The dual muffler and double wishbone rear suspension design of the new WRX sedan limits the amount of space available for large diameter piping. In order to clear the rear differential and lower control arms, the exhaust is routed close to the ground. One design flaw we see is the placement of the b-pipe-to-rear-pipe exhaust flange, which sits at the lowest point in the car just below the differential. A three-inch exhaust system will be hard to fit under the new chassis.
The 330x25.5mm rear rotors are a full-floating rotor design (like the fronts), which uses an aluminum hat and cast iron rotor ring allowing better rotor-to-caliper alignment for potentially less pulse and improved resistance to permanent thermal deformation. Production models will use much lighter caliper brackets and a steel hat for better durability, since the WRX uses an integrated drum parking brake inside the rear rotor hat. One major advantage of the AP Racing big brake kit is the addition of a directionally vented rear rotor, which is far more effective at keeping the rotors cool compared to solid stock rotors or straight-vaned designs.
In terms of balance, our four-wheel AP Racing set-up seems to be slightly more rear-biased since both front and rear rotors share the same 330mm (13-inch) rotor diameter and the rear piston area has increased proportionately to stock. The front pistons are now only 18 percent larger than the rears. In the case of most modern cars, like the 2008 WRX, minor changes in piston sizing for proper front-to-rear bias can be compensated for, as these cars now have Electronic Brake Force Distribution (EBD.)
Essentially, EBD acts as an automatic proportioning valve that redistributes hydraulic brake pressure to each wheel, optimizing braking force at each corner while maximizing traction during threshold braking. But the automatic biasing is only a minor stopgap and will not compensate for a gross difference in brake bias. This is especially so when the entire brake system has been modified with larger parts as the EBD operation is calibrated for stock brake components. In some cases, EBD might cause more of a problem, like in an overamplified brake system, which requires less line pressure for the same amount of braking force. Since EBD is not-self correcting, it might direct too much braking force to a given corner, thus making the ABS intervention work harder and longer.
Based on our street driving impressions so far, the four-wheel AP Racing brake upgrade has taken all the sponge out of the stock brake feel without much change in brake bias. The larger rotor diameter, more aggressive pad compound and stainless steel braided brake lines give an aggressive initial bite, but the pedal remains stiff and easy to modulate. This is well-suited for track applications, but may require some getting used to on the street.
Exhaust
Subaru's use of a new dual muffler design on the Impreza sedan platform is potentially troublesome for tuners. While the two tips might add a sense of value to the new car, the added weight and space constraints of the new rear suspension limit the size of aftermarket exhaust piping.
We replaced the small diameter stock exhaust system from the catalytic converter back with SPT's exhaust system. The direct bolt-on kit uses all the stock factory hangers and OEM thickness flanges so the stock exhaust flex bolts fit without any modification. The smaller, less insulated mufflers and the deletion of the post-cat resonator help reduce weight. SPT even went as far as using hollow tubing for the hangers to save weight. The new exhaust weighs 39 pounds, compared to the stock exhaust's 44-pound weight. The new 16-gauge 304 stainless steel and 409 aluminized mandrel bent tubing has an increased diameter of 2.5 inches all the way to the mufflers, compared to the 1.5-inch, 19-gauge stock B-pipe. The backpressure reduction amounted to a maximum 51 wheel-hp gain and now puts down a total of 224 wheel-hp to the ground with the K&N Typhoon air intake. One downside of the larger exhaust piping is the lack of ground clearance, especially near the rear differential, where the rear section flange comes uncomfortably close to the ground.
Next time, we'll dump most of this sissy cosmetic madness and revert to a fast, functional form. n
Pioneer's AVH-P4900DVD head unit is cheap, plays DVDs, won't break easily and has full video input/output capability. It was perfect for our SEMA show needs. Unfortunately, at the time of our build-up, no plug-and-play radio harnesses existed for the 2008 Impreza. This meant we had to splice together every single wire by hand to make it work. | 
Our 250W-max Pioneer 6.5-inch TS-C160R front component speakers (three pounds versus 2.25 pounds stock) fit easily into the door panels with a little reaming of the stock brackets. The tweeters fit without any troubles. Note the foam surrounds on the stock speaker brackets (right), which seal the speakers to the door panels. If you want to spend more cash, custom-cut MDF templates can also be used. | 
We yanked an MDF subwoofer box from our colleagues at Car Audio & Electronics mag and mounted our Pioneer Premier 10-inch slim subwoofer and amplifier. This ended up being the lightest and most easily removable solution. On top of the subwoofer is the remote amplifier gain knob, which is useful for scaring small children and innocent bystanders. |