The Subaru WRX is one of the most successful
and popular cars on the market today, and has been for a number of years.
It seems that just about every owner of the “pocket rocket” feels the need
to modify their car, be it for looks, the unique boxer engine sound, handling,
It is the performance factor that engineering workshop APS have concentrated on since the WRX was first released into Australia in 1994. Over this time, APS have developed a number of WRX performance systems, to fit each of the model year variants.
The most recent release from APS is the
RT spec system, which is their top shelf performance product, and also
their most powerful system ever for the WRX. RT stands for Road and
Track, and is a reflection of the all-round capability of the system.
The major components of the APS RT system
flow down from development of their club sprint WRX race cars. At
the heart of this system is a newly developed front mount intercooler,
and also a large roller bearing turbo.
Parts and Installation
My MY99 WRX already had the APS stage
1 system installed, so it was a matter of upgrading some components to
bring it up to RT system level.
The upgrade consists of:
The following components from the stage 1
system were retained:
Roller bearing turbocharger from IHI, model
Cast iron high-flow turbine discharge housing.
Front Mount Intercooler.
APS were kind enough to allow me into their
workshop during the installation to take some photos for this article.
I was genuinely surprised at the speed at which the installers worked;
there was no hesitation or head scratching, it was all hands on deck and
just straight into it. At one stage there was three guys working
on the car!
Unichip programmable ECU with electronic boost
Full 3” mandrel bent exhaust.
Aluminium cold air intake.
Front Mount Intercooler
the uninitiated, the intercooler’s job is to cool the air after it has
been compressed by the turbocharger, and before it enters the inlet manifold.
When air is compressed its temperature rises causing it to expand, so there
is a lower oxygen density and this reduces power output. Hot air
is the enemy of turbocharged vehicles, and significant power and reliability
gains are made by fitting an intercooler. In general, the bigger
the intercooler core area, the greater the cooling effect.
The standard intercooler is actually quite
small, and is mounted on top of the engine to enable it to be cooled with
air from the trademark WRX bonnet scoop. APS have developed a larger
top mounted intercooler to replace the standard one for their stage 1 system,
but for the RT system, they have gone one big step further with the development
of a Front Mount Intercooler (FMIC).
The FMIC is almost 3 times larger in area
than the standard one, so the air is cooled much more, making the intake
charge more dense which equates to higher power outputs from the engine.
It is mounted just in front of the radiator, inside the front bumper bar.
Some plastic material needs to be cut out of the bumper to fit it in, but
it is not that drastic, and no metal work is required whatsoever.
There are two choices for the surface finish
of the intercooler. Raw aluminium which is pretty much “in-your-face” as
you look at the car head on, or stealth black powder coated. I elected
to go for the stealth look.
There are two main intercooler manufacturing
techniques: flattened tube and fin (known as “tube and fin”) and also,
“bar and plate”.
Bar and plate is a sturdier, but also heavier
construction than tube and fin. Its origins stem from stationary
industrial compressor environments subject to high vibration, where ruggedness
is a more important consideration than weight savings. It is also
not as efficient a heat exchanger as tube and fin.
Tube and fin on the other hand has better
heat exchanging properties than bar and plate, and whose cores can be up
to 50% lighter in weight. Pressure drop in either type is dependent
upon a number of factors including the number of tubes or bars used, their
cross sectional area, and amount of internal finning. In the end,
intercooler design is a compromise between weight, durability, heat exchanger
efficiency, and pressure drop.
It should come as no surprise that almost
every modern car manufacturer uses tube and fin for the construction of
their (coolant) radiators. Porsche even specify tube and fin construction
for the intercoolers in their 996 twin turbo!
APS manufacture both types of intercoolers
for vehicles as diverse as 4 wheel drives, to full on race cars.
In fact their stage 1 top mount intercooler is made from bar and plate.
With the front mount, they wanted to keep the additional weight at the
front of the WRX to a minimum, so combined with its superior heat exchanging
properties, they decided that tube and fin was the optimum.
connecting hoses are made from bright red 4-ply reinforced silicone rubber,
which I reckon look pretty cool, and they really stand out when you lift
the bonnet to show off the system to your mates. There is a short
cast aluminium section that runs near one of the exhaust headers.
During the development of the FMIC, APS
performed some temperature testing on a race track using a data logger.
They found after many laps of the track on a warmish day, that pre intercooler,
the temp was around 130deg C. The intercooler was found to cool the
hot air back down to a temperature of around 30deg C.
Whilst at a standstill or in low speed
traffic, heat from the engine rises up through the Subaru factory intercooler,
making it much less efficient. This is known as “heat soak” and the
FMIC virtually eliminates this effect. The FMIC also benefits from
air being forced at right angles onto the core, in contrast to the top
mount, where the air is deflected onto the core by the bonnet scoop.
Another plus for the front mount is that power will be much more consistent
between hot and cool days compared with the original intercooler.
Cooler air also means that the onset of
detonation (uncontrolled flame front in the combustion chamber) will be
much reduced. This means that at the same boost levels, your engine
will be more reliable, and may even mean that a higher boost level can
be run whilst maintaining a good safety margin from the point of detonation.
Boost Pressure and Power
The RT system on my car produces a turbocharger
pressure of 15.8psi by 3250 RPM. This boost level is then held rock
solid, flat as a pancake, right up to redline. Yes, read it again,
right up to the rev limiter! That’s more like it!
In standard form, my car developed 90kW
at the wheels on the APS chassis dyno, and my stage 1 system produced 118kW.
The car with the RT system now develops a neck snapping 147kW at 6300 RPM!
The torque curve is essentially flat, being directly proportional to the
boost pressure curve.
The maximum boost pressure is actually
slightly lower than on my stage 1 system. This is because the IHI
VF22 flows a greater volume of air, so it actually provides more power
than the original turbo at the same boost level. Of course by flowing
more air, more fuel must be injected to maintain a safe air/fuel ratio.
When accelerating gently, it seems to me
that there is less induction noise with the IHI turbo. I asked APS’s
Peter Luxon why this might be so, and he told me that the larger turbo
is running less boost, so is probably not spinning as fast as the original
and smaller TD04 turbo. Also, he said that the rotating group may
be better balanced in the IHI turbo, both of which could be reducing the
induction noise. Under part throttle and while spooling up, the original
turbo used to make a kind of high pitch noise. The roller bearing
turbo by comparison is much quieter, and makes a nice sort of constant
noise like static on an untuned radio.
The power of the RT is a quantum leap in
comparison to the standard car. And its not just the peak power figure
that is increased, the entire power and torque curves are fatter over the
entire range. Compare this power figure with a factory 2 door Sti
which develops around 126kW on the same dyno.
I must admit that this worried me a little
in terms of clutch and gearbox longevity, but as long as you don’t do full-on
standing starts all the time, and show a little mechanical sympathy to
your car, there should be no problem. Keep in mind that the standard
Subaru clutch acts as a “fuse”, and in general will slip before the gearbox
On The Road
You can really - and I mean really - feel
the difference at the top end with this system. From the mid range
RPM, the car is transformed. I know I used similar words in my review
of the stage 1 system, but this is in a different league!
Generally speaking, boost feels as though
it peaks at about 3500 RPM. On the road, in 1st and 2nd gears, it
feels more like 4000 RPM before the action starts. In 3rd and 4th,
the car pulls like a thing possessed from anywhere above 3500 RPM.
The main thing people want to know is “How
laggy is it?”. The turbo is physically larger and therefore has more
inertia, so even with the help of the lower friction roller bearings, it
does take longer to spin up to speed. If you cruise along at a mid
range engine speed with a light throttle so the turbo is not boosting,
and then floor the throttle, the turbo does take longer to produce boost
than the original little TD04 did.
A fang up a tight twisty road reveals that
the system is no slouch coming out of tightish corners, even in 3rd gear.
And if any time is lost from the little bit of lag that is present, it
is certainly more than made up for when full boost is developed.
This was confirmed because I was behind a stage 1 kitted car with a very
good driver on this nice section of road, and I was persistently on his
What surprised me when I first drove the
RT car though is how smooth and progressive the power develops. You
don't feel such a kick in the back as the stage 1 system, but I can feel
that I'm going quicker – much quicker – in acceleration.
Another thing I like is when on the freeway
sitting at the speed limit, the throttle isn't quite so touchy as it was
with the smaller turbo. The little turbo was so close to boosting
at 3000rpm, you only had to touch the throttle and it was on boost, whereas
the VF22 needs an incisive prod of the throttle at highway speeds.
The system does however have a tendency
to be a bit jerky when changing gears in hard acceleration. It is
most evident in the two lowest gears. When you accelerate hard and
then change up, there is a quite a big jerk as you re-engage the clutch,
and before the turbo spools back up (APS have developed a larger blow off
valve which improves this). I also found that sometimes when lifting
off the throttle after moderate acceleration (without touching the brake
or clutch) the car lurches a little, a bit like an auto transmission shifting
down when you’re not expecting it to. It’s not severe, but I do notice
it. APS tell me that this is due to the injectors shutting down as
you lift off the gas, but it didn’t seem that noticeable with the stage
Fortunately, these are the only negatives,
and they take little away from the overall system.
To put the RT system into perspective,
I did a back to back comparison with an almost standard MY99 WRX.
It’s amazing how quickly you adapt to an upgrade and forget how the car
used to be. The standard WRX is almost the direct opposite in terms
of power delivery to the RT spec car. It car comes on boost just under
3000 RPM and feels strong up to around 4000 RPM but feels slow and flat
beyond this. The RT spec feels "relatively slow" up to 4000 RPM, after
which it absolutely takes off with an awe inspiring shove in the back.
The shove is constant all the way to the redline too.
It only feels slow below 4000 RPM because
of the huge power higher up. In actual fact, it accelerates just
as fast as the normal turbo below that range. This is backed up by
the dyno chart which shows the car has more power and torque from 3000
RPM and beyond.
The friend who’s car I swapped with was
very impressed with the RT spec. He took off at the lights with a
fair bit more chirp from the tyres than he was used to. Afterwards
he told me "Holy cow......I actually eased the clutch out gently to be
nice. I was amazed to hear the tyres spin! You'd have no rubber left
if I had dropped the clutch from 6 thou !!". G-Force junkies may
now join the queue.
I used a GTech meter to do some performance
times on a 20deg C day. On my first run, I ran a 13.22sec 400m time.
On my second run, I did a 400m time of 12.94sec.
Unfortunately I didn’t catch the 0-100
times because the GTech doesn’t save it if you’re doing a 400m run, it
just displays it for a second or so before continuing the 400m timer.
Note that in my experience, the GTech recorded times do tend to be a little
optimistic by a couple of tenths, so realistically the car is pulling consistent
low to flat 13’s for the 400m sprint.
I separately ran a 0-100km/h time of 5.21
sec, but I don’t think the launch was quite as good as the best 400m time,
because the clutch was getting a bit slippery!
The front mount intercooler provides vastly
improved intake charge cooling due to its size and position. It reduces
heat soak, provides greater tuning safety margins, and makes power more
consistently all year round. It also looks the part too, even in
stealth black finish.
The RT system delivers extremely quick
and reliable street performance. And if you’re serious about motorsport,
it’s a superb choice for sustained wide open throttle power delivery, without
fear of your engine destroying itself. It is probably about as far
as you can safely go without delving into the internals of the engine,
and modifying the clutch and gearbox. I have used the car for club
sprints with the WRX club, and also a couple of day long advanced driver
training courses consisting of around 50 hard laps, and the car performed
The RT system by APS makes the WRX a powerful
and formidable performer. I’d be tempted to call it “Godzilla” if
that name hadn’t already been used to describe a vehicle of this calibre.
The RT system is a legitimate all rounder, and is true to its name “Road
Many thanks to Patrick Felstead for
his effort and his positive feedback. Patrick's web
site is a wealth of information and well worth a visit.