So with our last go around of Project GD STI we had reached our goal of 500 wheel hp but we were not entirely pleased with our powerband. Our power peaked at 5800 rpm and fell like a rock after. The powerband was somewhat narrow as well. We wanted more area under the curve. So we had gotten and installed Precision Turbo’s 58mm inducer, the newest version of their excellent drop-in, stock replacement ball-bearing turbo.
Why were we trying to push the limit of stock position turbos instead of going to the standard of Subaru high power, the rotated mount? What a lot of people don’t realize is that going to a rotated mount turbo kit is more than just buying the turbo kit and putting it in, at least when you do it correctly in a way that won’t burn up a bunch of stuff when running on the track. Basically, you are putting the turbo where it was not designed by Subaru, especially from a heat management point. We have found that in track driving that the turbo will melt anything made of rubber or plastic up to 24″ away even with heat wrap and heat shielding.
This means that you end up relocating stuff like your wire harnesses and power steering lines as well as heat shielding them and many other parts in the area. That is if you do it right. We hardly see any rotated mount turbos installed with this sort of detail which might be ok for many years on the street but will be trouble in a session or two on the track. We prefer to cleanly use the stock heat shielding and take advantage of OEM engineering when it comes to dealing with the heat in this area even if it costs us a little in power. What we are doing here by keeping the turbo in the stock place is showing we can make more power with less lag than most people can handle while keeping everything factory original.
Since our old 1300cc injectors were topped out running at over 95% duty cycle we replaced them with some injector dynamics 1700cc injectors to give us more needed fuel.
We returned to our favorite local tuners Church Automotive Testing to have Shawn Church dial in our ECU for the new parts and to see what our engine would do with the new parts. Unfortunately, this was one of the hottest days in history in our area and the temperatures were well over 100 degrees which would preclude us from setting huge power numbers.
What we liked about tuning on this scorching day is that would nearly be a worst-case scenario of what the car would see in actual use. We also wanted to make our low-boost track map a little more conservative. We had run a track day at Willow Springs raceway on a high 90-degree day and had our coolant temp hit 220 degrees after about 4 laps where we had to back off. We had been running on 91-octane crappy California Pump gas as there were no E85 stations near the track so this was another worst case. E85 would have dropped our temps at least 10 degrees. Shawn first worked on improving our cold start and idle, then went to work on the pump gas side of the low boost map. He ended up running very close to the native wastegate boost of 12 psi and our engine was making about 4o5 whp, adding some ignition advance we made around 420 whp which was about 20 hp less than our previous thermal control pump gas track map. With even less boost and more spark advance (more spark advance makes the engine run cooler within reason), the engine should run much cooler under track conditions than before. Considering that the stock STI’s boost is 14.5 psi and we measured 245 hp to the wheels on this car when we started the project, we are making 175 whp more power than stock with 2.5 psi less boost than stock! This shows just how efficient our new motor is!