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.