What’s this? Another Subaru Impreza?
Well, just when you thought you’d seen everything there is to see under the
bonnet of the popular Subie, you come across this. Shannon O’Toole’s MY03 Impreza
RS equipped with a prototype Raptor centrifugal supercharger and a US-spec 2.5
litre STi block.
A one-off set-up guaranteed!
Shannon previously owned a Hyundai Lantra he considered
supercharging. However, the idea never got off the ground because Shannon says
the car didn’t really have the all-round potential to make it worthwhile.
Instead, Shannon saved his cash to put towards a brand new 2003 Impreza RS. The Impreza, he says, is a much better platform - well built and boasting
the traction and assurance of AWD. It’s a great base for tuning and just a
little bit different to an off-the-showroom-floor WRX.
Next, Shannon spent some time web browsing and came across www.raptorsc.com.au. Shannon liked the
look of the company’s blossoming products so volunteered his RS as a development
vehicle. He is now in charge of the development and marketing of the Raptor kit
to suit Subies.
To suit an all-out assault with forced induction, Shannon dropped in a
US-spec STi block which he had crammed with forged pistons. The block teams up
with the original RS SOHC, 4-valve-per-cylinder heads which were fitted using
3-layer metal head gaskets. An oil catch can prevents blow-by oil entering the
air intake and increasing the likelihood of detonation. At present, the standard
Impreza RS exhaust manifolds remain in service - and Shannon says these are a
major cause of restriction. A pair of Borla headers is planned.
The still-in-development supercharger installation is centered around a
Raptor V centrifugal blower. Shannon says the Raptor V is the smallest centri
available and is ideally suited to relatively small capacity engines such as the
EJ20 and EJ25. The blower has a recommended maximum output pressure of 15 psi
but Shannon is running 12.5 psi. This peak pressure is reached at 6400 rpm.
The supercharger of Shannon’s car is mounted using a custom 2-piece steel
bracket that’s 10mm thick. We’re told that production versions will be slightly
different. But more important is the direct belt-drive of the compressor. By
avoiding a traditional gear-drive system there’s no need for an oil lubrication
system. We’re told that production versions will also come with a dedicated belt
tensioner for the blower. Tensioning will take place 500km after blower fitment
and on a routine basis thereafter.
Shannon’s car runs a large K&N pod filter on the intake to the
compressor. At the time of our photo shoot, the K&N was in an exposed
location under the bonnet – we’re told a cold air induction arrangement will go
on shortly. The post-exhaust manifold pipework is 2 1/2 inch mandrel bent and
employs a Supercat 4 inch rear muffler. Shannon says he’ll probably upgrade to a
3 inch system to release every Watt.
A Hybrid air-to-air intercooler is fitted in the nosecone of Shannon’s car
but we’re told that production versions of the kit will come with a top-mount
intercooler to help maintain maximum throttle response. These kits will also
peak at around 8 psi boost.
Shannon first tried running an interceptor unit to accommodate the mods but,
as it turns out, this was a bit too ambitious... A Haltech E6X fully
programmable stand-alone system has now been wired in and tuned by ChipTorque.
Shannon says he’s looking into an adaptor harness to make the Haltech a plug-in
Fuel supply is provided by a Walbro 550 pump teamed with Nippondenso 550cc
injectors, factory reg and a Walbro pump. The Haltech-controlled ignition uses
the standard Subaru coils.
Receiving the supercharged output of Shanon’s machine is a modified solid
flywheel with a custom full-face clutch. We are told that the low-boost kits
should survive with a standard clutch. The standard RS gearbox is well within
limits – and the extra torque of the blower makes the standard ratios a doddle
at almost any rpm.
"The blower gives very smooth acceleration," says Shannon.
"At about 1500/2000 rpm you can feel it coming on and I find it’s best to
change at about 5500 rpm. The factory 6200-ish rev limiter protects the engine."
And peak power?
Well, the car has been recently tuned to deliver 139kW at all fours wheels on
a Dyno Dynamics chassis dyno - more than a current-model Australian-spec STi.
Note that this 139kW ATW output is achieved on 12.5 psi boost. The planned 8 psi
kits will likely give around 175kW at the flywheel with the factory exhaust in
place. The all-inclusive cost for the kit (minus installation) is expected to be
Given the body is 100 percent standard, Shannon says the 139kW ATW grunt of his RS comes as one hell of a surprise to
Aside from the under-bonnet modification, Shannon has made few mods to the
car - none have really been required. A FLYN front strut brace is the only
addition to the standard RS suspension and the four-wheel-disc brakes remain
stock. DBA replacement discs are on the ‘to do’ list.
Inside, the cabin is original Impreza RS with Shannon’s custom on-dash gauge
pod containing a volt meter and boost pressure gauge. A set of WRX pedals and
gear knob complete the indoor upgrade. Oh, and Shannon is an expert car audio
installer - he say it won’t be long before a pretty major beat system is built
into the car.
But first things first.
If al goes to plan, it shouldn’t be long before a Raptor-charged Subie
becomes a familiar sight!
ChipTorque +61 7 5596 4204
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