The nice smooth and powerful running is all to do with the volumetric efficiency. The most effective and economic result of the chemical reaction that gives the power takes place where the combustion partners are in the ideal ratio by weight. So, when the density of the air is greatest, the weight of air drawn into the cylinder is correspondingly greatest. Then the mixture approaches the ideal stoichiometric ratio, a mixture strength where every hydrocarbon molecule can find a corresponding oxygen molecule partner. More hydrocarbons left unreacted...too rich mixture, wasting fuel. Too much air... weak mixture, poor power, possible overheating, engine damage. The ratio for gasoline varies for true complete efficient combustion, depending on fuel blend, but is around 14.7/1 air/fuel.
This ideal scenario is best experienced on a warm, but humid, generally Summertime late evening run. Your motor runs smoothly, the World is in Harmony, your worries seem far away. Oh, forget all that theory, any clanker runs better after a couple of beers.
Ain't that so berger?
Swarfy.