Subaru 2004 Model STI Performance 


"On the road, it all means the APS STI is a more relaxing drive because you’re not constantly hunting back through the six-speed for a ratio that’s actually gonna spool up some herbs. The thing will actually accelerate in sixth now, which is really saying something."




News Flash! - SR55 goes 11.5 sec @ over 122 mph!


MZM's Full Weight Subaru WRX with '03 JDM STI Version 8 Type C Transplant peeled off consistent mid 11's in San Antonio in an internally stock 2.0L fitted with the following components:

APS SR55 Turbocharger APS Front Mount Intercooler
APS 3.5" Race Exhaust APS Dual Vent BOV
APS High Flow Cold Air Intake 101 Octane Unleaded Fuel
No other engine modifications such as NOS etc.

Click here for video action of MZM's previous 11.6 sec @ 121 mph run.

We are passionate about high performance for the All Wheel Drive Subaru. Drawing on 20 years of turbocharging and intercooler system design and production knowledge, our mission is to produce the very best high performance systems and components to significantly enhance 2004 STI engine power.

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:

  1. Side feed 480 cc injectors (350 flywheel horsepower at 11.0:1 A/F ration, 100% duty cycle). This is in stark contrast to the International Specification vehicle with its 565 cc top feed units.
  2. Although it does not appear at first sight to be so, the turbocharger's physical configuration is unique. APS is currently investigating upgrading the stock component with the SR Series range of ball bearing turbochargers.
  3. Most of the VF Series turbochargers are airflow limited to approximately 350 flywheel horsepower. In an optimum configuration, the VF 39 fitted to the US Spec STI will support approximately 370 horsepower.
  4. The stock ECU's operating software and loom hardware are unlike any other late model WRX.
  5. As is quite well known, the throttle butterfly is drive by wire.
  6. STI pistons are typically forged, however the US specification pistons are cast. Click here for pictorial comparison.

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.

Individual Components

AIR INTAKE Systems & Components

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Cold Air Intake System - 65mm
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Cold Air Intake System - 70mm I.D.

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Turbocharger Inlet Pipe Pod Air Intake System
INTERCOOLER Systems & Components
APS DR 525 Front Mount Intercooler System APS DR 725 Front Mount Intercooler System
SR 375 Dual Entry Top Mount Intercooler System - for Drive-by-wire models
APS Windscreen Washer Canister Kit
EXHAUST Systems & Components
APS High Energy Stainless Steel Turbine Inlet (Up) Pipe (for use with APS Equal Length Split Pulse Headers)
TURBOCHARGER Systems & Components
Ball Bearing Turbochargers

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APS Twin Scroll Rotated Mount Ball Bearing - 700 horsepower
Coming Soon
APS Twin Scroll Rotated Mount Ball Bearing - 560 horsepower
aps turbo
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P/N 990-060-650
APS SR56 Series Ball Bearing Turbo
SS Series Sleeve Bearing Turbochargers

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P/N 990-060-475
APS SS45 Series Sleeve Bearing Turbo
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P/N 990-060-460
APS SS42 Series Sleeve Bearing Turbo
TMIC High Volume Dual Vent Blow Off Valve FMIC High Volume Dual Vent Blow Off Valve
Tumbler Valve Delete Manifold Housing for Side Feed Injectors APS High Output Fuel System - Side Feed Injectors

STI Flywheel Power Graphs


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.

Track Update

1/4 mile ET: 11.97 sec @ 118 mph

Development Update

It's early days yet but the bar has been raised yet again. The above graph shows flywheel power and torque using the same equipment as listed above, though with the APS DR55 Rotated Mount ball bearing turbocharger.

Stay tuned for further updates!

Please check back soon for updated information