Subaru STI Performance

The North American specification Subaru STI is truly unique - unlike any other STI on the planet. In order to develop equipment that improves the performance of the US specification STI that is of the highest standard of engineering, APS has fully imported a US specification STI and complete engine for engineering and development purposes.

A comprehensive range of APS performance components are now available to satisfy the power hungry STI enthusiast.

The above chart shows flywheel power and torque data for the stock US Specification STI running on the equivalent of US 94 (R+M)/2 octane fuel.

The 2004 model US Specification STI is a unique model in many ways. Just some of those already identified are as follows:

APS is by no means suggesting these are the only differences. Quite possibly more will come to light in due course. In the mean time, the following current production APS components have been proven to be applicable in enhancing the engine performance of the US Specification STI.

STI Flywheel Power Graphs

Legend

1. Stock

2. Add 3" APS Turbo Back Exhaust System

3. Add Engine Management

4. Add APS Cold Air Intake System

5. Add APS DR 500 Front Mount Intercooler System

1. The stock curve is reasonably representative of Subaru's claims when run on 93 R+M/2 Octane fuel.

2. The second curve illustrates the gain (20 hp max) to be obtained by the installation of the full APS turbo Back exhaust system. The stock cat back components are quite efficient at stock power levels, hence opting for an aftermarket cat back system is of little benefit in terms of power increase.

3. The third curve illustrates the benefit achieved through installation of the re-calibrated APS plug-in engine management unit. As can be seen from the boost curve, there has been significant remapping in this area. However, the most significant feature of the boost curve is the way it returns to stock at redline. This is simply the result of insufficient air mass flow through the stock air element assembly/turbocharger compressor.

4. The fourth curve illustrates the effect of installing the APS Cold Air Intake System. Although boost has not altered, peak power rises and more importantly is held out to redline. This is a result of significantly reducing the restriction created by the stock air cleaner assembly. This permits later gear changes and results in superior acceleration by keeping the engine more within its maximum power band.

5. The fifth curve illustrates the result of installing the APS DR500 Front Mount Intercooler System. This has effectively eliminated any system pressure drop between the air cleaner element and the throttle body. In other words, from the turbocharger compressor's point of view, this is as good as it gets! As can be seen from the curve, the engine is producing 370 hp (flywheel), however the peak point has dropped to 5,000 RPM - A sure fire indicator of insufficient compressor mass flow. There is only one way to go from this point - and that is to install a larger turbocharger.

Development Update

Outstanding results have been obtained using straight forward bolt-on equipment to significantly improve the power and torque of the US Specification STI.

The vehicle improvements were:

Fuel	94 R+M/2 Octane
Turbocharger	APS SR50 Series Ball Bearing Turbo (note SR50 superseded by SR55 turbocharger)
Intercooler	APS D/R 500 Front Mount Intercooler System
Exhaust	3.5" Race Exhaust System
Fuel System	APS Improved
Engine Management	APS Plug-In
Blow Off Valve	FMIC High Volume Dual Vent Blow Off Valve

The above components resulted in an outstanding 413 horsepower and 124 horsepower improvement over stock at 6700 RPM. In addition, the SR50 produced more boost pressure at lower RPM than the stock unit. This translates to more torque, earlier in the RPM range.

Unlike the stock engine, where power falls away after 5800 RPM, the SR50 equipped STI just keeps making ballistic horsepower at high RPM.