2006 Subaru WRX - Test Bench

Studying the MAF data from the ECU, we see that the "short-ram" style K&N intake system indeed had the highest power output, but it also had the perceived lowest air flow. Also, the "cold-air" style Injen intake, which is the longest intake present, did indeed have the highest peak torque, but it also had the lowest airflow of all the systems at peak torque.

Although the different geometry intakes did show some of the characteristics that we'd expect to see from theory, the fact that each intake made more power with less air means power was coming from the change in air/fuel ratio, since boost and ignition timing did not change by a significant amount. Ultimately, bolt-on aftermarket intake systems make power by tricking the ECU into thinking less air is delivered into the engine.

If the ECU thinks that there is less airflow, fuel delivery is trimmed back, making the mixture leaner. In reality, the same amount of air is being ingested by the engine but differences in intake geometry and MAF placement have fooled the ECU's reading. Compared to the OEM's super rich conservative tuning, slightly leaner mixtures will result in more power. The K&N intake made the most power by tricking the ECU into thinking the least amount of air was delivered. If the ECU was retuned for peak power, instead of reliability, our test results would be significantly different and the changes in intake geometry, (ie. piping length, diameter, flow rate) would reflect differently.