The EVO is one of those cars almost everyone buys knowing that they will eventually modify it. With the older-generation EVO VII and IX, you'll be hard-pressed to find a stock one for sale. Almost every one of them has some type of aftermarket part on it, but the X is a bit of a different story. Being a brand-new car on the market, owners are a bit hesitant to mod them because the chance of losing their warranty is high.
I can agree with that argument for about 10 seconds. Then I step on the pedal and a surge of 50-plus wheel horsepower over stock pushes me back into my seat, and I can't find any reason why an EVO X owner would putt around stock. It's just so easy to unlock so much power for so little money while never worrying about reliability.
The AEM intake that I installed costs approximately $280 and a reflash and dyno tune from Mike Welch at Road Race Engineering is about $500. You really can't beat that value. Other cars on the market won't pick up that kind of power even with thousands of dollars invested.
How and why it's possible for the EVO X to make such power with just an intake and retune was answered by my tuning session at RRE, and the subsequent dyno charts will explain.
After three consistent pulls, the EVO X put down a baseline of 261 whp and 277 ft-lbs of torque . If you look past the numbers, you can see boost and air-fuel ratio are also plotted on the chart. Granted, it's difficult to notice variations in the orange AFR line, but you can see that it runs along the 10.0 AFR mark, which is extremely rich for a turbo car. There's a reason why, though. Running that rich provides a good safety barrier (or margin of safety), so even if the car were to lean out by a full point or two (1.0 to 2.0 AFR) it's not going to knock and blow the motor up. Secondly, Mike explained to me that the stock ECU has aggressive timing maps set from the factory, hence the need for lots of fuel and its ability to cool down the intake charge. The next interesting plot to note is boost pressure (purple line). Maximum boost peaks at 4000 rpm, hitting 22.5 psi then quickly trailing off to 14 psi by redline. As you can see, there are many areas for improvement.
The decision to go with the AEM intake was made mainly because it comes with a CARB-exempt sticker (making it a legal upgrade in California) and that other than the AEM logo on the filter box, it looks stock. Don't be fooled into thinking that because the filter isn't exposed it won't make as much power as an open element. The only difference between the two is noise. If you want the constant whooshing sound from your turbo, then an open element is for you; if you've grown tired of that noise (like me) and want a low-key setup, then a setup like the AEM intake is best.
 Once installed, the AEM intake (without any tuning) instantly netted an additional 20 whp and 24 ft-lbs of torque. Very nice gains, indeed, and it's across the entire rpm range. You'll notice that the boost pressure has jumped up significantly. In stock form, 4000 rpm saw 22.5 psi whereas with the intake it hit almost 25 psi and a steady gain of 2 psi was achieved throughout the rpm range.
The rise in boost pressure can be attributed to the way the vacuum lines are plumbed with the AEM intake. Also, the larger diameter of the piping results in a leaner AFR, as noted from 5000 rpm onward. These two factors are largely responsible for the increase in power and torque.