[Preorbtat]

This topic has been addressed many times in a car forum i frequent.

Both answers are right. With a standard drop in filter, no you won't see much of a difference if any because you are still getting the same warm underhood air and using the same intake piping.

As for increasing fuel economy, yes, some filter systems do indeed do it. It's not the fact that the MAFS (mass air fuel sensor) is reading how much air is incoming, but how much it is reading. A MAFS does not read 100% of the incoming air, just a small portion that gives a general idea of how much is incoming. air does not flow uniformly when traveling through the intake, more like a fluid.

because of this, the MAFS usually won't read the correct amount of air. if it reads more then it should in comparison with the whole amount of air incoming it will add more fuel to compensate which causes a rich burn.

But, the best fuel to air mix is toward the slightly lean side. Almost all car manufacturers won't set up the ECU to run the engine this way to make the engine for a bunch of reasons. one being that with a lean fuel mix, you run the risk of more premature ignition and spark knock. So the ECU's are usually set up to run far away from this fuel:air level.

Going back to the MAFS, if it reads less then it should in comparison with the whole air going through the intake, then the ecu will think there is less air going in then what is really going in. thus supplying less fuel and running the engine more toward the lean side. this will give you a slight bit more power using less fuel then normal.

But if your car does start running to far lean, most cars will give you a check engine light because the O2 sensors on your Cat will notice to much O2 in your exhaust.