Power Pages: Subaru WRX 02/02

In the first part of our Power Pages WRX we explored the effects of adding an air intake, exhaust and fuel tuning. We learned that the WRX in stock trim runs a very rich 10.4:1 air/fuel ratio, and getting the engine to breath easier has a big effect in the horsepower department. So far we have also learned that the water boxer engine is not too difficult to enhance with bolt-on components. As the cylinder heads are parallel with the vehicle's frame, we will soon discover how difficult it is to play with different camshaft profiles in an upcoming segment.

The Engine
With the addition of the XS Engineering air intake, exhaust and A'PEXi AFC we found an additional 21.8 hp and 22.8 lb-ft of torque. This round we go deeper into the power within the EJ20's turbocharger. The factory unit is an IHI pumping out a maximum boost pressure of 11 psi. Its size is very deceiving; it may look small, but this baby was able to put out an incredible 190 hp at the end of our last session.

Performance Components
Since the vehicle is already equipped with an air intake, exhaust system and fuel computer, it was time to see what the stock turbo was capable of producing. The turbo has already been given some breathing room with the above mentioned components, so we decided to open up the exhaust system even more with an XS Engineering front pipe/mid pipe comb while upping the boost level to 15 psi using an A'PEXi AVC-R boost controller.

The mid pipe is offered with a 2.75-in. tubing diameter. The bolt-on unit is constructed of 16-gauge mild steel to take on the extreme heat produced by the turbocharger. All the bolt-on flanges are made of 3/8-in. steel, and the factory O2 sensor bung compliments the factory location for the sensor. This unit also removes the second catalytic converter in the three-cat system.

Just like the XS Engineering front pipe, the mid pipe removes one of the OEM catalytic converters, leaving only the first cat out of the three left in the exhaust system. This unit is also constructed of 16-gauge mild steel and 3/8-in. flanges. To ease operating stress on the pipe, XS incorporated a flex pipe to take on the engine's movement.

Another valuable power-producing component, not new to Power Pages, the A'PEXi AVC-R boost controller will play an important role in upping the factory boost levels on the WRX. With so many options built in to this unit, we're able to control boost by the change of gears, speed and engine rpm. The AVC-R can also be used as a textbook boost controller, which is exactly how we used it on this test vehicle.

Dyno Testing
From our first three installments, the end result in horsepower gains was measured at a maximum level of 190.6 hp, which was realized in the 6000 rpm range. The WRX redlines at 7000, but the max horsepower level was always found in the 6000 rpm range. So, with our next three installments, we were hoping that the power would pick up after 6000 and pull all the way to 7000 rpm.

In most cases, the front pipe and mid pipe would have been installed first, but with these bolt-on pieces opening up the turbo's breathing room most of the power is found due to the boost increase of these components removing the factory catalytic converters. Since we installed the boost controller first and turned boost up to 15 psi we made sure that after adding the front pipe and mid pipe the boost will not surpass the 15 psi mark. With the boost controller installed and set at 15 psi, the turbo was able to greatly increase power in the high rpm area, which aided the 6000 rpm mark where the turbo's power fell off on our baseline test. Max horsepower jumped to 210.6 hp, with most of the power increase showing from 4000 to 7000 rpm. Torque levels also saw a great improvement, jumping up to 196.3 lb-ft, almost surpassing the 200 mark.

Now that we know boost would not go higher then 15 psi without us physically turning it up, the front pipe was installed and tested. The front pipe provided a peak gain of 7.1 hp over our previous run while actually breaking the in the 200 lb-ft mark with 202.4 lb-ft of torque. Although the peak provided a high gain, most of the power was realized in the midrange, from 3500 to 5000 rpm, where we saw as much as 10 horsepower.

Basically, the mid pipe can also be considered a straight pipe. Straight pipes and turbochargers go hand in hand when making power. Since the mid pipe removes the third cat, we knew midrange power was going to make the difference. Just like the front pipe, the mid pipe was able to pick up plenty of power, ranging from 5 to 13 hp, while still able to produce an additional 2 hp at the peak. Torque levels were extremely high landing an additional 20 lb-ft of torque in comparison to our previous run.

Conclusion
A peak gain of 219.7 hp at 6000 rpm and a maximum of 222.4 lb-ft of torque at 3800 rpm were discovered after our three new power adders. Overall power gained was 29.1 hp and 34.8 lb-ft of torque. This was all accomplished on the stock turbocharger. So, as it turns out, w were able to greatly improve power after 6000 rpm. For our next session, we will see what advantages we can discover by swapping out the O.E. intercooler with a front mount unit. We will also be looking into what we can gain from a replacement turbocharger. Since this time around we had to extend cool-down time as our horsepower levels increased and detonation became audible, we believe we should see a significant improvement from adding a more efficient intercooler.