Since the Subaru Impreza WRX appeared in Australia
more than a decade ago we’ve seen countless workshops getting stuck into
modification. But few – if any – are as impressive as the Middleton Rally Team
(MRT) XB power-up kit for the newly released MY06 WRX.
Forget entering debates on other upgrades that
might offer more top-end power – this baby will blow most other Rexes to the
side of the road before they’ve hit boost! The typically laggy and soft
bottom-end performance of the previous models is now just a distant memory – the
MRT enhanced MY06 WRX is a torque monster!
The Latest WRX and MRT’s Mods
The MY06 Subaru Impreza WRX is quite a different
beast compared to earlier versions. While the previous 2-litre engine struggled
when off boost, the latest model features a 2.5 litre donk with a relatively
high 9:1 static compression ratio combined with a relatively small TD04L
turbocharger. The result is much improved flexibility and torque spread,
although the engine isn’t as willing to rev as the superseded 2-litre – peak
power now arrives 400 rpm earlier than previously. Vital stats are 169kW at 5600
rpm and 320Nm of torque at 3600 rpm (up 1kW and 20Nm on the MY05). Subaru
officially recommends a minimum of 95 RON fuel.
So what does MRT do to enhance the already
improved all-round performance of the new Rex?
Well, the entry-level XA kit comprises a
replacement rear muffler (which bolts to the rearmost flange of the factory
exhaust), aftermarket panel filter and an engine management tune using EcuTeK
software. These changes increase power from 120 – 125kW (standard) to around 135
– 145kW at the hubs (as measured on MRT’s 4WD Dynapack dyno). This upgrade costs
AUD$2290.
But the next step up is the MRT XB kit (as
tested). The XB kit includes a full-length 3 inch exhaust with a twin-pipe dump,
air intake mods and dedicated ECU tune.
Interestingly, the factory MY06 WRX exhaust flows
better than previous models thanks to an increase in pipe diameter from 48.6 to
54mm. Still, stepping up to a 3 inch (76mm) pipe represents a near doubling in
cross-sectional area so you’re pretty well assured a reduction in backpressure...
The new system is made from high-grade stainless steel for maximum
durability.
Air intake restriction is eased thanks to an
aftermarket panel filter teamed with some stealth modifications to the
pre-airbox ducting. In the standard car, air enters a pick-up above the
radiator, flows into a resonant chamber inside the left front guard and into the
airbox. MRT modifies the in-guard resonator so that it becomes a secondary air
pick-up – this reduces restriction and lowers the pre-turbo intake air
temperature.
The final part of the upgrade is an engine
management tune. MRT uses EcuTeK software to re-flash the factory ECU via the
OBDII port which means there’s no need to cut or splice any wires. The EcuTeK
system gives adjustment over a huge range of parameters – everything from inlet
cam timing to the new electronic throttle control system. MRT won’t divulge
specific air-fuel ratio or ignition timing numbers but they will reveal that
peak boost pressure (achieved in a mid-range rpm) is increased from about 13 to
15.5 psi and tapers to about 14 psi near the redline. Ninety-eight RON fuel
should be used for maximum performance but MRT’s relatively conservative tune
lets you get away with 95 RON so long as you drive intelligently.
Driving the XB Kitted MY06 WRX
The XB’d MY06 WRX drives completely
differently to other 2-litre WRXs we’ve sampled.
The tweaked 2.5 offers genuinely useable torque
off idle and by around 3000 rpm you’re surging ahead with a meaningful amount of
boost. The low rpm response and torque feels very similar to, say, a
turbocharged Audi A4, Saab 9-3 or Volvo S60 - high praise indeed! Part-throttle
performance is strong and, interestingly, you can feel boost pressure snowball
and road speed will increase while maintaining a constant throttle. Throttle
response is quite sharp (for a turbo engine) and there’s the flexibility to
shift from 1st to 3rd gear – something that would have any
other WRX haemorrhaging...
Mid-range torque is very impressive but the
top-end does feel a bit flat as the standard turbocharger nears the limit of
efficiency range. MRT’s dyno figures show gains of 35 – 40kW over the standard
output of 120 – 125kW at the hubs - a gain of around 30 percent. Sure, it’s easy
to find modified WRXs with more power but in the real world of driving, the
accessibility of the XB enhanced MY06’s performance more than makes up it.
And what about price?
Well, after you’ve spent AUD$41,000-odd on a MY06
WRX the MRT XB kit will set you back almost another 10 percent – AUD$3990 fitted
and tuned. It’s not cheap but, if you can afford it, we reckon it's money well
spent. We can’t remember the last time we came away so impressed by a WRX (and
that’s saying something given how many we’ve driven!).
If you’ve ’been there and done that’ with 2-litre
WRXs, we strongly suggest you take a XB kitted MY06 for a test drive. It’s
guaranteed to impress.
The In-Development XC Kit
At
the time of writing, MRT is working on a ‘stage three’ XC upgrade for the MY06
WRX. It is expected that the standard turbocharger will be replaced and a
modified fuel system will be necessary.
If
MRT’s XC kitted Forester XT upgrade is anything to go by (see Forest Fire),
we reckon you’ll be looking at a power gain of more than 50 percent – while
still retaining much of that tremendous low-to-mid range grunt. We can’t wait to
test that one!
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