For the next step we installed DC's downpipe. Like the exhaust, it also features all stainless construction, CNC flanges, a 3-inch diameter, and clean welding. The downpipe bypasses the cat so this is more of a racing-use-only mod that is not legal for use on the street. Bypassing the cat also made the check engine light come on as the rear 02 sensor is now sending a signal to the ECU so the ECU believes that the cat has failed. A check engine light eliminator like the ones sold through Summit Racing and various other sources will fix this issue. On the dyno the downpipe boosted the power to 268-wheel hp, a whopping gain of 21 hp. This gain is more of a synergistic one shared by both the exhaust and the downpipe. Once the restriction of the downpipe was removed, the engine was able to exploit the full breathing potential of the exhaust. The gains created by the downpipe were mostly at high rpm, above 5500, with a slight 2-5hp loss below that rpm.
AEM's cold-air intake was next up on deck. Many Subaru tuning experts told us that a cold-air intake would not show much of a gain in power. We found this not to be the case. The cold-air intake places the air filter completely out of the hot engine compartment behind the front fog light where it can breathe cool dense air. It uses AEM's new ultra-efficient dry flow filter. The dry flow filter can be washed and does not require oiling. It was developed to meet stringent OEM filtering requirements and traps over 98 percent of all dust particles before they can enter the engine. This is better than any current aftermarket air filter. The AEM intake has lightweight, all aluminum construction and is tuned for best power output. The intake really worked well with the engine cranking out 284 whp, a peak gain of 16 hp. To be honest, about 6 of these hp was created in a tiny blip only 250 rpm wide at the very top of the powerband but a fat 15 or so hp was realized in most of the area from 5500 rpm on up. We had some concerns about the blip as we knew we were reaching the limit of the stock fuel system and we were wary for signs of the engine starting to lean out.
DC's beautiful header was installed next. Sharing the same 304 stainless construction as the rest of the DC pieces, the header features equal-length runners and a nice fully mitered merge collector. Merge collectors are usually found only in very expensive custom and semi-custom headers. DC's robotic mitering helps make this sort of quality available at mass production prices. The header has thick warp-resistant CNC flanges and a crack-resisting slip joint. Like an ber-expensive custom header, the slip joint is a double slip joint to make it leak free. Like a stainless race header the DC header immediately turned beautiful shades of purple and straw yellow, looking very cool.
On the dyno the header posted impressive gains in the midrange with a 15hp gain from 3000-5500 rpm. There was a slight loss in power at the very top, erasing the blip in power that the cold-air intake provided for a peak power reading of 278 hp. However, this loss of power is deceiving because it only occurred in a small 250 rpm range. The power at the top of the powerband was now getting to be inconsistent and we are worrying that this is a sign that the engine's stock fuel system is not keeping up with the flow demands imparted by these bolt-on parts.
The last of power adders installed this month is DC's up-pipe. Many Subaru experts had told us that the up-pipe would not make any more power but we were determined to test DC's piece as it was 5mm larger in diameter than the stock part. The up-pipe features the same quality of construction of the other DC parts. After we bolted it on we were surprised that it made the car quite a bit louder. Usually stuff done before the turbo, in our experience, has little effect on the noise level but this up-pipe made a bigger effect on the noise level than the downpipe did! The up-pipe boosted our power level to 285 hp. It also restored the 250 rpm blip in the powerband. The downpipe caused a small 2-3hp loss below 4500 rpm and created a 3-5hp gain above 5500 rpm. A small gain but a gain nevertheless. One thing for certain, in XS Engineering's experience we are right at the maximum power level that the stock injectors and fuel pump can support. We cannot ask much more from the engine until those issues are addressed.
We were quite impressed that our simple bolt-ons freed up 39-wheel hp. With indications that we are running out of fuel we look forward to what can be extracted next. In our next installment, Eric Hsu will work his tuning magic on the ECU as we upgrade the fuel system for more boost and power.