When we arrived at the state-of-the-art Tomei headquarters, south of Tokyo, the EJ25 was already strapped down in the dyno room. A series of run-in and warm-up procedures were done before going for a full boost, full power run. With over 450 ps and 60 kg being developed, this engine has received some serious attention from the guys at Tomei. Having had much experience in the past by building the motor in the Prova-Tomei Impreza demo car, they knew exactly what needed to be done to take the EJ one step further. The main goal was to not only create a more powerful package but to fine-tune its response and midrange characteristics that are so often ignored in track-going engines. It all started with the base EJ25 engine, which was stripped down and prepared for the build. For this project a special set of internals were built, starting with the fully counterweighted and balanced competition crank, the H-beam connecting rods, and the special forged 99.75mm pistons. Once this was done, the little boxer heads were taken care of with some top-of-the-line Tomei parts. To fit the special beryllium valveguides and valve seats, the heads were heated in an oven for some time so that the alloy would slightly expand. Once this was done, the guides and seats were dipped in liquid nitrogen so the metal would contract, allowing for an easy fit into the heads. Once back to room temperature they would embed themselves perfectly into the head. The stock valves were used since they were well up to the job, even on a highly modified engine, while some porting was done to guarantee the best flow. Specially milled Tomei Poncams were thrown in at 250 degrees and 9.6mm lift on the intake, and a slightly more aggressive 256 degrees and 9.8mm lift for the exhaust side. These were then mated to Tomei adjustable cam pulleys along with a Tomei timing belt and guide. The 0.7mm think metal head gaskets were used to seal the heads to the horizontally opposed block giving an 8.5:1 compression ratio, ready to take the 1.6 bar of boost. This force-feeding was taken care of by the Garrett GT3076R mounted on custom 4-into-2-into-1 equal length header.
Boost control is looked after by the HKS external wastegate while gases are dumped into the Tomei custom front pipe and then onto the one-off titanium system, built by Hatanaka-san at Tomei. Tomita-san took care of fabricating the custom aluminum piping kit for both the intercooler piping as well as the short intake pipe where the K&N cone filter is fitted. A Calsonic front-mount radiator has the job of cooling the intake charge before sending it onto the stock intake plenum. Fuelling is taken care of by a Bosch motorsport fuel pump that sucks gas from the 20-liter racing tank in the trunk and sends it on to the twin Tomei billet rails via braided lines. These feed the four 850cc/min injectors while the factory FPRs do a good job of keeping pressure under check. Fuel and ignition maps are all handled by the MoTeC M600 ECU, which was custom wired into the Impreza. A massive oil cooler with its own custom air guide, along with the racing radiator, keep the engine within operating temperature, even when pushing hard out on track. The monster midrange torque of the EJ25 is sent through the Cusco twin plate metal clutch to the Cusco Dog engagement gearbox. This five-speed transmission has been used extensively in rally competition and is perfectly suited to a time attack car like this Team Tarzan Impreza. Easily fitted inside the factory Subaru gear housing, the Dog gearbox has more evenly spaced ratios to help get the most out of the increased power while cutting down in shift times dramatically. When we saw the car being tested at Fuji Speedway, Yamada-san was savagely banging the gears in, making it sound just as fast as a sequential box. Cusco went all-out on the driveline with a one-way limited-slip differential up front and a one-and-a-half-way limited-slip differential at the rear, while the center differential is fitted with a special Cusco Active Control DCCD unit so Yamada-san can adjust the front/rear torque split according to varying tracks and conditions. A carbon-fiber (CFRP) propeller shaft helps keep rotational masses to a minimum, furthering the response. With such an impressive mechanical base it was then onto Voltex in Suzuka who took care of the exterior of the car.