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Thought it would be useful to provide a guide which sizes injectors to your engine horsepower target.


Definitions:

Brake specific fuel consumption (BSFC): BSFC is a measurement of an engines efficiency regarding how well it uses the energy available in the fuel it consumes. It is measured in lbs/hp/hr or pounds of fuel used for each horsepower.

When sizing up your injectors for a particular engine, BSFC numbers along with engine horsepower potential is used to calculate how much fuel will be required of them.

Some BSFC numbers: (since we are only dealing with EFI engines I will only add these)

Naturally aspirated engines using EFi: .45-.55 BSFC
Forced induction engines using EFi: .60-.65 BSFC

Duty cycle: is the amount of time that the fuel injectors are switched on. A duty cycle of 0% means that the injector is not on at all, a duty cycle of 100% means the injector is on constantly.

lbs/hr x 10.2 = cc’s/min
cc’s/min / 10.2 = lbs/hr

Lets begin

Based on the amount of power your engine will produce, and how efficiently it makes that power in terms of fuel consumption that being BSFC, you can determine how much fuel each injector will need to flow, given one injector for each cylinder.

First provide a realistic estimation of how much your engine will make. Multiply this number by the BSFC number, which will give you pounds of fuel used per hour (POFUPH). Then divide POFUPH by the number of injectors you are planning to use to find the amount of fuel each injector will need to supply.

Example

200 hp x .65 BSFC = 130 lbs/hr

130/lbs/hr / 4 injectors = 32.5/lbs/hr

32.5/lbs/hr x 10.2 = 331.5 cc’s/min

Therefore we would need an injector that flows 331cc/min for each of the 4 cylinders.

However, it is important to note that this will indicate what the injectors’ flow at it’s maximum duty cycle. Injectors will not last long if they are forced to operate at or near their maximum duty cycle. The industry standard is not to push injectors past 80% duty in order to keep them operating safely. Therefore an extra value is added into the equation being 0.8 and this is divided by the final injector requirement.

331.5cc/min / 0.8 = 414.4cc/min

Bear in mind that all injectors are rated at their static pressure, which is usually 43.5psi (NB SE fuel pressure 53-61 psi) , if you apply a higher fuel pressure to the injector, i.e. by using an aftermarket fuel pump then a smaller injector could potentially flow at higher injector flow rate:

For example. We have an injector that flows 414cc/min at a pressure of 43.5psi, if we have to run at 60psi as opposed to 43.5psi , how much would the injector flow at?

(New pressure / static pressure) x injector flow rate cc/min = new flow rate

(60 / 43.5) x 414.4 = 571.1 cc/mm

This formula shows that raising the pressure to the injector yields higher flow rates, however there is a point of diminishing return. Meaning that as the pressure continues to raise flow rates don’t increase due to the physical size limitation of the injector.

It also worth to note that the higher the fuel pressure is, the harder it is for the injectors’ magnetic field to actually pulls the injector open. If magnetic strength is overcome by fuel pressure it will cause the injector to lock, stopping flow and will not continue to flow until fuel pressure subsides. This is a bad thing when your engine is producing lots of power and heat at that moment.

It is best to find an injector with a flow rate that is sufficient at its normal fuel pressure rating to supply enough for the engine. This will prevent any problems with high fuel pressure clamping the injector nozzle shunt.