Throttle Bore and Length
The bore of the throttle body makes a significant difference to the way the engine performs. If the throttle bore is too small then this will restrict airflow at high RPM but if the bore is to large then this will lose torque in the lower to mid-range RPM. Therefore it is critical to consider what the engine will be used for and what sort of performance characteristics are wanted from the engine.
The length of the inlet tract also plays a large part in the performance characteristics of an engine. If the inlet is too long then this has the same effect as having to small a throttle. This is due to the buildup of friction between the inlet walls and the air slowing the air down and restricting its flow.
If the inlet is to short then the engine will suffer from poor low to mid-range torque due to the lack of airspeed and dynamic filling of the cylinder.
The overall inlet length from the inlet valve to the start of the inlet tract also plays a part in maximising performance, this is due to a pressure wave that is created when the inlet valve is closed but the air in the inlet tract, the momentum of the air hitting the shut valve starts a pressure wave that travels up and down the inlet tract. The intention of a tuned inlet length is to have the pressure wave arrive back at the inlet valve as it starts to open, this then provided a dynamic fill and increased the engine volumetric efficiency.
It is import to note that the tuned inlet length is based around a given engine speed, therefore, the inlet length should be determined based on the same parameters as the exhaust and camshaft so that it creates a tuned system to maximise the performance benefits at the given RPM.
The general rule with the injector position is the higher the revs the further from the inlet valve is should be. This is to allow the air and fuel more time to mix prior to it getting to the cylinder.
There are of cause some limitations with this rule and this is at low engine speeds. If the injector is too far from the inlet valve then the fuel can condense against the walls of the inlet before it reaches the cylinder. If the application is for a race engine that will always be used at high RPM then this is less of a problem, but with road cars that need good mid-range drivability then the injector positioned closer to the head would improve the drivability.
In extreme cases, injectors can be installed outside of the inlet tract altogether and set to inject straight down the runner.
The interaction between the injector and the throttle plate can also be beneficial. As the air passes over the throttle plate it is disturbed and causes turbulence which helps with the mixing of the air and fuel.