Having an electronic fuel-injection system on a small motorcycle may seem high-tech, yet the system is actually pretty basic when compared to the latest electronic fuel-injection developments. While the Mikuni and Keihin throttle-bodies used by the different manufacturers are fed by an immersed fuel pump in the fuel tank (which pressurizes an injector housed in a throttle-body), most high-performance road motorcycles have injectors mounted into the inlet port of the cylinder head. The fuel pump on these systems uses the fuel tank as part of the circuit, which is constantly feeding the injector with pressurized fuel and returning the unused fuel back to the tank.
This port electronic fuel-injection system enables higher-pressure injection, which is necessary to supply the right amount of fuel for high-rpm road motorcycle engines.
The technology doesn't stop there though. Toyota has actually enlisted Yamaha to develop DFI (Direct Fuel Injection) systems for the top-end of their small car engines. This direct injection system incorporates a second injector that is mounted directly into the cylinder head, which enables a much leaner mixture to be dispersed into the cylinder while still maintaining a richer portion of that mixture around the spark-plug electrode. This prevents the fuel mixture detonating and is up to 15 percent more efficient than inlet port injection.
The difficulty is that direct fuel-injection requires a pressure of 1400psi (30 times more pressure than most motorcycle electronic fuel-injection systems) to inject the fuel quick enough at high revs. The only way to achieve this is by a mechanical fuel-pump that is driven straight off the engine, as opposed to the electronic pumps on most motorcycles currently on the market.