Fuel Tuning
One of the most important and basic engine tuning aspects is dialing in the AFR or air-fuel ratio. This is the proportion of fuel-to-air in the combustion chamber at the time of ignition. Even slight changes in the AFR can have a dramatic effect on power output, drivability and emissions. An AFR of 13:1 means that there is approximately 13 times more oxygen going into the engine than fuel; hence the "thirteen-to-one" ratio. For a given amount of fuel, you need a set amount of air to burn it completely. When the two are present in a ratio that burns both completely, it's called a stoichiometric mixture, or stoich (14.7:1). This is why a stoich mixture is considered "ideal" AFR for gasoline engines.
Unfortunately, 14.7:1 is no magical AFR that will net maximum power. Ideally, an engine should be tuned to have different AFRs under different engine loads and RPM.
Chances are, you've heard the terms "rich" and "lean" before, but do you really know what they mean? It's pretty simple, actually. A higher AFR is a lean mixture, a low AFR a rich mixture.
A rich mixture is one in which there is more fuel present than there is oxygen needed to completely burn the mixture. Slightly rich mixtures leave extra fuel in the chamber and can be good for cooling the intake charge and warding of detonation, but on the downside, rich mixtures can cost us horsepower, decrease mileage, increase harmful emissions, and leave carbon deposits on the valves and combustion chamber. In extremely rich mixtures, the excess fuel can even run past the cylinders and rings to contaminate the oil and possibly damage the cylinder walls and rings.
A lean AFR, on the other hand, is one in which there is more air present than needed to completely burn all the fuel, leaving extra oxygen in the charge. This increases the likelihood of all the fuel being burned and is why mileage increases and emissions are generally lower. On the downside, lean mixtures burn more slowly and at higher temps, which can cause burned valves, detonation and other problems, especially under high load conditions (wide-open throttle).
Tuning the fuel boils down to setting some targets for your AFRs and doing your best to hit those targets. For naturally-aspirated cars, AFR's should usually range from 14.7:1 at idle and very light throttle, 14:1 to 13:1 at part throttle, and around 12.5:1 at wide-open throttle.
The AFR targets for a forced-induction car are a little different. Normally, the off-boost targets will be the same as naturally aspirated engines, but under boost they usually are tuned to run rich, which leaves extra fuel in the chamber for cooling and reduces the chances of detonation. On crappy pump gas (91 octane), most tuners we know tune using conservative AFR targets under boost (around 11:1 to 12:1).
There are different methods for tuning fuel delivery, but all work in one of several ways: increasing the fuel pressure (which increases flow), increasing injector size, modifying the signal to the injector or adding injectors. The first is normally done via a rising-rate fuel pressure regulator, the second is obvious and the third is done electronically by magical boxes that cost a small fortune and the fourth is a no-brainer. We've listed them along with a few terms and parts that are essential to tuning fuel.