| Subaru WRX STI - Engine Bolt-Ons & Making It Grip
In our last installment of Project STi we tackled the car's weakest area, its handling, and made significant improvements by installing Whiteline's Basic Handling Pack suspension kit. Although we were very favorably impressed with the kit's enhancement of Project STi, we needed more. We wanted more than just a better handling car, we wanted one that had the potential to put the hurt on the competition in Time Attack events as well as a decent ride because this car also serves as a daily driver.
Although the stock STi dampers are valved more stiffly than the base model WRX, we wanted something with higher spring rates and more damping, especially with our sticky tires. When pushed hard, our car would exhibit float, body roll, and poor transient response. The STi had an edgy feel to it where it would break loose when turning in without giving much warning to the driver.
We also wanted to lower our ride height to lower our center of gravity and to reduce weight transfer to the outside tires without reducing bump travel. What most people don't realize is that simply lowering many cars reduces bump travel to the point where the car rolls onto the bumpstops as soon as hard cornering is established. This causes severe over- or under-steer depending on which end of the car runs out of travel first. In order to handle well the car must be kept off the bumpstops. This is the first golden rule of good handling that most people ignore.
We improved all of these annoying issues and got our desired lower ride height with the installation of Whiteline's Group 4 coil-over suspension system. We once again went with Whiteline for various reasons after deliberation. Whiteline is an Australian company and the WRX is the Australian equivalent to the Honda Civic. Because of its popularity, Whiteline puts a lot of R&D effort into their Subaru suspensions. The roughness of Australia's rural roads also imparts influence into Whiteline's suspension designs. Their stuff is built around getting the maximum performance while minimally affecting ride quality.
The Group 4 suspension uses a monotube high-pressure damper. A non-inverted monotube damper pressurizes its fluid with nitrogen gas separated from the oil with a floating piston. Unlike normal shocks, the gas and oil are not allowed to mix. This works to provide more constant damping as a foamy oil/gas mixture does not provide predictable damping.
Monotubes also work better than the more common twin-tube damper because their higher operating pressure reduces cavitation of the fluid inside. Cavitation is foaming of the fluid caused by the rapid movement of the shock shaft and piston assembly through it. Foamy fluid does not provide consistent damping. Pressurizing the oil with 12 bar, or 168 psi, of nitrogen gas just about eliminates the foaming and provides consistent damping, even under severe conditions.
Monotubes also dissipate heat much quicker than a twin-tube and are more durable due to a larger piston diameter with its increased bearing area. Being non-inverted (not upside down) is also a plus as it negates the need for regular servicing like inverted designs, including the stock STi struts. It's also a good part of the reason why with the Group 4 suspension the ride is smoother and excess friction is limited.
The Group 4 damper is also externally adjustable from the top thanks to the non-inverted design. This way the adjusters are easily accessible and stay out of the way of road grime, water, and salt that tend to corrode or foul bottom-mount adjusters. The rear shocks' adjustment knobs have extensions so the adjustment can be carried out from the top of the rear seat. Easily accessible adjustment knobs are important as they allow for quick adjustment at the track in the pits enabling the driver to make the most out of the test time. The damping adjustment affects mostly the rebound, which is the most important element as far as adjustability is concerned. The damping adjustment is global across the shaft's velocity range.
The Group 4 dampers feature a huge 46mm piston. A shock piston this size is usually reserved for off-road truck racing applications. The advantages of a large piston are many. A large piston has a much bigger bearing area. Since the bearing area of the piston absorbs most of the shaft's side loading common on MacPherson strut suspensions, this is critical for long life. A 46mm piston has twice bearing area of a typical twin-tube damper.
Additionally, a large piston means that more fluid is displaced as the shock responds to bumps. A higher fluid flow though the valving means that the valving is more sensitive and damping is controllable even at very low shaft speeds and small movements. This equates to a much higher level of control. It also means that the damper is more responsive to adjustments. A larger piston also means that more gas volume, which makes room inside the damper for shaft displacement, can be packed into a shorter package. This overcomes a monotube's primary disadvantage that it must be longer to get the same amount of travel as a twin-tube.
Lastly, a damper is largely an energy conversion device, converting the kinetic energy of the spring's oscillations into heat. The larger the piston, the bigger the diameter of the damper's tube results in greater oil capacity, hence the better integration between stationary and working temperature of the oil. Remember, the hotter the oil gets the less control over damping it has.
Another cool feature of these dampers is that the damper body is shortened so the car can be lowered without losing bump travel. This is an important feature that allows a lowered car to stay off the bumpstops in a corner while maintaining reasonable spring rates. The dampers feature modular construction and can be taken apart for rebuilding or revalving if so desired.
The Group 4 shocks are a coil-over type and come with a tender spring in addition to the main spring. The tender spring helps keep the main spring tensioned and in place when the car is at full droop or when going over uneven bumps. Tender springs go a long way toward reducing the clank and rattle of typical coil-over systems. The front springs are 5 kg/mm while the rears are 4 kg/mm, about a 20 percent increase over the stock spring rates.
We also installed a set of MRT caster/camber plates that Whiteline modified at our request for the front suspension. The modified plates allow for 1-degree positive caster more than any other plate on the market. The top strut shaft mount in the caster/camber plate features a spherical bearing that eliminates the stock mount's sloppy rubber. Although this transmits a bit more noise, the lack of slop here helps keep the alignment true even when highly loaded, such as when cornering. It also ensures that every bit of up and down wheel movement passes though the damper and is controlled.
With the caster/camber plate in place we returned to West End Alignment and realigned the car to run 2.5 degrees negative camber and 6 degrees of positive caster, two more degrees of caster than we were able to get stock. This gave us a much-improved on-center steering feel and much sharper turn in. The car's once rubbery reflexes now felt sharp. If you remember from the first installment, the rubbery, slow, and unresponsive steering was the biggest dislike of this car in stock trim. With more negative camber and the lack of flex in the upper mount, understeer was greatly reduced as well.
Our driving impression with the Group 4 coil-overs is very favorable. In some ways the ride was much improved over the standard STi shocks and springs. Although the ride was firmer, the freeway chop and the seasick float of the stock suspension were reduced. The car had much improved steady state cornering and was much more neutral. The best thing under track conditions is the car's sudden transition to oversteer was further reduced and became even more predictable with less of an on-edge feeling even though the chassis was much more responsive to steering input. This is one of the best off-the-shelf coil-over system for the STi and perhaps the best riding system for everyday street use that we have tried. In upcoming segments of Project STi we will be trying a few more of Whiteline's tricks.
Now it is time to add some power. We decided to first subject Project STi to a barrage of the usual bolt-ons. We contacted AEM for their cold-air intake and DC Sports for a cat-back exhaust, downpipe, up-pipe, and header. The first step was to run a baseline test of the car. For dyno testing, XS Engineering has one of the only 4-wheel chassis dynos in our area. XS uses a Dynamic dyno and all of the testing for Project STi will be done on this dyno in an effort to get the most comparable results. On the baseline run the STi pulled 246 hp at the wheels.
Next we bolted on DC's cat-back exhaust. The exhaust is impressive, constructed of polished 304 stainless steel with smooth robotic mig welds. 304 stainless is one of the best materials for exhausts as it is far more corrosion resistant than 409 stainless or aluminized mild steel commonly found in stock exhausts. The entire exhaust is stainless, even the hangers. The tubing has smooth mandrel bends with no backpressure-causing crimps. The tubing is 3-inches in diameter, considerably larger than stock. All of the flanges are CNC machined from solid stainless stock for a warp-free, good sealing surface. The exhaust has straightthough perforated core mufflers for super low backpressure. On the dyno however, we were surprised that the horsepower only increased to 247 hp, a peak increase of only 1 horsepower. We did note that at 5500 rpm the gain was close to 10 hp and there were slight gains across the bottom range of the powerband. We know that the DC exhaust is a good design that can support much more power and we felt that the engine's main restriction at this point was elsewhere.
For the next step we installed DC's downpipe. Like the exhaust, it also features all stainless construction, CNC flanges, a 3-inch diameter, and clean welding. The downpipe bypasses the cat so this is more of a racing-use-only mod that is not legal for use on the street. Bypassing the cat also made the check engine light come on as the rear 02 sensor is now sending a signal to the ECU so the ECU believes that the cat has failed. A check engine light eliminator like the ones sold through Summit Racing and various other sources will fix this issue. On the dyno the downpipe boosted the power to 268-wheel hp, a whopping gain of 21 hp. This gain is more of a synergistic one shared by both the exhaust and the downpipe. Once the restriction of the downpipe was removed, the engine was able to exploit the full breathing potential of the exhaust. The gains created by the downpipe were mostly at high rpm, above 5500, with a slight 2-5hp loss below that rpm.
AEM's cold-air intake was next up on deck. Many Subaru tuning experts told us that a cold-air intake would not show much of a gain in power. We found this not to be the case. The cold-air intake places the air filter completely out of the hot engine compartment behind the front fog light where it can breathe cool dense air. It uses AEM's new ultra-efficient dry flow filter. The dry flow filter can be washed and does not require oiling. It was developed to meet stringent OEM filtering requirements and traps over 98 percent of all dust particles before they can enter the engine. This is better than any current aftermarket air filter. The AEM intake has lightweight, all aluminum construction and is tuned for best power output. The intake really worked well with the engine cranking out 284 whp, a peak gain of 16 hp. To be honest, about 6 of these hp was created in a tiny blip only 250 rpm wide at the very top of the powerband but a fat 15 or so hp was realized in most of the area from 5500 rpm on up. We had some concerns about the blip as we knew we were reaching the limit of the stock fuel system and we were wary for signs of the engine starting to lean out.
DC's beautiful header was installed next. Sharing the same 304 stainless construction as the rest of the DC pieces, the header features equal-length runners and a nice fully mitered merge collector. Merge collectors are usually found only in very expensive custom and semi-custom headers. DC's robotic mitering helps make this sort of quality available at mass production prices. The header has thick warp-resistant CNC flanges and a crack-resisting slip joint. Like an ber-expensive custom header, the slip joint is a double slip joint to make it leak free. Like a stainless race header the DC header immediately turned beautiful shades of purple and straw yellow, looking very cool.
On the dyno the header posted impressive gains in the midrange with a 15hp gain from 3000-5500 rpm. There was a slight loss in power at the very top, erasing the blip in power that the cold-air intake provided for a peak power reading of 278 hp. However, this loss of power is deceiving because it only occurred in a small 250 rpm range. The power at the top of the powerband was now getting to be inconsistent and we are worrying that this is a sign that the engine's stock fuel system is not keeping up with the flow demands imparted by these bolt-on parts.
The last of power adders installed this month is DC's up-pipe. Many Subaru experts had told us that the up-pipe would not make any more power but we were determined to test DC's piece as it was 5mm larger in diameter than the stock part. The up-pipe features the same quality of construction of the other DC parts. After we bolted it on we were surprised that it made the car quite a bit louder. Usually stuff done before the turbo, in our experience, has little effect on the noise level but this up-pipe made a bigger effect on the noise level than the downpipe did! The up-pipe boosted our power level to 285 hp. It also restored the 250 rpm blip in the powerband. The downpipe caused a small 2-3hp loss below 4500 rpm and created a 3-5hp gain above 5500 rpm. A small gain but a gain nevertheless. One thing for certain, in XS Engineering's experience we are right at the maximum power level that the stock injectors and fuel pump can support. We cannot ask much more from the engine until those issues are addressed.
We were quite impressed that our simple bolt-ons freed up 39-wheel hp. With indications that we are running out of fuel we look forward to what can be extracted next. In our next installment, Eric Hsu will work his tuning magic on the ECU as we upgrade the fuel system for more boost and power.