Project Subaru STI Part 6

The turbine housing inlet is ported out to 52mm, the size of the typical aftermarket bigger diameter uppipe. The bypass port for the internal wastegate in the exhaust housing is unshrouded and enlarged to improve flow and help reduce the chances of boost creep.

Unlike many "upgrade" turbos, the FP Green uses the correct compressor and turbine housings for the wheels with the correct volute and diffuser configuration. This ensures that the wheels work at their design efficiency. Many upgraded turbos put huge wheels in the stock housings and if you look inside the housing of a turbo upgraded like this, the big wheels carve into the housings diffuser, sometimes leaving the housing with little or no diffuser at all. These sorts of upgrade turbos usually have a compressor efficiency of less than 50 percent and have compressor discharge temps of over 300 degrees! Not the best thing for your motor.

Turbines upgraded like this often have a less than optimal approach angle for the exhaust gasses impinging on the turbine wheel. This kills the turbine efficiency. It produces a wheel that flows well, but produces much less shaft power - making for a very laggy turbo. A poorly upgraded turbo often has only slightly more power than stock, and sometimes even less with quite a bit more lag. Forced Performance uses the proper housings for its bigger wheels so the compressor and turbine can function as designed.

The center section of the turbo is reworked as well to help reliability at higher boost pressures with FP's vastly improved thrust bearing. Thrust bearing failures in my experience are the most common ones experienced in well set up high-performance turbo systems, especially when running over 15 psi of boost. FP gets around this by doubling the oil flow to the thrust bearing and installing a thrust bearing with 3x the surface area as stock.

Finally FP changes the wastegate actuator with a part with a cracking pressure of 15 psi vs. the stock actuator's 6 psi. This usually results in much faster spool and better throttle response as the wastegate will stay closed until the boost pressure is closer to its rated pressure.

The FP turbo came ready to bolt on with all of the required fittings needed to adapt oil and water lines to its center section. We did, however, need to remove the stock TGV valves from the intake manifold. TGV valves are devices that Subaru has installed in the STi's intake manifold. They are butterfly valves that induce a swirl into the intake flow at partial throttle for emissions purposes. The trouble is that these valves also block about 40 percent of the manifold's intake port; and the actuator for the valves hits the compressor housing of the FP turbo.

We removed the valves and the actuators; welded up the holes for the valve shafts; and ported out the passages for the valves, converting the intake manifold to a straight unobstructed passage to the cylinder head. From what we have been told, this operation frees up about 5 to 8 wheel hp and improves throttle response. From looking at the blockage the TGV valves create, we believe it.

The 3-inch inlet of the FP turbo presented another issue: the stock intake hose would not connect to it. The stock inlet was also a highly restrictive piece, full of kinks and necked-down areas that would restrict flow. At FP's recommendation we contacted Perrin Performance for one of their inlet hoses specially designed for the FP Green's 3-inch inlet.

The Perrin inlet is molded from thick multi-ply silicone. It eliminates the kinks and neck-down effect, improving flow. It also connected directly to our XS Engineering air intake with little effort.

Perrin also supplied us with several of their pieces to help us with our performance goals. First was their 3-inch stainless steel downpipe. The Perrin downpipe has a divider that separates the flow from the turbine and the wastegate. The wastegate discharge flow is separated from the turbine flow and reintroduced into the exhaust farther downstream. This improves flow for both the turbine and the wastegate port and helps with turbo spool and response.

Subarus, especially track driven ones, have a problem with oil consumption and blow-by contaminating the intake charge. To prevent this, we installed one of Perrins' catch cans on our car. The Perrin can separates out oil from the blow-by gases and traps it before it can contaminate the intake charge. This helps reduce oil consumption and reduces the chance of detonation due to oil contamination of the intake charge.

When running our racing downpipe with no cat at the track, we used to get an annoying check engine light for an unresponsive rear O2 sensor. We installed Perrin's rear O2 sensor emulator to get rid of this problem. The Perrin emulator fools the ECU by sending the ECU a simulated signal of a properly working 02 sensor. No more check engine lights.

To help us stay cool on the track even on typical 120-degree Willow Springs days, we installed an ARC radiator and an XS Engineering 160 degree thermostat. The ARC is a racing all-aluminum radiator with nearly twice the cooling capacity as the stock unit. Now our coolant temps stay within acceptable ranges. We feel that running our engine hot on the track probably contributed to its current oil burning state. Now our water temps will stay more stable.

After XS Engineering reflashed our ECU with one of their off-the-shelf programs for a parts combo identical to ours, it was time for us to go out and see what she could do.

With our new group of parts, we were more than impressed with our test drive. The acceleration pinned the driver to the back of the seat, the nose of the car reared up and the steering got light. Sometimes there was wheelspin from the four tires. It was amazing. It felt like a genuine 12-second street car. It felt like we gained 50 to 60 more hp. The negatives were minimal. Due to our engine's large 2.5-liter displacement, the close spacing of our 6-speed gearbox and our light flywheel, the increase in turbo lag due to our larger turbo seemed minimal. Our drivability remained so good that our package seemed like it would be something the factory would offer in a higher output STi option.

The gains were so great that we were mulling over suspension changes to match the suspension to our newly increased power. Unfortunately, due to scheduling, many weeks and a few track events passed between our installation and the dynoing of our parts. The owner of the car was complaining that his car felt weaker and was burning more oil. We drove it and the car did indeed feel weaker.

We rushed a dyno session with XS Engineering, and found that we were only a few hp greater than our previous test. To be fair to Forced Performance and Perrin, we are going to freshen our tired motor and try to get some new test number more in line to what we felt when we first installed the turbo in our next installment.