The amount of fuel injected is determined by the time the injector is open (controlled by the ECU) and the physical size of the internal passage ways inside the injector.
Injector sizing is commonly identified by stating the flow rate, which is the amount of fuel that flows through the injector when held fully open for one minute. Flow rates are usually quoted at 3 bar fuel pressure. Doubling the pressure increases the flow rate by approximately 30%, injector dependent.
Example: Through a 500 cc/min injector flows 500 cc (½ litre) of fuel per minute at 3 bar. This would increase to 650 cc/min at 6 bar.
What size will you need?
The engine power divided by the number of cylinders, gives you hp/cylinder.
As a rule of thumb you will need 5.5 cc fuel per hp.
For example a 600 hp eight cylinder engine requires an injector flow of at least
600/8 x 5.5 = 412 cc/min per injector.
Low Ohm versus High Ohm Injectors
The impedance (resistance) of different injectors can vary from 0.5 ohm to 16 ohm. This requires different operating currents to open them.
High ohm injectors need low current to open. They do not need current control as the high resistance ensures that the current does not build to dangerous levels.
Low ohm injectors need maximum current to open and then a much smaller current to remain open. MoTeC uses a 'Peak-and-Hold' system of current control where the injector is allowed to build to a high current flow to open and then controlled to a quarter of its peak value. This allows accurate control of the injector while reducing the heat generated in the ECU.
Low ohm injector have a much faster opening time. This is important at idle and low throttle openings where the required opening times are small.