Finally, a range of high performance direct replacement turbochargers that have been specifically designed for the Subaru WRX/STI by the Subaru performance specialists - APS.
APS has designed and produced what is arguably the ultimate in bullet proof turbochargers for the Subaru Impreza enthusiast.
Unencumbered by the limitations of current off the shelf WRX turbochargers, APS has totally engineered a "no-compromise" series of turbochargers from the ground up to match the exact requirements of high performance Subaru Impreza engines.
High strength compressor wheel design, ball bearing construction and high nickel content turbine housing means complete reliability to pressure ratios over 3:1 (30 psi!). And since nobody likes to wait, the APS S/R Series turbochargers are designed for fast spool-up to provide crisp turbocharger response.
Best of all, the S/R range of turbochargers are complete bolt on. Simple hook up of all factory connections and full compatibility with high performance inlet/exhaust means that the installation of your APS S/R Series turbocharger is truly no hassle.
If you want ballistic power and outstanding turbocharger reliability in a truly bolt on high performance unit, then the APS S/R Series turbocharger the perfect choice.
Available in a range of 3 turbocharger power output sizes to over 550 hp (410 kW), APS provides an S/R Series turbocharger that is optimum for engines from mild 2.0L through to wild 2.5L. The APS S/R Series Turbochargers are complete with all ancillary equipment for a truly bolt-on performance upgrade to the Subaru WRX/STI.
Why Twin Ball Bearing?
Demands for improving acceleration response and for the reduction of so-called turbo lag are popular amongst performance enthusiasts who wish to take advantage of the enormous gains in power and torque delivered by turbochargers. In addition, bullet-proof reliability is required particularly at high turbocharger boost pressure levels as well as at extreme exhaust gas temperatures commonly found in high performance turbocharged engines.
In order to achieve crisp turbocharger response, a number of advances in turbocharger design have been utilized over the past decade. Primarily through the use of modern metals/ceramics in order to reduce the mass of the rotating assembly. However, significant gains have been made by reducing the friction of the rotating assembly - and this has meant a departure from traditional turbocharger designs.
Traditional turbocharger design employs a conventional plain bearing that runs on a film of oil. This is known as a floating metal bush.
The diagram above shows the turbocharger main shaft supported by floating metal bushes. Oil is fed through the bushes and forms a cushioning layer between the turbocharger shaft and the supporting bush. The shaft relies on a constant supply of fresh, clean oil over a very wide contact area in order to maintain sufficient clearance from the bush itself. A similar approach is used to support the turbocharger main shaft from thrust loads as well.
Whilst floating metal designs have served us well in the past, the frictional forces are relatively high. This results in sluggish turbocharger response and can be somewhat fragile in nature under extreme operating conditions.
Nissan attacked this very issue some 15 years ago on the GTR Skyline by developing a turbocharger bearing system that forms the basis of the true high performance modern turbocharger.
By utilizing robust ball bearings at either side of the turbocharger main shaft, this did away with the floating metal and thrust bushes.
As seen in the diagram above, the turbocharger shaft is supported by two ball bearing assemblies. These again are fed with engine oil, but no longer rely on a thin film of oil over a wide area to support the turbocharger shaft.
The result is an outstanding reduction of frictional torque on the rotating turbocharger assembly in contrast to the old fashioned floating metal bushes. The improvement in turbocharger response, particularly in the lower to mid turbocharger speed range is phenomenal.
The graph above shows frictional torque versus turbocharger speed of both old fashioned designs and modern ball bearing turbochargers. Clearly evident are the improvements with ball bearing turbochargers - especially at the low speed range of under 60,000 RPM where friction losses are reduced by 40% to 50%. This translates directly into a quantum leap in turbocharger response.
To show this in another way, the above chart demonstrates the mechanical efficiency advantage of the dual ball bearing design compared to journal (floating metal) and hybrid ball bearing turbochargers. The improved mechanical efficiency of the dual ball bearing design results in exceptionally crisp and strong throttle response over that of lesser turbocharger designs. This produces an improved response that can be converted to quicker 0-60 mph times.