air filter upgrade

Engine control systems sense air mass, not density. That's how they control the mixture so well.

Well... the ones with mass airflow type do. My '02 OBS employes an alternate MAP-type system which uses manifold pressure, atmospheric pressure, air temperature, throttle angle, and vehicle speed to guess at how how much air is being injested without actually measuring it. The cleaner-running '05's have gone back to the mass airflow type, which is clearly the superior system whenever tight control over fuel delivery is a requirement.