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.