Do you ever wonder why your car runs better on a cold winter night versus a hot summer day? Well, the cooler the ambient air the more oxygen it has. And since you need oxygen and fuel to make horsepower, the more oxygen you have the more horsepower you can make. When a turbocharger compresses the ambient air, the heat generated during the process can be raise the temperature as high as 450+ F. That is why it is very important to utilize an efficient intercooler.
The key to a good intercooler is its ability to remove the heat from the compressed air without causing too much restriction (pressure drop). The key to APS' D/R Series intercooler is incorporating the largest possible core while maintaining the structural integrity of the vehicle. The APS D/R Series FMIC is massive, measuring 9.5x28x4.5 inches with a surface area of 266 square inches. By utilizing a large intercooler core with a large surface area the charge air can be efficiently cooled without causing a large loss of pressure.
When size matters, the APS unit is the largest intercooler available on the market. The system is 4.5 times larger than the factory top-mount intercooler. An efficient intercooler like the APS system can reduce charge air temperatures from 475 degrees to a more engine friendly 120 degrees. Not only will the cooler charge air decrease the chances of detonation but will also increase horsepower dramatically.
The last component that was installed on our test vehicle was the APS' SR series ball-bearing turbocharger. Traditional turbocharger designs incorporate a conventional plain bearing that runs on a film of oil. Although the design has worked well over the years, the drawback of this type of design is the high frictional forces, a culprit of slow spool-up time.
APS chose to utilize a twin ball-bearing turbocharger to promote faster spool-up. By reducing friction, due to the ball-bearing design, faster spool-up time is realized without sacrificing durability of the turbo. Ball-bearing turbos have been used in racing for decades but it hasn't been until recently that they have been made more affordable. The APS turbo is also water cooled to help cool the turbine and bearing assembly, in turn increasing the longevity of the turbo.
Compared to a non water-cooled turbocharger, cooking of the protective layer of oil can occur and possibly damaging vital turbo components. The water cooling helps reduce the chances of cooking due to the lower bearing and turbine temperatures after shutdown. APS offers three different sizes of turbochargers-SR30, SR40 and SR55. Going with the bigger is better adage, we chose the SR55 for our STi tester.
In bone stock trim, a WRX STi generates roughly 250 horsepower and 245 lb-ft of torque on a DTS chassis dyno. On our STi tester, we installed, in addition to the turbo, APS' air intake, 3.5-inch race exhaust, front-mount intercooler, larger injectors and an Ecutek ECU. With these additions we got 351.8 horsepower and 320.8 lb-ft of torque, an increase of 100 horsepower and 75 lb-ft of torque. The tuning combination was all performed on 91-octane pump gas. We would not be surprised to get another 50-plus horsepower on 100-octane. Now if that doesn't stimulate your senses you must be an extreme sports junkie.