MY01 WRX Power Up - Part 2

In Part One we discovered that the MY01 Impreza WRX (this particular example, at least) wasn't very responsive to just exhaust and air filter changes. In fact, with only these two components of the APS '210kW' kit installed, peak power did not move at all from the car's 103kW (at all four wheels) Dyno Dynamics base figure. One explanation for the apparently odd behaviour (the new exhaust looked like it would flow a heap better than the standard unit) is that the engine management may need to be altered if the better gasflows are to be taken advantage of. For example, during all dyno tests, the car's air/fuel ratio had so far been running at an indicated 10.0:1 (and, most likely, richer) at full throttle loads in the mid-range and above. This is much too rich to give optimal power, and is probably responsible for holding back the gains that would otherwise have been so far seen.

The high performance exhaust, however, did give a reasonable improvement in the bottom-end. This was largely due to a quicker boosting and leaner air-fuel ratios in that part of the rev range.

Following the fitment of just the exhaust and filter insert, we then watched Adelaide's Turbo Tune workshop install the second half of APS's '210kW' kit; namely, a replacement top-mount intercooler and the pre-programmed Unichip interceptor. Note that the interceptor module is also used to control a water spray system for the replacement intercooler.

We were relying on the remaining modifications to achieve big things... and we weren't disappointed!

Intercooler

After the exhaust and filter, the third step of the APS kit installation was to ditch the standard top-mount air-to-air intercooler for the APS replacement unit.

The factory MY01 'cooler uses the same type of layout found in earlier WRXs. That is, a forward-facing bonnet scoop catches airflow and directs it through a top-mount air-to-air core. A rubber seal on the underside of the bonnet ensures that air from the bonnet scoop is forced through the intercooler. The MY01's intercooler core (ie not including its end tanks) measures approximately 415 x 155 x 65mm. Stripped of all its pipe and hose attachments, it has a mass of approximately 3.2 kilograms. (Intercooler mass is an important indicator of a cooler's heat-sinking ability - which is vital in normal on-road driving.)

Inside the standard intercooler are very fine turbulence fins, which serve - in theory - to improve overall heat exchange capability. In addition, dual air entries are fitted to the rear tank of the intercooler. These are aimed at maintaining an even heat and pressure distribution across the width of the core.

With the factory intercooler removed from the car, you can clearly see the plastic Y-piece that connects the turbo compressor outlet to those twin air entries. In previous model WRXs, this plastic pipe is notorious for causing restriction and blowing apart - and it doesn't look any better on the MY01. Note that - like the Australian-delivered MY00 - the blow-off valve on the new WRX bolts straight onto the leading end-tank.

In summary, the standard Subaru intercooler looks like a slight improvement over those fitted to early WRXs. However, it does not make use of all the available space - as you'll see...

The first most noticeable aspect of the APS replacement intercooler is how heavy it is. At around 8.5 kilograms, the APS top-mount has vastly more heat-sink capacity than the 3.2 kilogram stocker. Part of this intercooler's substantial 166 percent greater mass can be attributed to its larger core: it measures around 442 x 115 x 115mm (compared to 415 x 155 x 65mm). However it's heavier construction is mostly because it uses a bar-and-plate design, which has a lot more aluminium in the core.

Note that the APS core is massively thicker than the factory part - 115mm versus 65mm. This added depth takes up some of the otherwise wasted space above the gearbox. The extra core width and thickness allows more tube area for the charge-air to pass through, giving potentially less flow restriction. Like the standard intercooler, the new replacement also carries internal turbulence fins to aid its heat exchange abilities.

In addition to the intercooler core upgrade, the APS '210kW' kit also provides you with this 3-ply Y-shaped silicone hose. This replaces the plastic factory part, offering potentially improved durability and airflow.

Interestingly, some of the factory hardware associated with the Subaru intercooler needs to be transferred over to the APS unit. The two cast elbows that form the dual air entries must be used, as does the short intercooler-to-throttle hose. A small diameter OE breather pipe is also bolted to the front of the APS intercooler. Due to its vastly increased thickness, however, it's no surprise that the APS core requires the fitting of a different underside-mounting bracket; this, of course, is supplied in the kit.

Sooo with the APS exhaust, air filter and top-mount intercooler now fitted, Turbo Tune gave the MY01 Rex some more power runs on the dyno. The first thing we noticed was that manifold boost pressure now spiked to 15.2 psi in the mid-range and settled back to around 12 psi in the top-end (up from 14.5 and 11.5 respectively). From this, it would appear that the new intercooler core did give less restriction. But did that equate to more power?

Not really.

In this graph you can compare the APS-intercooled power curve (pink) along with the standard, exhaust'd and air filter'd runs (plotted in red, blue and green respectively). The power curve with the new intercooler installed shows a small peak power gain of around 2 percent over standard. The new mid-range boost spike also reveals a very slight improvement in that part of the curve. Note, however, these gains are so small that they might be due to just normal dyno run variations.

So why was there no more power from the better intercooler?

Significantly, as before, air/fuel ratios remained unmeasurably rich all the way through the higher revs. (The richest that the air/fuel ratio meter probe can read is 10.0:1.) Given that an obviously superior exhaust and intercooler had made almost no difference to the measured peak power, we were starting to think that the management system definitely needed tweaking to release any decent power gains.

In any case, a chassis dyno is a far from ideal place to identify the real-world abilities of an air-to-air intercooler. This is true for a couple of reasons. Firstly, a dyno fan cannot replicate the aerodynamic flows across the front of a moving car - in terms of both air speed and turbulence characteristics. With the cooling flow for the WRX's intercooler completely gained from the bonnet scoop, it's realistic to expect that the atmospheric flow through the intercooler on the dyno will be quite different to that achieved on the road. Secondly, in normal on/off boost driving, the intercooler acts in a quite different way from its required behaviour during a single power pull on the dyno. The best environment to measure the heat-sink and heat-exchange ability of an intercooler is on the road (or track).

So, despite the lack of any decent dyno power improvement, we'd still suggest that the APS replacement intercooler gives notably superior airflow, heat-sink and heat exchange properties over standard. This seems logical given its increased dimensions and much heftier weight. It's just a shame that at this stage those benefits weren't able to shine through.

Unichip Interceptor

The Unichip is a programmable interceptor module that adjusts fuel, ignition and boost parameters. It works by intercepting the input and/or output signals passing to and from the factory ECU, altering them so that the ECU delivers changed fuel, ignition and boost behaviour. The Unichip is not the same as an aftermarket replacement chip or a fully programmable standalone engine management system.

One of the biggest attractions of an interceptor module is that it retains 100 percent factory ECU functionality. That means that you still get factory quality cold starts, air temperature compensation, knock sensing and - amongst other things - closed-loop mixture control under light throttle cruise. On the other hand, an interceptor such as the Unichip cannot make an effective fuel changes during light-load closed loop operation (where the ECU self-corrects from the input of the oxygen sensor); but in this application, where high load changes were being chased, that doesn't matter.

The Unichip in the '210kW' kit arrives pre-programmed from APS - all it needs is to be wired in. The main Unichip module is connected into the existing wires that lead into the factory ECU, along with two so-called "X drivers" - one to control boost pressure and one to control the (yet to be fitted) intercooler water spray. As a general aside, you should note that the MY01 uses a new engine management system, which is significantly more advanced than that in previous WRXs.

With the Unichip now connected, the MY01 again hit the Dyno Dynamics all wheel drive chassis dyno in search of what had - so far - been an elusive power gain. But what happened next was truly spectacular. From early in the mid-range, we watched the new power curve (plotted here in red) absolutely blitz those seen in every previous run. As you can see, the Unichip gave a towering peak power measurement of 134kW - around 30% up on everything measured so far. And yes, all of this took place without altering the dyno settings or any other funny business. We made sure of that.

So how could the Unichip achieve so much, you ask?

Well, firstly, we should point out that the huge power gain seen here is the cumulative result of the fitting of the new exhaust, air filter, intercooler and Unichip. The improved airflow through the engine and the (likely) reduced charge-air temperature gave terrific potential to generate additional power. In this case, however, the Unichip was required to optimise the way that the engine management system reacted to these mods.

As part of achieving this, APS has programmed the Unichip to run the engine leaner at high loads. As seen in the lower part of the above graph, the Unichip'd engine runs to a richest air/fuel ratio of 10.7:1 - and that's attained only at around peak power rpm. This compares to the factory ECU, which - on our demo car - had previously been sending air-fuel ratios off-the-scale rich (beyond 10.0:1) from mid-range rpm onward.

The other Unichip-induced change that we observed during our dyno runs related to boost pressure. With the Unichip installed, boost spiked to around 15.2 psi (as seen before) but it then held to a more substantial 14 psi. Note that this is a full 2 psi more than any prior runs. Lastly, we'd imagine that the Unichip altered the MY01's ignition timing. Although we didn't sample timing on-the-fly, we'd assume that APS has programmed the module to make changes in this area.

In any case - whatever has been programmed into the Unichip interceptor module - it sure seemed to do the trick. We could hardly believe how it had turned everything around.

Unichip-Controlled Intercooler Water Spray

The final stage of the APS '210kW' MY01 kit was to wire one of the Unichip "X drivers" to an intercooler water spray. The driver switches on the water spray system, with the turn-on point being dependent on engine revs and boost pressure.

Included in the APS kit is a XA Ford Falcon windscreen washer kit, produced by Flexible Drive Agencies (part number W5010). The boxy plastic bottle was mounted just in front of the MY01's 12V battery, while its water pump was connected into the existing washer hose. This new addition takes on the role of the factory washer bottle/pump assembly.

The APS kit utilises the MY01's factory washer bottle and pump assembly for their intercooler spray system (presumably because of the larger capacity of the bottle). Note the supplied warning sticker that advises to fill the bottle with clean water only - you don't want to have windscreen detergent bubbling up over the intercooler core...

Once the Unichip decides to activate the Subaru pump, water is pushed through a length of rubber hose to a pair of plastic spray nozzles. These are inserted through two raised areas in the plastic panel on the underside of the bonnet - which is exactly where the intercooler spray nozzles are fitted to the Japanese-market MY01 STi. Note that the hose leading to the nozzles is also equipped with a one-way valve to prevent water back-flow.

With just the intercooler water spray added since our last dyno runs, we weren't expecting much more of a gain - and we were right. As you can see, the new power curve (shown in black) maxed out at around the same 134kW. The shape of this curve very nearly overlays the previous configuration (plotted in red). Having said that, the intercooler spray system isn't necessarily a waste of time. As we stated when testing the effects of the replacement intercooler, a chassis dyno is not a good place to quantify gains in this area.

Conclusion

The MY01 WRX is not a straightforward vehicle to power-up. Unlike previous models, you cannot just add a number of bolt-ons and enjoy big gains.

Judging by our demo vehicle, the new Rex uses a very stubborn engine management system. All it does is run ultra-rich at full noise, virtually eliminating any stand-alone power gains from typical exhaust, intake and intercooler upgrades. From this, it appears that - without making management software changes, installing an interceptor or a programmable aftermarket ECU - you'll be unlikely to produce hardly any more peak power than standard.

And that's a very important point to remember.

The complete APS '210kW' kit proved to perform very well. It increased measured power at the wheels by 30 per cent, which equates to (as near as dammit) the advertised 210kW flywheel figure. For AUS$4620 (plus freight but including fitting), the kit represents quite good value for money. Add the cost of a typical high-performance exhaust, aftermarket air filter, upgrade intercooler with water spray and some kind of management change and you're automatically looking in the ballpark of AUS$5000 (depending on your specific selections).

In addition to its reasonable pricing, the APS ‘210kW’ kit has been tried and tested in a large number of MY01s – the company makes the point that the durability of each component was a priority. Indeed, the well-developed kit is safe buying for anyone who doesn’t want to endure any kind of hiccups during the enhancement process (such as the all-too-common droning WRX exhaust system). Another notable benefit is that the Unichip arrives pre-tuned, so there’s no risk of engine damage while somebody hunts for the optimal maps. Parcelled and delivered with every necessary component (gaskets, brackets - you name it), APS dealers should be able to fit the entire kit in less than one day.

In summary, the APS '210kW' kit is recommended buying.

Contact:

Turbo Tune
+61 8 8297 1030

Air Power Systems
+61 3 9720 9170
http://www.airpowersystems.com.au/

So Why Not Just Buy the UniChip? After having read this two-part series, you'll have realised that just adding a high performance exhaust, filter and intercooler has almost no effect on the MY01 WRX's peak power. Not on the dyno with our demo car, anyhow. But that doesn't mean that these bits are worthless. Each of these modifications gives the potential to generate much more power. It's just that - in this case - the Unichip module was required to gain any real advantages from their fitment. Without the installation of the aforementioned components, the Unichip alone would certainly not have given such a big power increase. Not on the same boost level, anyhow. So don't be fooled into thinking "the usuals" - an exhaust, intake and intercooler - are suddenly a total waste of time! Go ahead and buy them - but also budget to include some kind of management system change in your new shape WRX upgrade.

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