Originally Posted by Phimosis

The computer uses a mass air flow sensor in the intake and O2 sensors in the exhaust. It tells it how much air is going into the engine, which then allows the computer to decide how much gas to send in with it. If it is too much fuel, the O2 sensor will detect that it is too rich and decrease the dwell time on the fuel injector to lean the mixture. That way the A/F ratio stays around 12.0:1. This does not make the most power! The best ratio is around 14.0:1 air to gas ratio, but that mixture runs quite hot and you can melt pistons in a performance engine. You can get away with that lean mixture in a Toyota Prius, but not in an angine that can spin 7,500 rpm and make close to 100 hp/liter.

With that said, if youre in 120 degree heat, the air is less dense and the MAF sensor will detect that. It will decrease how much fuel is injected to maintain that 12.0:1 A/F ratio. You'll make less power and use less gas.

If you put headers, exhaust and intakes on your car and gain 40hp, it's because those less restrictive parts are letting more air into the engine. The MAF will sense that and increase how much fuel is injected in order to maintain the 12.0:1 A/F ratio. Then, Viola! More power at the expense of more gas.

If you are getting better gas mileage, it's either because 1) you're making less power. Or 2) your A/F ratio is running leaner than what Nissan wants it to and may be at risk of damaging your engine.

I too learned the addage that headers will decrease backpressure and increase fuel mileage, but that was in the 70's in an era of carburation and restrictive intake/exhaust setups. In those cars, going to a free flowing exhaust would, in fact, lean out the jetting and you would get more power per fuel used because the carburator can't adjust the A/F ratio unless you manually change the jets in it.