This is a little more complicated than a simple formula.  If you know how much horsepower you want to make, you should be able to just calculate how much fuel you need right?  Well, there are more things to consider.  The first thing is crankshaft horsepower vs wheel horsepower.  Crankshaft horsepower is what you see in all the magazines or online articles talking about the new Camaro, Challenger, Mustang or whatever.  There was a change of how horsepower was measured back in the 60’s, but now basically they take an engine loaded up with accessories and put it on an engine dyno to get the power numbers that you see published.  Now if you take one of those new 426 horsepower Camaros to a chassis dyno you are sure to see a number lower than 426, this is referred to as wheel horsepower (whp) or brake wheel horsepower (bwhp). The reason you will see a lower number on the dyno that you drive onto is because of what is called drivetrain loss.  Things like the transmission and rear end differential are mechanical devices that give you certain benefits like changing gears and transferring power 90º from the driveshaft to the axles but they come at a cost. These drivetrain components take power from the engine to operate so it isn’t uncommon to see a 15% reduction in power at the wheels from what you see at on the engine dyno (or in our case from a magazine).

The next thing is something called Brake Specific Fuel Consumption or BSFC.  This is a number that is dependent on all sorts of things, but basically for naturally aspirated you will be somewhere around .50.  For supercharged engines you will probably be around .60, and for Turbo setups you will probably be around .65.  The reason you need a larger injector for boosted applications is because the injector flows less with the pressure differential is less.  If you have a naturally aspirated engine at 58 psi of fuel pressure you have 58 psi pushing fuel through that injector or more if the intake is in vacuum.  If you are running 20 lbs of boost, the pressure difference between the fuel and the intake isn’t 58 anymore, its only 38.  At that reduced pressure difference the injector will flow less, meaning you will need a larger fuel injector.

The fuel you are using has a lot to do with the injector size you need also.  If you’re running gasoline, either from the pump or race gas, you can run a much smaller injector than if you’re running alcohol because of the energy density.  Meaning if you want to run E85 in your turbo car you are going to need ~45% bigger fuel injector than if you ran gasoline.

The number of cylinders your engine has also has an effect on what size injectors you need since you will be dividing the totally fuel consumption by the number of cylinders.  If you had a 300 hp 4 cylinder engine you will need injectors twice the size as a 300 hp engine that has 8 cylinders.  This is assuming you have 1 injector per cylinder, if you have a throttle body injection setup you will need to calculate your injector size based on how many injectors you have an not by cylinders.

Fuel pressure also influences the flow rate of injectors.  For example, the LS1 engine uses a fuel pressure of 4 bar (58 psi) which is higher than the typical 3 bar (43.5) fuel systems.  If you use a fuel injector designed for a 43.5 psi system in a 58 psi system the flow rate will be higher meaning you can get away with a smaller injector.  If you are tuning your engine computer’s injector flow rates, you will need to make sure the tables you are using for the injectors are for the same fuel pressure.  If you use Mustang injectors in an LS1 that don’t use the same fuel pressures, your flow rates will be incorrect.

The last thing is something called duty cycle.  The duty cycle of a fuel injector mean the percentage of time the injector is flowing fuel compared to the amount of time it isn’t. The recommended ‘safe’ duty cycle is 80% meaning the injector is only on 80% of the time. Injectors can become unstable at high duty cycles and the flow rates may become uncontrolled. Where it gets tricky  is when you have a high rpm engine, because the amount of time the injector has to operate gets smaller at higher RPM. The time the engine takes to complete the combustion cycle is shorter as your rpm goes up.  For example if you have an engine with a rev limiter at 5,000 rpm it takes 12 milliseconds to complete the 4 strokes (intake, compression, combustion, exhaust). At 5,000 rpm your injector only has 12 milliseconds to operate, meaning at 80% duty cycle it could only stay on for 8.6 milliseconds.  If you had an engine that made the same horsepower, but revved to 7,000 rpm, you would only have 8.6 milliseconds to operate, meaning only 6.9 milliseconds of on time at 80% duty cycle.  You will need a larger injector for a 7,000 rpm engine than a 5,000 rpm engine even if they make the same exact horsepower, because it will have to flow the same amount of fuel in 6.9 milliseconds instead of 8.6 milliseconds.