Body TuningEnthusiasts often forget about the effects of vehicle aerodynamics. After all, it’s a lot easier to find a dyno to measure power improvements versus finding a wind tunnel to measure changes to a vehicle’s coefficient of drag or coefficient of lift. Even so, the reality is that well-engineering aftermarket body components may allow for increased performance in two ways. First, wider fenders may accommodate a wider wheel-and-tire combination. The wider tire translates into a larger contact patch which improves nearly every measurable performance aspect of the vehicle. Traction and handling are vastly improved with a larger contact patch at each corner. Second, air dams, wind splitters, undertrays, canards and wings can be used to eliminate front-end lift while providing downforce at both the front and rear of the vehicle. For this demo car, APR Performance selected its SS/GT aerodynamic widebody kit to serve as the foundation. This kit widens the body by 80mm, allowing the use of 18×10 or 18×11 wheels with tires measuring up to 295mm in section width. This kit includes the widebody front bumper with brake/oil cooler vents, carbon-fiber wind splitter with front undertray, carbon-fiber canards, widebody front fenders (40mm) with wheel turbulence ports, widebody side skirt, widebody rear door, gas door and fender combination (40mm); and widebody drag-reduction rear bumper. Since APR tracks this STi, it has also been equipped with an APR Performance GTC-300 WRX-Spec carbon wing for additional downforce. Formula GT3 mirrors and a SEIBON CWII-style carbon-fiber hood finish off the body enhancements. After all the pieces were in place, TPR Auto Body shot the body in Code Red BASF paint.
Power TuningA 500-plus horsepower stroker engine was definitely an option for this STi. However, APR wanted a power solution that would relate to most Subaru enthusiasts. For this project, the decision was made to leave the longblock untouched, while maximizing the efficiencies of the fuel, turbocharger, intercooler and exhaust systems. Fuel demands are addressed with a Walbro in-tank 255 liter-per-hour pump. This pump supplies a quartet of 850cc/minute Denso injectors that take instruction from an HKS F-CON V Pro engine management system. The HKS V Pro allows the HKS Pro Dealer (in this case, SP Engineering) to properly adjust fuel delivery and ignition timing for maximum reliable performance and fuel economy. Setting up a car to run laps requires careful selection of the turbocharger. Throttle response and mid-range power have to be weighed against top-end power production. In the case of the EJ25, there are also the power handling capabilities of the factory pistons that must be considered. Making anything over 400 wheel-horsepower is best left to an EJ25 engine equipped with aftermarket forged pistons and upgraded connecting rods. It’s also a great idea to upgrade the cylinder heads and camshafts when an EJ25 build is undertaken. After weighing all these factors, a decision was made to use a Garrett 3071R turbocharger. This GT ball-bearing turbocharger packs big power in a small package. The turbo relies on a T25 footprint for the turbine housing that is home to a GT30 turbine wheel. On the compressor side, a 71mm outside diameter compressor wheel transitions to a 53.1mm inducer. According to Garrett, the ultimate horsepower potential of the turbocharger is about 460 flywheel horsepower or about 390 horsepower at the wheels. Where the GT3071R is really meant to shine is in the response department. A fast-building boost curve allows this EJ257 engine to generate its peak torque at just 4,900rpm (354 lb-ft) while its peak power (346 wheel horsepower) checks in at 5,500rpm. This performance is realized on 100-octane unleaded gas with the boost pressure set in the 1.3kg/cm2 range. This boost pressure is held steady thanks to the combination of the HKS wastegate and EVC combination.
The Garrett GT3071R features a GT30 frame with a 71mm outer diameter and 53.1mm inducer, as well as a 60mm turbine wheel.While the factory top-mount intercooler is adequate for short power bursts at stock power levels, it’s not up to the task of chilling the airflow of a track-spec STi generating over 400 horsepower at the flywheel. For this task, an HKS front-mount intercooler provides superior cooling as it’s not only larger than the factory intercooler, it’s also located in the proper place (the front air stream of the vehicle). Since the front-mount location of the intercooler slightly disrupts airflow to the radiator, the factory heat exchanger was trashed in favor of a high-performance radiator from PWR. To cool the engine oil, a GReddy oil cooler is on call.