The Physics of Injectables: Hypodermic Horsepower
Shooting up a little extra performance isn’t just for cyclists and baseball players, you know.
Engines are designed to run on commonly available fuels, but as every street racer knows, injecting a little something extra can help performance. (Certain athletes seem to have picked up on this concept, too.) Auxiliary injection systems provide tremendous power gains in a relatively easy-to-install package. What is injected generally falls into one of three categories: coolants, oxygen enhancers, or additional fuel. Here’s how they work.
MAYBE TRY THIS AT HOME: Your engine is designed to handle the amount of power it makes when stock. All short-term power boosters put additional stress on internal components. If you’re thinking of injecting anything extra into your engine, you may need to beef up your twirly bits.
When you spray water into an engine’s intake manifold, the water vaporizes, lowering the temperature of the intake charge and making it denser. A denser charge means more air and more fuel reach the cylinders, which translates to more power. The lower charge temperature also greatly reduces the likelihood of knock or detonation, allowing either a higher compression ratio or more boost pressure. While there’s little benefit to water injection in naturally aspirated engines at normal temperatures, in a turbocharged or supercharged engine where boost turns up the heat, the temperature reduction can be dramatic—possibly more than 100 degrees. This is particularly beneficial if the engine lacks an intercooler. In these systems, injected water is typically mixed with alcohol to prevent freezing, which is why they are also commonly known as alcohol or methanol injection. Regardless of the name, water injection is a useful safeguard for those either boosting or increasing the existing boost on an otherwise stock engine.
GAIN: Depending on how aggressive one wants to be with the boost increase and how much water one chooses to inject, the power increase can range anywhere from 10 to 25 percent.
The power output of any engine is limited by the amount of oxygen it can ingest. That’s why most go-faster solutions—larger displacement, higher rpm, forced induction—focus on pumping more air. But you can achieve the same result by adding a substance that contains oxygen. The most common of these is nitrous oxide, or N2O, which is 33 percent oxygen (atmospheric air is about 21 percent). Injecting it into an intake manifold increases the oxygen that ends up in the cylinders. The nitrous itself doesn’t make more power; it helps the engine make more by allowing it to burn more fuel, which you must make sure it gets. Nitrous’s benefits to power output compound because it is stored as a liquid in a pressurized tank. As it evaporates, it also cools the intake charge, increasing charge density, further boosting the oxygen intake.
GAIN: A typical 10-pound bottle can provide a 100-hp boost for about two minutes, but that go-fast time drops as the power jolt increases.
Because diesel fuel starts to burn shortly after it’s injected, there’s little opportunity for the fuel and air to mix thoroughly. Diesels leave about one-fifth of the air in their cylinders unused, which is one reason they make less horsepower than gasoline engines. This problem can be addressed by injecting another fuel, such as propane, into a diesel’s intake manifold, giving it plenty of mixing time. The combustion process still starts when the diesel fuel is injected, but the leaner mixture of propane helps consume the otherwise wasted air. The propane and air mixture also increases the speed of combustion, which improves efficiency. Injecting alcohol into a diesel nets a similar result.