You are what you eat, and the same goes for your engine.
You are what you eat. The phrase is at least how the public has distilled the doctor's orders to eat healthily in order to feel good. The thing is, engines are a lot like humans in the sense that they need fuel, air, and a way to get rid of the waste. Just like humans, the fuel that comes in dictates how much performance is doled out. Back in the day, carburetors used to do the job of feeding the engine so that it could belch out power. Its job was to mix the fuel and air at the ideal ratio so that it could turn into piston-pushing explosions inside the engine.
Ideally, the fuel air mixture is 14.6 parts air to one part of gasoline. To adjust the mixture, a knowledgeable mechanic or DIY owner needed to get under the hood and tweak the carburetor with a screwdriver. While this setup works to keep an engine running, it's an inefficient system because it isn't easily adjustable. What's more is that sending a precise mixture through the channels of the intake manifold is a lot like pumping pancake batter through the air conditioning vents of a home in order to try and get it to a hot pan in the kitchen. Okay, so maybe not that bad, but you get the idea. In order to make an engine more efficient, and take advantage of all the computing technology that cars carry, a different sort of injection system is needed.
Enter the fuel injection system. It comes in many forms, but the main ability of the system is to alter the mixture of fuel and air on the fly, allowing more lean or more rich mixtures to be combusted in the engine in response to changes in engine load, oxygen content of the air, temperature of the air, and the presence of a turbocharger or some other method of forced induction. The most sophisticated of the bunch are direct injection systems, which pump fuel directly into the cylinder instead of having it come in through the intake valve. Many high-performance cars like the McLaren F1 and cars with newer engines feature this technology because it eliminates almost all wasted fuel and keeps the engine running very smoothly.
This may be great for your daily commute, but what happens when you want to squeeze more power out of your engine? Aside from the more simple methods like exhaust and intake upgrades or more complex commitments like turbochargers (where fuel injectors help by adding more fuel to balance out the greater amount of air in the cylinder), there is always the water injection route. It seems counter intuitive to put a liquid used to quell fires into a machine that is purpose built to combust, but water injection can have a huge effect on adding performance to an engine in a few ways. First, the water helps to cool the incoming air, which makes the air denser. A higher air density means more air gets into the cylinder, creating a bigger explosion.
The water also helps engines with high compression ratios or aggressively tuned forced induction systems by cooling hot points inside the engine that can cause premature combustion. This makes the engine run more smoothly and in turn gives the car more power. The first production road car to feature this technology is the BMW M4 GTS, which makes 16% more horsepower than a standard M4. Like an athlete, a car's engine must take in a proper balance of nutrients in order to keep things running properly during normal use and at max capacity when on the knife's edge. The injection system is usually something you never have to worry about unless you know how to properly tune it to unleash more power and fun.