning.com Brake Lines
Motul RB600 Brake Fluid

Interior
Defi Gauges (Boost/EGT/Oil Pressure/Fuel Pressure)
Defi-Link Control Unit II
Turbosmart E-boost2 Boost Controller
Innovate XD-16 Wideband Air/FuelRatio Gauge
Zero/Sports Triple Meter Gauge Pod

Wheels/Tires
Enkei NT03+M (17x9.5 Front & Rear)
Bridgestone Potenza RE-01R (245/40/17 Front & Rear)
Rays Engineering Lug Nuts

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By Adam S. Geczi
17 Articles

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