top of page


The following fuel injector calculator works differently than most found on the internet, in that it does not require the user to make a guess at BSFC. Airflow is an easier number to get your hands on. I prefer to size the fuel system to the maximum airflow the turbo is rated for, so that you're covered in the event of a runaway boost situation, at least as far as fuel capacity is concerned. It can also be used to approximate HP goals by simply dividing Dynojet WHP or Engine HP by 10. 

Injector size may seem pretty straightforward. It's often in the "name" of the injector, or you might find it on a spec sheet that came with the injectors. Ethanol, with its higher viscosity, throws a monkey wrench into things however. All injectors are derated somewhat when running ethanol. For e85, derating the injector by 10% (multiply injector flow by 0.90) is a reasonable starting point for most injectors. Injector size can be entered in cc/min or in lbs/hr. 

Injector Rated Pressure is available from the injector reseller or manufacturer. It's almost always 43.5 psig. 

Base Fuel Pressure is also differential fuel pressure. You can test increased base fuel pressure here to see if it gives you the additional headroom you might need on a particular set of injectors. An injector's maximum differential pressure should be available from the seller or manufacturer. Higher differential fuel pressures are harder on the pump, so be sure your pump can support it. 

Fuel specific gravity varies in a pretty narrow range but is important. Pump gas is typically assigned a specific gravity of 0.76. Ethanol and Methanol are 0.79. And the specific gravity for most race fuels can be found online. 

Target AFR is what you actually intend to run for AFR, or what the AFR is in your datalogs. It will need to be in the correct scale for the fuel in use. For example, E85 AFR when using E85, not gas scale AFR. 

The injector size units you choose to use will determine which of the two results you use at the end. The other will make no sense. Fuel SG is ignored in the lb/hr calculation since the weight of the fuel is already known.

bottom of page