After a session of RHD practice we were ready to drive the P25 again, this time seriously. We noticed the tenacious grip, supple ride, powerful brakes, and broad powerband immediately. More importantly, this STi did not exhibit the spastic nature of understeer and the sudden, violent transition to oversteer that is an STi trademark. The car had a progressive buildup of slight understeer that could be countered with trail braking or lift throttle. Stronger rotation could be induced with left-foot braking. Power-on oversteer could be easily invoked with the right foot and it was progressive and easily controlled. The WRX's tendency to rear up under hard acceleration was greatly diminished. The normally rubbery STi steering felt much more communicative and sharp. We were amazed that this soft-riding STi was the best handling WRX we have ever driven. In fact, we felt that the P25's ride was better than Project STi's stock suspension to begin with.
Knowing our EVO-biased preference in chassis dynamics Whiteline had the balls to bring out an EVO IX with some of their mild suspension upgrades for us to do a direct back-to-back comparison with the P25. To our amazement, the P25 felt better than the EVO for all attributes other than the EVO's superior fast steering ratio. We were impressed by Whiteline's confidence in letting us drive the P25 against a car that they knew we feel is one of the best handling cars in the world, knowing full well we were going to report our findings and opinions as we saw it.
As we intend to start our Project STi with a mild suspension upgrade, Whiteline brought out a 2006 STi with their Stage One Handling Pack suspension kit for us to evaluate. The Handling Pack is designed to greatly improve handling with no detrimental effect on ride comfort. The Handling Pack consists of a steering rack bushing kit, front and rear adjustable sway bars, front and rear camber adjusting bolts, and caster/anti-lift kit. Whiteline's Stage One '06 is noticeably better handling than a stock STi. It had better on- and off-power cornering balance and much less understeer. Steering response and body roll are greatly reduced as well. It was not as good as the P25 by any means, but it was impressive compared to a bone stock STi considering the short list of parts used.
Whiteline's Handling Pack sway bars are 22mm in diameter for both the front and rear, up from the stock 20mm. This difference in diameter equates to approximately a 45 percent increase in stiffness over stock. Both bars are adjustable to give the ability to tune the balance of the car to the driver's preference. The holes closer to the inboard side of the bar make it stiffer, the outboard holes softer. If you want more oversteer you can make the rear bar stiffer, the front softer, or any combination of the two biases towards more rear roll stiffness. For more understeer, you would make the opposite adjustment. The bars feature forged ends, which are superior in strength compared to the more common welded-on ends, and are CNC bent from a high grade of spring steel. They feature urethane bushings and a powdercoated finish.
The STi has very soft and compliant steering rack mounts. This is especially true with the 2004 cars. The bushings are so soft that steering wheel centering is a problem and the steering has a rubbery feel. Whiteline replaces the rubber mounts with ones made of hard polyurethane. This small part makes a huge difference in what we think is one of the most irritating attributes of the STi. Now, instead of feeling vague, the steering is much more crisp and direct. We feel that every single STi should use these bushings, especially the pre-2005 models.
To be able to adjust a little more negative camber into the suspension, Whiteline supplied us with their camber bolts. The camber bolts have an eccentric cam built into them to allow a small degree of camber adjustability. The bolts are strong and, unlike inferior quality camber bolts, they do not slip, even when using sticky R-compound tires. Adding negative camber is important because it helps keep as much of the tire's contact patch on the ground as possible when the car rolls through a corner. Too much negative camber, however, can have a detrimental effect on tire wear. To get a greater range of adjustment for our Project STi, we replaced both the upper and lower strut bolts with the adjustable pieces.
The most interesting part in their Handling Pack, and the part which we think is responsible for the greatest improvement in balance under acceleration and braking, is Whiteline's caster/anti-lift kit. The STi has a large amount of anti-dive and anti-lift geometry built into the suspension. Subaru's intention was that this would help control body motion under acceleration and braking, allowing the use of stock springs. Unfortunately, what this does is cause a large change in wheel rate when acceleration and brake torque is applied to the suspension, subsequently stiffening the suspension under these conditions. A stiffer front suspension causes more weight transfer to the outside front wheel under these conditions and increases understeer.
By relocating the lower control arms' rear pickup point, the amount of anti-lift and anti-squat is reduced to nearly zero. As an interesting point, most true race cars have zero anti-lift and squat designed into their geometry. We feel that this kit is responsible for the Whiteline car's great improvement in on-throttle handling that reduces the stock STi's sudden swing from understeer to oversteer.
The Whiteline parts also cant (give slant to) the lower control arms forward to give the front suspension more positive caster. Positive caster causes the outside front wheel to gain negative camber in a turn, right when you need it the most. This is a good thing. Think of a parked chopper with the wheel flopped to one side. That's an extreme example of negative camber gain with positive caster. When the wheels are pointed straight ahead there is no gain in negative camber, good for even tire wear. Positive caster also helps steering feel and straight-line stability. The Whiteline parts give one half of a degree of positive static caster to the suspension which can run up to 1 degree of additional caster when the wheel is turned. We installed Whiteline's Comfort version of the kit that has 70-durometer urethane bushings. Later, when we get more serious with the suspension development, we will replace this part with the 90-durometer race version of the part, which rides rougher but has less deflection.
After installing the parts, which was a relatively simple afternoon of work, we headed off to West End Alignment to have Darren Nishmoto work his magic to Project STi's suspension. We set our vehicle's suspension to Whiteline's recommended 1.5 degrees of negative front camber and 1 degree negative rear camber, and zero front and rear toe-in. We measured caster and found it to be right around 3.5 degrees.
Like the 2006 model we drove in Australia, we were very surprised with the improvement in Project STi's handling. For very little effort and money our cornering grip is greatly improved. The car turns in much better and the stock annoying plowing is reduced. Now the vehicle understeers while under throttle and the car also rotates better when trail braking. Body roll is reduced further, making the chassis feel crisper and more responsive. As a final touch, the rubbery steering feel is nearly gone.
The ride, noise, and vibration characteristics are unchanged from stock and we feel that the tire wear will improve and we will be less likely to tear the tire treads due to the understeer as before. Seriously, the engineers at Subaru should look at these changes and incorporate them into the newer cars. But then we would not have so much fun improving on their work will we?