Optimum gas flow and minimal exhaust back pressure is essential for high power. The APS high output stainless steel exhaust system really performs.

Careful attention to tube diameter, smooth radius mandrel bends and muffler design is essential for high performance. With 20 years of experience in exhaust system design, we have built a superb exhaust system.   

Exhaust system design parameters immediately after the turbocharger are critical not only for outright performance, but for optimum torque and turbo response. The optimum internal profiles cannot be achieved with twin pipe or bell mouth pipes. APS has engineered the High Flow Turbo Outlet housing for extreme gas flow and high exhaust gas temperatures which are normal in high performance turbocharged engines. Cast in high temperature ductile iron ensures optimum internal shape and maximum durability under high rpm, load and boost conditions. 

The end result is an exhaust system that produces high outright power with stronger mid range torque than fabricated turbine outlet style exhausts. All this with improved turbo response (less lag) when running large turbochargers and maximum durability.

"How much horsepower will that exhaust system add?"

This is an often asked question, to which there is no real answer. An exhaust system is only a point of possible restriction, which needs to be reduced as required.

For example, in a best case scenario, total removal of the complete turbo back exhaust system from a stock MY01 225 hp WRX will result in a power increase of approximately 20 hp. On the other hand, on a 425 flywheel hp WRX, utilization of the correct exhaust system would literally represent 65 of the 200 hp increase over standard.

A good starting point for the correct choice of an exhaust system, the following should be used as a useful giude:

1. Street Use - Legal Emissions/Noise - Less than 4.0 psi back pressure
2. Street Use - No Legaility Issues - Less than 2.5 psi back pressure
3. Race Use - As low as possible, however no more than 1.0 psi back pressure

The following chart provides exhaust back pressure data for a given engine power level for the stock WRX exhaust system as well as the range of APS High Output exhaust systems. In addition, the chart provides exhaust noise test results as per the SAE J1169 noise test.

Selection of the correct WRX or STI exhaust systems is the appropriate combination of back pressure, catalytic convertor and noise to suit the engine's power output. The bottom line is, if this data cannot be provided by an exhaust manufacturer, look elsewhere.

For example, California and various other states in the US mandate J1169 SAE stationary noise tests. In Australia the test requirements are ADR 28/01 stationary noise test and it too is based upon SAE J1169. In fact, the above tests have identical test procedures and only the required limit varies:

The test involves positioning a type 1, SIA compliant ANSI S1.4 - 1983 noise level meter, specifically as illustrated in the diagram below. No other positioning is relevant! Noise tests are extremely specific in their scope and must be conducted very carefully!

Engine RPM must be held steady at 75% of rated peak horsepower RPM.

Given the latter figure is 6,000 RPM for the US spec. WRX/STI, the test speed is 4,500 RPM for these vehicles.

International specification vehicles (including Australian) vary as follows:

Again, it must be emphasised, all facets of a noise test must be specifically followed for the test to have any meaning.