
The Parts
Our first objective is to make more power and torque by further improving the exhaust flow. Although gains in the WRX were made with an aftermarket cat-back exhaust, there was still a restriction in the exhaust to contend with. The factory downpipe ensures emissions compliance thanks to its inline catalytic converter. However, it also poses a bottleneck in the exhaust flow. To illustrate the benefit of a free flowing turbocharger, we turned to TOMEI Powered for a solution. The stainless-steel downpipe features a two-piece stainless-steel construction composed of 76.3mm piping with a slip fit joint for reduced weight. This downpipe weighs in at just under eleven pounds, less than half the weight of the stock unit.

Test 1: TOMEI Powered Downpipe
Swapping out the downpipes on the WRX will typically take between two and three hours. You may need to modify a wrench to reach all of the bolts on the bellmouth of the downpipe at the turbocharger, as Subaru uses a stud with nuts on both sides to secure the downpipe. We took a box end wrench and ground it down until it was thin enough to fit around the nut on the turbo flange. The larger diameter tubing and straight through cat-less design of the TOMEI Powered downpipe provides a more free flowing exhaust path leading from the turbocharger to the cat-back system. In conjunction with the downpipe install, we downloaded (from the Cobb Tuning website) a flash tune optimized for a bolt-on downpipe. On the dyno, the downpipe made measurable gains, increasing peak output by 15.75, to 289.28 whp. Horsepower gains on the WRX were observed from 4,500 RPM through redline, with the greatest gain of 29 horsepower noted at 6,600 RPM. While peak torque only increased by just shy of one lb-ft , noteworthy gains of up to 19 lb- were realized from 4,500 RPM through redline.The TOMEI Powered downpipe weighs a lot less than stock, and makes 15.75 more peak horsepower when paired with a Cobb AccessPort flash. The greatest gain of 29 horsepower occurred at 6,600 RPM. Keep in mind, this is a cat-delete pipe and is not street legal.
Test 2: Perrin Performance Intercooler

The Perrin Performance Front- mount Intercooler produced the greatest gain, raising peak power by 31.51 horsepower and realizing a maximum gain of 49 horsepower at 5,700 RPM. Bill Knose of Delicious Tuning adjusted the fuel and ignition tables using a Cobb AccessPort.
Test 3: TOMEI Powered Equal Length Headers
Moving on to the exhaust manifold, expect to invest at least three to four hours for the installation. In order to remove the stock manifold and up pipe, it is necessary to drop the sub frame. The TOMEI Powered equal-length manifold featured spot on fitment and a clean presentation. Although the unequal-length manifolds are generally considered to be more responsive, the mistimed exhaust pulses cause the engine to sound like its only firing on three cylinders. Proponents of this sound refer to this as the “Boxer engine rumble.” Equal-length runners do not suffer this condition, and since the exhaust pulses are channeled in order through the 4-2-1 manifold design, the turbocharger showed marked improvement on the dyno. After Knose optimized the tables once again by way of the Cobb Access Tuner so ware, the peak boost pressure actually dropped by 1.6 psi, yet the engine produced additional horsepower from as early as 2,250 RPM through 6,000 RPM. The broad RPM range and gains throughout o er usable power without any perceptible loss in response. The greatest horsepower gain of 35.4 and the greatest torque gain of 43.77 lb-ft both occur at 4,250 RPM. Peak power of 327.48 whp was noted at 5,970 RPM, while peak torque came in at 341.53 lb-ft at 4,200 RPM.The TOMEI Powered equal- length exhaust manifold with up pipe helped to increased power and torque production without any loss of response and without sounding like it’s running on only three cylinders.
HP Level | HP Increase | % Increase | Total | |
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Baseline | 273.53 WHP | – | – | – |
Test 1 TOMEI Downpipe | 289.28 WHP | 15.75 | 5.8 | 15.75 |
Test 2 Perrin Performance Front Mount Intercooler | 320.79 WHP | 31.51 | 10.9 | 47.26 |
Test 3 TOMEI Powered Equal Length Manifold | 327.48 WHP | 6.69 | 2.1 | 53.95 |
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FINAL POWER | 327.48 WHP | – | 19.7 | 53.95 |
The Bottom Line
The downpipe, intercooler and manifold collectively added 53.95 horsepower, raising peak power figures to 327.48 whp. That’s a 19.7 percent leap in output compared to the previous figures that the WRX put up after the first Test & Tune segment. In the next installment, the WRX will get a fuel delivery upgrade composed of a pump and injectors. Then it will receive a turbocharger upgrade and a retune. When we started this Test & Tune session on the GVB WRX, the goal was to aim for the 340 horsepower mark on 91 octane fuel. Without the turbo and fuel system upgrades, the WRX is only 12.5 horsepower from reaching its goal. The new question to ask is “How much will the WRX end up making?” Stay tuned.
Parts Price List | ||||
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TOMEI Powered Downpipe (#431106) | $390.00 | |||
Perrin Performance Front Mount Intercooler (PSPENG436) | $1,700.00 | |||
TOMEI Powered Equal Length Manifold (#193105) | $790.00 | |||
* * * * * | ||||
Total MSRP | $2,880.00 | |||
DSPORT Street Price | $2,448.00 – $2,592.00 |