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2006 Subaru GDB Impreza STI - JDM Impreza Savant

Street tough guy to circuit assassin

Steve Enomoto
Apr 11, 2013 SHARE
Impp 1302 01 o+2006 subaru GDB impreza STI+cusco front bumper Photo 1/12   |   IMPP-130200-STI-

This tale takes place in Tokyo before the economic bubble burst in the early '90s, when Japan was experiencing prosperity, especially the car tuning scene. It's deep past midnight at a well-lit parking lot adjacent to the expressway and street racers start to assemble, all seeking nocturnal stimulation. Tonight's stage is the Tokyo Metro "Shutoko" highway, best characterized as repeated bursts of high speed on a narrow and curvy highway with capricious obstacles galore. In this scene, the racer's weapon of choice is the front-engine, rear-wheel-drive (FR) platform, but a gradual shift in paradigm has seemingly begun taking effect. There is a new phenomenon, or rather a trending occurrence, where all-wheel-drive vehicles dominate everyone and everything effortlessly, even in the factory stock form. It has become socially evident that a powerful technology has finally caught up to this urban subculture, stressing the old-school racers with two ultimatums: either continue honing their driving skill in an attempt to rise above the performance of the AWD machines or simply succumb to the almighty new wave.

When the Subaru WRX and Mitsubishi Evolution AWD vehicles started to take presence in Shutoko, it wasn't exactly openly accepted. Instead, it was automatically rejected with contempt. Many street racers in Japan mocked the drivers for relying too much on fancy gadgetry to get them around turns, regardless if it was at ridiculously higher lateral g's like never seen or heard of before. The very people hating on the AWD machines were, of course, the guys with FR-configured vehicles who desperately clinched to keep their tails from washing out during turns, or the front-engine front-wheel-drive (FF) guys who constantly fought not to understeer into a guardrail. Although difficult to face the music, nobody dared to argue with the physics that all four wheels accelerating the vehicle had significantly more efficiency than just two. Nevertheless, the false stereotype of the AWD drivers being geeky and over-obsessively computer literate was deposited into the scene-but all that was soon to change.

Impp 1302 06 o+2006 subaru GDB impreza STI+cusco titanium strut tower bar Photo 2/12   |   2006 Subaru GDB Impreza STI - JDM Impreza Savant

As these vehicles started to gain ground and become increasingly utilized in professional motorsports, they shattered records left and right. The road to acceptance had been paved for the racers. Mockery in the form of anger triggered primarily by fear of change turned out to be a subconscious behavior of jealousy, and eventually led to acceptance and respect. The entire game changed but the psychology involving the human/machine interaction has become more sophisticated than ever. It wasn't just a simple affair of the person manually controlling the automobile anymore, but had developed into the fact that the person had to understand how the vehicle mechanics reacted to the computer inputs. There on, it was totally up to the driver to effectively use that principle to pilot the vehicle to new heights. So yes, the new electronic assistance made the vehicle fast no matter how bad the driver, but imagine if that same driver mastered the computer/vehicle/human relation, it, or rather they, would be unstoppable. The simple matter of fact is that the automobile world had just stepped up its game, and the Subaru Impreza was one of these pioneers in its own respective aspect. Ken Yamamoto's '06 Subaru GDB STI is one of these machines that ruled the underworld of highway racing before, and throughout its trial and tribulations, has climbed the ladder into professional racing and is still advancing.

One example of the technologically advanced features of the Subaru Impreza WRX STI is that it's equipped with numerous differentials, the critical one being the center unit that serves as the front- and rear-wheel torque split. As times progressed, these center differentials became progressively more controlled by a single computerized brain, ravenously processing signals from over 20 different sensor input feeds from various locations of the vehicle. Subsequently, this brain would automatically decide how the torque split should be applied; its objective is to optimally lay down the traction to the ground on virtually any type of road surface. So it really didn't matter whether the road was super slick black ice or really sticky hot tarmac, the machine certainly knew how to enter and exit a turn faultlessly. It is also well known that Subaru engineered this feature not just for performance, but with the consideration of upping the level of safety as well. Sticking with the beefy STI center diff, Ken also chose a front and rear Cusco limited-slip differential with a custom pressure ring set to adjust the sensitivity of the LSD for his ride. Other driveline components include an HKS twin-plate clutch and a Subaru JDM STI six-speed transmission, however we sense that a sequential gearbox may be in the works.

Impp 1302 04 o+2006 subaru GDB impreza STI+HKS racing blow off valve Photo 3/12   |   2006 Subaru GDB Impreza STI - JDM Impreza Savant

The manufacturer Subaru is the automobile division of Fuji Heavy Industries and, in current times, is most widely known for producing AWD platforms equipped with horizontally opposed boxer engines in its manufacturing plant located in the Gunma prefecture of Japan. And just like the phrase, "If you build it, they will come," from the movie Field of Dreams, flat-four boxer-loving shops flocked to this area and set up shop, catering exclusively to these kinds of vehicles. Car Station Marche is one of these shops nonetheless rated highly in the region specializing in all genres of vehicles dedicated to the AWD Subaru Imprezas and Mitsubishi Evolutions. Much like how a street car evolves into a full-blown race machine, one of their own Imprezas have made it to the big leagues.

The morphing of a street demon to a dedicated track car just doesn't happen over a period of a few weekends and a large lump sum of yen, but takes an immense amount of dedication and sacrifice to reach this kind of status. When asked about the primary goal for the vehicle, Ken replied that he had none. The vehicle will be ever changing, until the day the machine is retired or the owner is too frail to pilot the thing.

The spec intensity in terms of attention to detail of the machine makeup must definitely be praised much like a vivacious gospel choir on an Easter Sunday morning. The engine is an EJ207 base with HKS forged internals built by Masanori Kuwahara, a respected engine/suspension engineer. The headwork concerning this rotating assembly is literally too much detail to list from the conventional port and polish to the advanced funnel format valve seat construction using all specialized HKS components. The bottom end received HKS parts as well, with a special combination of piston, connecting rods, and crankshaft to an increased bore of 92.5mm and EJ25 STI Spec-C stroke length of 79 mm. The previous average and ordinary 1,994cc engine displacement was nurtured into a torque-enabled 2,126 cc. The HKS GTII turbine gobbles up the air well above atmospheric pressures, and the 670cc Bosch fuel injectors and an HKS Twin Power ignition system are left to deal with the guilt to burn the excessive calories. The HKS F-Con V Pro also interprets the air/fuel mixture and fine-tunes as per its course layout. There were even talks of implementing a twin-charged system in the future, which means that an HKS GTS8550 Supercharger might be added to the already torque-filled turbo engine. Much like the new Subaru BRZ, Ken knew that the vehicle's handling would greatly improve if the engine was lower to the ground, so custom mounts were fabricated to drop 15mm from the factory position.

The lower the center of gravity, the more precise the vehicle handling characteristics can be experienced. Why do you think the F1 cars have everything, including the engine and driver inches off the ground? Top-of-the-line 30-way adjustable Endless Super Function suspension coilovers fitted with custom X-Coil R spring rates were chosen for this particular vehicle, along with Cusco front and rear sway bars to minimize lateral roll. The rest of the suspension components may not sound fancy; however they are considered the most crucial and include a mix of Subaru OEM and STI variants used and dialed in to each circuit specifications by Kuwahara. OEM components can be the best choice at times over aftermarket products, since auto manufacturers have a larger budget for R&D as well as employ the best designers in the world to produce each component. The rest of the chassis is stiffened up with a Cusco titanium strut tower bar, Marche original bracing and 12-point cold steel rollcage.

Impp 1302 07 o+2006 subaru GDB impreza STI+spare tire Photo 7/12   |   2006 Subaru GDB Impreza STI - JDM Impreza Savant

Staggering Yokohama Advan RZ wheels in 17x9.5 at the front and 17x9 strapped in either Advan AD08 or A050G5 compounds, depending on course temperature and surface conditions. The front wide setup caters to the perfect balance between under and oversteer all managed with the three differentials working in sync with one another. Another front/rear ratio balanced equipment is the front Endless six-pot brake calipers and rear Brembo STI calipers all regulated by the combination of varying Endless race pads. The wheel assembly sits flush with the custom 8mm front and 5mm rear wide fenders. Moreover, a Cusco front GDB lip with the addition of custom composite panels, all topped off with Voltex side skirts and rear dry carbon wing accentuate the factory Subaru body exterior.

Clearly, the high aspirations along with the nightly experience gained ripping the streets served as a great investment for the transformation to the professional race status. Ken is only seen at such proving grounds as the Fuji International Speedway and Tsukuba Circuit with the big dogs now, taking on the top tuned GT-Rs and Porsches in the country. He has also gained respect and acquired great support from HKS personnel, which is considered a high honor for any enthusiast. The future of AWD vehicle technology is constantly one-upping the past but we have a feeling at this rate that Ken's Impreza can hold its ground for decades to come. The tale of the '90s Tokyo Shutoko racers is based off chronological events, but it's up to you if you believe it or not.

Impp 1302 08 o+2006 subaru GDB impreza STI+nardi classic steering wheel Photo 8/12   |   2006 Subaru GDB Impreza STI - JDM Impreza Savant

Behind the Build

Name
Ken Yamamoto

Impp 1302 05 o+2006 subaru GDB impreza STI+voltex side skirts Photo 9/12   |   IMPP-130200-STI-

Age
47

Location
Gunma, Japan

Occupation
CEO of Precision Machining Company

Hobbies
Subaru Imprezas only

Build time
Still counting

Feedback
Kuwauser@cs-marche.com

Motivation
Regaining youth

2006 Subaru GDB Impreza STI

480 whp / 466 lb-ft of torque (est.)

Engine JDM Subaru EJ207 engine; HKS 264 (in/ex) camshafts; Full-Race spec headwork; HKS timing belt, 2,126cc stroker, pistons, titanium-plated piston rings, H-Beam connecting rods, HKS crankshaft, Marche pulleys, custom 15mm engine mounts, HKS Racing Suction + Marche Airbox III, Walbro Fuel Pump 255-lph (2), Sard fuel pressure regulator, custom fuel collector tank, Bosch 670cc fuel injectors, HKS exhaust manifold, Sakumasu SPL HKS GTII turbine kit, HKS Racing blow-off valve, Marche up-pipe, HKS downpipe, HKS Type-S intercooler, HKS titanium exhaust, HKS M45i spark plugs, HKS twin power ignition, HKS F-Con V Pro engine management, custom modified oil pan, HKS oil filter, Xtreme engine oil

Impp 1302 02 o+2006 subaru GDB impreza STI+JDM EJ207 engine Photo 10/12   |   2006 Subaru GDB Impreza STI - JDM Impreza Savant

Drivetrain JDM STI six-speed transmission by Marche; Cusco limited-slip differential modified by Marche; HKS twin-plate clutch; Marche semi-sequential shifter kit

Suspension Endless Super Function Suspension and X-Coil R springs; Cusco sway bar (front and rear) and Titanium strut tower bar; race alignment by Masanori Kuwahara

Wheels/Tires Yokohama Advan RZ 17x9.5 (front) and 17x9 (rear); Yokohama Advan AD08 and A050 255/40-17 tires; Advan lug nuts

Brakes Endless 6-pot front calipers; JDM Brembo STI rear calipers; Endless front CC-RG or N84M brake pads, rear CC-RG brake pads, brake Lines, RF650 brake fluid; Marche custom brake duct

Exterior JDM Subaru STI body; Cusco front bumper; Marche custom front spoiler; custom Mitsu Shinoda 8mm front and 5mm rear wide fenders; Voltex side skirts and dry carbon GT wing

Impp 1302 11 o+2006 subaru GDB impreza STI+HKS titanium exhaust Photo 11/12   |   2006 Subaru GDB Impreza STI - JDM Impreza Savant

Interior Recaro SP-GII driver seat, TS-G passenger seat, and seat rails; Takata racing harness; Nardi classic steering wheel and hub; NISMO GT shift knob; Defi oil pressure/boost pressure/oil temperature gauges; Innovate air/fuel ratio gauge; HKS Circuit Attack Counter Lap Logger; Marche custom rear seat bracing; Marche 12-point steel rollcage

Gratitude "M. Kuwabara, the engine and suspension engineer; M. Sakamoto, the lap timer and mechanic; M. Sakumasu, the HKS turbocharger designer; T. Sakazume, the HKS director of electronics; Car Station Marche."

Impp 1302 12 o+2006 subaru GDB impreza STI+voltex GT wing Photo 12/12   |   IMPP-130200-STI-
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Sources

Yokohama Tire
800-722-9888
http://www.yokohamatire.com
Car Station Marche
http://www.cs-marche.com
By Steve Enomoto
15 Articles

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