Anybody that raises their FP and expects the car to run richer is a fool (unless they have the necessary programming changes). The car will run rich until the adaptive fuel trim cells are updated, and then the trims will be applied to closed loop AND open loop.........at which point your car will run leaner when your foot is on the floor, guaranteed. Everytime I see the magazines say, "you'll need an adjustable regulator to dial in your AFR", I roll my eyes.

A gasoline engine burns gasoline in the presence of oxygen (see How Car Engines Work for complete details). It turns out that there is a particular ratio of air and gasoline that is "perfect," and that ratio is 14.7:1 (different fuels have different perfect ratios -- the ratio depends on the amount of hydrogen and carbon found in a given amount of fuel). If there is less air than this perfect ratio, then there will be fuel left over after combustion. This is called a rich mixture. Rich mixtures are bad because the unburned fuel creates pollution. If there is more air than this perfect ratio, then there is excess oxygen. This is called a lean mixture. A lean mixture tends to produce more nitrogen-oxide pollutants, and, in some cases, it can cause poor performance and even engine damage.