We had installed an AEM EMS back in
Sept. 2008 on this 2008 STi. Customer asked for more power with minimal turbo lag and we recommended a Tomei ARMS 7760 turbo along with RC750cc injectors & re-tune. Below is the dyno graph for both setups. We achieved 306WHP peak HP compared to 274WHP on OE turbo both running 17psi boost. The spool characteristic of the ARMS 7760 turbo is similar to OE but maintains 17psi at the redline.

After looking at the air intake temperatures (sensor located right on the Top mount intercooler), we decided to switch to a front mount intercooler as the temps climbed up to 190F. When cold, sensor was reading 70F, by the end of the run it was at 170F-190F range. We custom fabricated a front mount intercooler kit after looking at the options that were on the market. We managed to cut some corners and the turbo lag compared to TMIC was only 200RPMs!!


We gained 55ftlb torque & 50WHP at 4000RPM (see above dyno graph), and 9WHP peak HP and hit 315WHP.
Intake temperature issue was also solved (the sensor is now right on the charge pipe 2" away from throttle body) with minimal turbo lag.
Upon delivery of the car, customer was not happy with how his OE intake manifold looked compared to the bright red intercooler piping, decided to order a Cosworth intake manifold and have it powdercoated bright red to match piping. After install, we had to retune the car, it ran richer unexpectedly, and we had to take about 8-10% fuel and the power above 3500 RPM dropped about 20-25WHP. Cosworth manifold would have made extra power if we were running a GT30R or bigger turbo along with headwork. Looks like this is not a good choice for OE or smaller bolt on turbos.

One thing to look at is the 225WHP (300 crank HP) mark on the dyno charts. Subaru USA states car makes 305 crank HP @ 6000RPMs.
our 225WHP tests:
OE turbo + TMIC 5000RPMs
Tomei turbo + TMIC 4300RPMs
Tomei turbo + FMIC 3700RPMs
Tomei turbo + FMIC + Cossy int.manifold 4000RPMs