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Originally Posted by kikkegek
Some say cold air holds more oxygen, so you get more power from each stroke of the engine.
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That is true.
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Some say warm air performs less, so you have less power, so you get better mileage.
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Not really true, but could be viewed that way.
Colder air does hold more oxygen, so it has the potential to create more power. But to make that extra power you have to use extra fuel. Hot air, on the other hand, uses less fuel (at a given throttle opening) and therefore makes less power, but you have to open the throttle further and use more air/fuel mix to make the same power. Quite the conundrum.
The ecomodding communities have been going back and forth over this topic for years now. Hotter air gives fewer pumping losses, but colder air is more efficient. Computer retarded timing removes any benefit of a hotter mixture, but computer controlled injectors can create an always-stoich mixture. Cold air holds more oxygen, but hot air atomizes fuel better. Back and forth, on and on. Unfortunately, there are a LOT of factors at work here, beyond just the temperature of the air.
Cold air generally makes more power at WOT because it allows/requires more fuel and because the carb/throttle body butterflies are no longer the primary restriction on the engine. At anything less than WOT the jury is still out. You
may see an increase in mileage using cold air, but you
may not. It depends on tons of factors like fuel vaporization, length and restriction of intake tract, engine operating temperature, etc. But something to keep in mind is that air that's too cold doesn't atomize fuel well, so cold intake air can often lose power over warm air.
There just simply isn't a "cold air is more fuel efficient" or "warm air is more fuel efficient" statement that can be made. If you're looking for fuel efficiency you should first look at timing, gear ratios, lubricants, exhaust efficiency, etc. before you worry about intake air temp being any sort of a factor at all.