cold air is better

cold air particles=more dense

hot air particles = less dense/spread out

the more air you can cram into your combustion chambers = the better/more efficient combustion will be. if you get say a cubic foot of hot air into a space, and cubic foot of cold air into the same space, the cold air will have more mass/more particles because it is more dense.

thus also explaining why forced induction systems work so good. you can get TONS more air into your cylinders, therefore the combustion is alot more complete than naturally aspirated engines. this results in alot more horsies