In multi-point injection systems, the air required for the combustion of the fuel is led from the air filter, through the throttle valve, and into the common manifold, or plenum chamber. From here, individual intake runners, or pipes, branch off to each cylinder. All of these pipes are of equal length.
The design of the intake system determines how large an air mass can be drawn into a cylinder at any given engine RPM. This is where most advantage can be taken of electronic fuel injection, since it can achieve exactly uniform distribution of the air delivered to the cylinders.
With unobstructed passages to each cylinder, the cylinder fills with air as efficiently as possible. The breathing of the engine, or its volumetric efficiency, is improved. The more air crammed into the cylinder, the denser the air-fuel mixture, when ignition occurs.
This increases the thrust on the piston, and increases engine power output too.
The temperature of the air influences how dense the air-fuel mixture will be. Cold air is denser than hot air, so it has a greater mass in any given volume.
This is why, on most intake systems, the air entry to the intake is away from engine heat - to ensure the entry of cool air.
Since the manifold carries air only, it doesn’t need to be heated by coolant. Filtered air arrives at the intake port, as cold and dense as possible, ready for mixing with the fuel from the injector.
