* Owner: William Chon
* Dyno & Facility: HB Speed Dynomite Dynamometer
* Accessport Tuning: Jon Drenas
Pros
The '08 Subaru Impreza STI, powered by a newly revised 2.5-liter Boxer engine, is rated at a factory 305 hp and 290 lb-ft of torque. Among many of the changes from the previous model STIs include an increase in power output, thanks in part to variable-valve timing on both the intake and exhaust cams.
Cons
'07-'08 Subarus (and some '06 WRXs) were tuned from the factory to run stoichiometric air/fuel ratios (14.7:1) through peak boost to about 4,000 rpm-great for fuel economy and emissions, but bad for power and drivability. These '07-'08 ECUs also have multiple timing tables that add, subtract or even multiply the timing values each motor runs. There are tables within the factory ECU that can add or subtract to themselves, or multiply the total timing values (bad). "If the vehicle is not properly tuned," begins HB Speed owner and tuner Jon Drenas, "the ECU can take the tuner's intended timing value, let's say 11 degrees at peak torque, and using knock feedback and other algorithms, add 8-10 degrees-effectively running 19-20 degrees. The problem is that the ECU is not quite fast enough to pull out that timing if it knocks hard (which it would) and eventually would blow the motor."
Notes
As with any vehicle tested in 2NR, we performed routine maintenance, including an oil change and spark plug check/replacement, before strapping onto the dyno. In stock trim, the '08 STI delivered 245 hp and 262 lb-ft of torque at 15.13 pounds of boost.
Pros
The Perrin exhaust's polished, 3.5-inch quad tips are fed through three-inch piping to minimize exhaust restriction, while its resonated center section prevents a monotonous drone when driving on highways.
Cons
Removal of the factory exhaust requires plenty of time and patience. Ask for assistance when installing the new exhaust to help speed up the process.
Parts
Center section, mid-piping, rear section, V-band clamps, mounting hardware, instructions, warranty
Tools
10-, 12-, 14- and 17mm socket, 10-, 12-, 14- and 17mm open end, ratchet, extension, crescent wrench and WD-40.
Installation Time
60 minutes
Notes
Perrin claims an increase of 8-10 hp/lb-ft of torque using their cat-back system as a simple bolt-on, but we decided to take things one step further, and tune with a Cobb Tuning AccessPORT. With Drenas at the helm, the STI posted a gain of 30 hp and 45 lb-ft of torque over baseline, with boost peaking at 16.67 psi. The largest gains were recorded at 4,400 rpm to 5,200 rpm, where the STI posted an improvement of 55 lb-ft of torque and 62 hp at 6,200 rpm.
Pros
The Invidia V2. downpipe features a large, four-inch stainless-steel bell-mouth and divorced wastegate. Using its one-piece design eliminates the factory catalytic converter to improve horsepower and boost response. The unpolished bell-mouth design offers a subtle look that's sure to ward off unsuspecting eyes that would otherwise be snooping around in your engine bay (i.e. cops).
Cons
The fitment of the downpipe was a straight forward affair, but the factory gaskets will need to be replaced, as they are prone to cracking or ripping upon removal, causing massive exhaust leaks.
Parts
Downpipe
Tools
12-, 14- and 17mm socket, 12-, 14-, 17- and 22mm open end, ratchet, extension, crescent wrench and WD-40
Installation Time
50 minutes
Notes
To show the true potential of the downpipe, we installed the unit without tuning the AccessPORT, and noticed a gain of 13 hp and 15 lb-ft of torque from 2,000 rpm to 5,700 rpm over our previous run. At 3,500 rpm, the EJ25 netted 15 hp and 25 lb-ft of torque, but also lost power from 5,750 rpm to redline. The drop-off in power is a good example of why additional tuning becomes critical when installing aftermarket products.