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Rapid RT Spec
RT Spec Review
by Patrick Felstead


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, or performance. 

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: 

  • Roller bearing turbocharger from IHI, model VF22.
  • Cast iron high-flow turbine discharge housing.
  • Front Mount Intercooler.
The following components from the stage 1 system were retained: 
  • Unichip programmable ECU with electronic boost mapping module.
  • Full 3” mandrel bent exhaust.
  • Aluminium cold air intake.
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! 

Front Mount Intercooler 
For 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. 

The 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 does. 

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 tail. 

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 1 system. 

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. 

Acceleration Runs 
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 faultlessly. 

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 and Track”. 

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.