While our American readers are new to the flat-four turbo engine of the MY01 WRX, it's a familiar friend to many Australians into high-performance. Just bolt on a big exhaust, slap on a pod filter, wind the boost up and let 'er rip, mate.
Well, that's how easy it used to be to make a Rex haul arse...
To find out how the current MY01 model responds to modification, we oversaw a test session at Adelaide's Turbo Tune workshop. They had a 10,000 kilometre MY01 WRX in perfect health and were about to install a complete APS (Air Power Systems) '210kW' kit. This AUS$4620 (plus freight) package comprises a new high-performance exhaust, air filter, top-mount intercooler (with water spray) and a Unichip signal interceptor.
Rather than simply install the entire kit and look at the net power gain, we decided to test the effects of each component incrementally. In this - the first of our two part series - we'll begin by looking at the advantages of the exhaust and air filter.
The first vital step of the testing was to attain a repeatable base power figure. American readers should note that all of the accompanying power graphs are derived from a Dyno Dynamics four-wheel-drive chassis dynamometer - not a DynoJet unit. To obtain accurate and comparable figures, each of our graphs was taken after the readings had settled; they were not one-off bests. The same tank of 96-octane premium unleaded was used throughout the testing and dyno room temperature ranged from about 14 to 17 degrees C. The bonnet was dropped down and third gear was used for each run.
As you can see, Subaru's quoted 160kW flywheel figure plummets to 103kW at all four wheels on Turbo Tune's Dyno Dynamics chassis dyno. This power figure is pretty much line-ball with other MY01 WRXs tested on this same dyno - this car was in fine health. In addition to power, you can see that the air-fuel ratio is plotted on the lower part of the graph. Note that this standard car runs very rich (at least 10.0:1 AFR) at full load and mid-high rpm. Interestingly, turbo boost pressure peaked at 14 psi in the mid-range and fell to around 11.5 psi in the top-end.
Exhaust
Despite the fact that a high-performance exhaust is only part of APS's overall kit, we decided to see what power gains came from fitting it alone. Indeed, one of the first - or only - modifications many people will make to their MY01 WRX is a high-flow exhaust system.
But, firstly, let's take a look at what's being thrown away...
Up front - straight off the back of the turbocharger - the factory exhaust carries over the same dump pipe and primary cat converter assembly seen on previous Australian-delivered Rexes.
And, just like previous models, the dump pipe doesn't look like it would flow too well at all. With a flat metal plate butting up near the wastegate outlet, all of the exhaust gasses are forced to mix and jostle to escape down the single 57mm ID pipe located adjacent to the turbine. Note that, unlike previous WRX models, the MY01 does not have its 02 sensor located in the first bend of the system.
Moving along, the same 57mm ID pipe diameter continues out the back of the primary cat converter. It stays that diameter all the way back to the twin tail pipes. Throughout the standard system, it's fair to say that Subaru has done a pretty good job at maintaining pipe diameter. Those bends that we could see without removing the attached heat shields looked to maintain a reasonably near-constant internal diameter. Not far behind the primary cat converter is Subaru's secondary cat converter. The factory 02 sensor screws directly into the top half of the converter's body.
A little bit further downstream is what appears to be a gas expansion chamber. We assume that it's not another cat converter because of the unusual way it's mounted off one side of the pipe.
Finally - after the pipe passes over the left rear driveshaft - the system terminates in a large muffler. This is equipped with twin 48mm ID outlets.
In summary, the standard MY01 WRX exhaust system does leave substantial scope for improved flow - but it's nowhere near as restrictive as, say, that fitted to a Nissan S15 Silvia 200SX.
Nevertheless, the APS '210kW' kit's new exhaust goes whole-hog in replacing the standard system with a full-length mandrel bent stainless steel design.
Again starting at the back of the turbo, the APS exhaust system uses a heavy-duty cast iron dump pipe. Yes, not thin mild steel tubing, but a full custom casting! They call it their high flow turbo discharge housing. This housing matches the turbo flange pattern and flows smoothly into a 73mm ID round passage. A threaded 02 sensor fitting on the first bend identifies this as the same dump pipe as used on all pre-MY01 WRXs. When being fitted to the MY01, however, this 02 sensor hole is filled by a threaded plug. The length of the plug is such that its base ends up more or less flush with the inside of the pipe. This keeps in-pipe turbulence to a minimum. Note that a stainless steel heat shield screws into bosses on the outside of the pipe. This shield serves to reduce under-bonnet heat - specifically, the heat that rises into the intercooler above.
Bolting to the 3-bolt flange at the base of the dump pipe is the first section of APS's stainless steel pipe. Carrying on the 73mm ID, this length of tube incorporates two shallow angle bends instead of one large one. This helps to maintain good gas flow.
Local emission laws require the fitment of a cat converter to any vehicle that came so equipped. The APS kit passes Australian emissions laws, with the exhaust system using a single 73mm cat converter. As with any cat converter, this doubles as a muffling device. It also offers minimal restriction due to its large diameter and its relatively long distance from the turbo.
Next comes the APS post cat converter pipe. This section flows through a long hotdog-style straight-through resonator and has a couple of very gradual bends along its length. Note that the inner pipe diameter drops from 73mm to 60mm immediately prior to the trailing flange. This is contraction in diameter is quite common to see where tailpipe dB is deliberately being suppressed. A threaded fitting at the leading end of the pipe accepts the MY01's 02 sensor.
The next part of the APS system takes us to the very tip. More of that smaller 60mm ID stainless pipe then takes a couple of gentle bends (much gentler than factory) over the back axle and leads into a centre-offset straight-through muffler. The inside of the big 73mm ID tip is also perforated.
Well, that's the structure of the original and new exhausts, but what did they newie do for power?
Back on the dyno with its APS system, the MY01 WRX gave a much more distinctive exhaust beat without being boomy or overly loud - a very pleasing note was being emitted. The car sure sounded like it was making more power - but was it? Looking at our dyno graph, which compares the standard (red) power and that achieved with the big exhaust (blue), you can see where the exhaust made the biggest gains. Interestingly, up to 19 per cent more power had been gained in the bottom-end of the rev range. In the top-end, however, maximum power appeared to be unchanged.
So what's going on, you ask? Well, we also expected a reasonable peak power gain with the new exhaust system. Looking at its dump pipe, pipe diameter, the quality of the bends, the cat and mufflers, the APS system looks very impressive. APS themselves claim that their system reduces the standard car's exhaust restriction from 9.6 to 3.5 psi: a major improvement.
In our test car's case, we suspect that - as can occur on various other current-model vehicles - the MY01's engine management was being upset by the dramatically altered combustion behaviour. Ignition timing and air-fuel ratios that didn't allow the advantages of the large exhaust to show must have been offsetting any potential peak power gains. Unfortunately, we can't tell if the car was running richer than standard in the top-end because the Bosch tailpipe sensor that's used in the Dyno Dynamics system cannot go richer than 10.0:1 in its measurement. With the new exhaust fitted, boost pressure behaved pretty much as it did in standard form - spiking to 14.5 psi (up from the standard 14) and levelling out at around 11.5 psi. Note that this small change in mid-range boost pressure is indicative of the turbocharger spooling up faster than standard. This - combined with leaner air-fuel ratios - has given the car its stronger bottom-end.
So much for what we thought would be a sure-thing peak power gain!
Air Filter
With less restriction through the exhaust system, the air intake was the next part of the APS kit to be installed. We chose the intake as the next incremental mod because a lot of people will modify their exhaust and intake hand-in-hand - prior to lifting the boost or making any other changes.
The MY01 draws its induction air from a forward-facing scoop that's mounted above the radiator. Air then travels to the driver's side of the engine bay, enters a resonant chamber inside the guard and enters the bottom half of the airbox. Once it's passed through the filter element, the top half of the airbox and the airflow meter, induction air then travels to the turbo compressor through a pipe running beneath two branches of the intake manifold.
Interestingly, Subaru has moulded the MY01 WRX's airflow meter into the top half of the airbox. This means that you can't simply ditch the airbox and bolt on a pod-style air filter.
Because of this, the only remaining straightforward intake mods are the fitment of an aftermarket filter element and/or modifying the airbox and the ducting leading into it. The APS '210kW' kit gives you a K&N element to replace the stock part.
As you might imagine, this was a breeze to install - just undo a few clips, swap the filter elements and fasten it all up again. With this 2-minute modification performed, we then set off to see if it had made any difference to measured power.
As can be seen by the green line, there was no significant change in the power being developed. That's not really surprising - the pressure drop across most factory filter elements is very, very tiny. Having said that, however, a K&N filter does offer one practical benefit. Being able to be washed (with the appropriate K&N filter cleaner) and then re-used, you don't have to keep shelling out AUS$37.90 for replacement OE air filters. Indeed - if you intend on keeping your MY01 for long enough - the K&N insert offers more of a financial gain over a power gain.
So there you have it. This particular MY01 WRX sedan has - so far - proven very difficult to extract more power from without making boost or engine management changes - so far we've achieved a gain at only lower revs. But remember, we've also fitted only half of the kit - and in the next part of this series, we'll cover the installation of the replacement top-mount intercooler (complete with water spray) and the Unichip signal interceptor.
And let us tell you something - next week we start talking about a l-o-t of extra power!
Contact:
Turbo Tune
+61 8 8297 1030
Air Power Systems
+61 3 9720 9170
http://www.airpowersystems.com.au/
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