2006 Subaru WRX

Back when we still had our bone stock 2006 Subaru WRX project car we undertook a little intake test to see just how aftermarket air intakes work. The question wasn't so much whether an intake made power or not but was more about how it made power. We tested the stock intake box, an Injen cold air (or long runner intake assembly), and a K&N Typhoon short intake on Crawford/i-Speed's Dynapack dynamometer. Each intake was tested with the ECU cleared of all its learned long term fuel trim, boost and igniton timing adaptations and dyno'd three times to let the car learn on the dyno at WOT, allowing the ECU to learn only performance oriented mapping trims. The final pull was again with the ECU reset, just to confirm if anything else had changed. The runs for the stock curves were the baseline performance curves, taken with the car pulling into the shop after running many street miles on the stock ECU. After a hard reset and a "dyno" learning cycle, we see the "Stock Learned" curves are more performance oriented. Physical measurements of each intake's external and internal geometry were also made to see if the power gains were from flow considerations in the hardware itself.

For each pull, we logged every single channel available from the ECU. The most important of these that would affect power output would be mass air flow, ignition timing, intake air temperatures, air/fuel ratio, and, of course, boost. We didn't really care about additional external sensors since all that really matters to a car is what the ECU sees in its own little microcosm. -JC]

Since the inception of the internal combustion engine, one tenet has always been true for engine performance: the more air you can flow into an engine, the more fuel you can burn. In the early days of engine tuning, when engines were controlled by mechanical means through carburetors and distributor caps, this tenet held true. Bigger carburetors were used to allow more air to flow into the engine and, subsequently, carburetors were re-jetted to get more fuel in. Nowadays, the same tenet does not necessarily hold true in modern cars with their advanced engines and complex engine control systems. Manifold air pressure (MAP), mass air flow (MAF), exhaust gas oxygen (EGO), and other sensors monitor today's engines very closely, requiring smart choices when tuning and modifying your sport compact car.

Herein lays the premise of an aftermarket intake system - increase air flow to increase fuel flow and thus, overall engine performance. Most intake systems do this by tuning the engine to take advantage of resonance to ram air into the cylinder. Using acoustic models, like Helmholtz resonators or the pipe organ theory, we see that changing intake pipe lengths changes the resonance frequency and thus, the engine speed when the intake system is tuned.

Based on these models, long intake systems lead to lower resonant frequencies, while short intake systems lead to high resonant frequencies. Using a 2006 Subaru WRX as our test platform, the long intake system (a.k.a. cold-air intake) has a tubing center line length of approximately 30.75-inches, while the short intake system (a.k.a. ram air) uses a tubing length of approximately 7.75-inches. The stock system will set the baseline for the comparison with a tubing length of 16.50-inches.

Horsepower
Ignoring the power output of the stock system with the long-term trims (most likely learned for emissions and fuel economy purposes, not for performance, since most street driving is not done close to WOT), the maximum horsepower output for the two aftermarket systems and the stock system were very close together, with the average spread being only about 3whp. Assuming the dyno is correct and repeatable, we see that the ram air style intake system from K&N has the highest peak power, followed by the Injen system, and then the stock system: