Calculating the Right Fuel Injector

How do you know what the right size fuel injector would work for your application. Well, this webpage shows you how to calculate the correct size of injector based on the horsepower (Hp) that you are hoping to attain. First here's the basic formula:

Fuel requirement in lbs./hr = (Max HP x BSFC) / (number of injectors x duty cycle)

Note: to convert from lbs./hr to the Metric measurement of cc/min, use this equation: [(lbs./hr) x 60] / 6.177 = cc/min

Max HP is a realistic horsepower estimate at the crankshaft or known value from engine dyno testing. Chassis dyno horsepower figures can only be used once you factor in the drive train losses, which can vary from vehicle to vehicle. Ask your chassis dyno operator to calculate the drive train horsepower loss for your vehicle. Add the drive train horsepower loss to the drive wheel horsepower to closely estimate crankshaft horsepower. BSFC or brake-specific fuel consumption is the amount of fuel consumed per unit of power produced. It is an indication of the efficiency of the engine configuration and calibration.

Actual BSFC is a function of compression, camshaft timing, cylinder head design, tune, ambient conditions, etc. The lower the BSFC number, the more efficiently the engine is making power. Engine dyno testing can provide exact BSFC data. To estimate the fuel requirements of your engine, use the examples below that best match your engine type. The reason we use a higher BSFC value to calculate fueling requirements for a supercharged engine is because of the parasitic losses or the power required to driving the supercharger that is never seen at the crank. In other words, a supercharged engine that dyno tests 450 hp at the crank, may actually be making 490 hp, but the supercharger and drive assembly is absorbing 40 hp, so you net out 450 hp.

Also, the heating effect of pressurizing the intake charge in a non-intercooled system also increases the fueling requirement of a super/turbocharged engine. Always remember that too lean of a mixture can result in spark knock, high combustion temperatures and engine damage. It's smart to be slightly on the rich or safe side.

 

Engine Type Gasoline Alcohol
High Compression 0.45 to 0.55 0.90 to 1.10
Low Compression 0.50 to 0.60 1.00 to 1.20
Super/Turbo Charged 0.55 to 0.65 1.10 to 1.30

There is one other parameter involved in properly sizing fuel injectors: duty cycle. This is the percent of time that the injector is actually open (which is also referred to as pulse width) vs. total time between firing events. When an injector is open 100% of that time, the injector is in what is called a static condition. For road-racing engines that are at maximum power for extended periods of time, the desired maximum safe duty cycle is 0.85. This ensures that the injector is closed a sufficient time to keep it from overheating.

For a typical street engine that spends less than 1% of its time at maximum power, you could argue that a higher duty cycle could be used to calculate fueling needs. Typically we would not do this because again we want to error on the safe side. Some may ask why not just install the biggest injector you can find. Well it's the same analogy of putting an 850cfm carburetor on a Chevette motor, overkill at best, more like a controlled leak. One other thing to remember is that an injector can only open and close so fast, this is called minimum dynamic flow range. If the ECM, in an attempt to lean out a rich mixture, selects a pulse width that is shorter than the injector's minimum dynamic flow range, the injector becomes inconsistent in its ability to supply the required fuel. This results in poor engine performance, surging and stumbling. In other words bigger isn't always better.

Let's calculate the fueling requirements of a few engines to illustrate what we have been talking about.

For the first example let's take a stock Ford 5.0L Mustang motor that makes an advertised 215 hp and look a the very conservative approach Ford used to calculate the injector size for the factory engine by using the O.E. typically safe 0.80 duty cycle limit.

     Fuel injector size = (215 hp x 0.55) / (8 x 0.80) = 18.5 lbs./hr
     or the ACCEL/DFI p/n 150119 injector


Now let's upgraded the engine with more efficient GT-40 type components that will lower the BSFC and use a more realistic 0.85 duty cycle limit. Ford says this combination of GT-40 parts will produce about 275 hp. What injector size is required to support this?

     Fuel injector size = (275 hp x.50) / (8 x 0.85) = 20.1 lbs./hr
     or the ACCEL/DFI p/n 150121 injector


Until now your only choice would have been to go with a 24 lbs./hr unit, which would be fine if the engine was making about325 hp, but not ideal for 275 hp. Remember the comment about realistic horsepower; don't kid yourself! Now let's factor in an adjustable fuel pressure regulator as a tuning tool for this setup. By adjusting fuel pressure you can change the flow rating of a given injector. The calculation is simple, as long as you know the static flow rating of an injector at a specific pressure. For example ACCEL/DFI p/n 150121 flows 20.0 lbs./hr at 2.7 BAR or 39.6 PSI, which just happens to be where the stock Ford nonadjustable fuel pressure regulators are preset. As a point of reference, most GM factory fuel pressure regulators are preset at 3.0 BAR or 44.1 PSI. If we were to increase the fuel pressure from 39.6 PSI to 45 PSI, what will be the new flow rating of the ACCEL/DFI p/n 150121 injector?

     New flow rating = [square root of (new pressure /old pressure)] x old flow rating

     New flow rating = [square root of (45 PSI / 39.6 PSI)] x 20.0 lbs./hr = 21.3 lbs./hr

This increase in flow rating would support about 15 additional horsepower on our GT-40 engine. An adjustable fuel pressure regulator is an excellent tuning tool as long as the fuel pressure does not exceed 55 PSI, which is the limit that the stock fuel line fittings are designed to handle. So let's say we increase the fuel pressure up to 55 PSI, then the ACCEL/DFI p/n 150121 injector would be flowing 23.6 lbs./hr. But because ACCEL/DFI offers p/n 150123 that flows 23.1 lbs./hr at 39.6 PSI and 150124 that flows 24.3 lbs./hr at 39.6 PSI, radical increases in fuel pressure are not required to find the perfect match for your engine. The key is to make power efficiently, choosing the correct injector for your intended needs and using the adjustable pressure regulator as a fine tuning tool.

For the third example let's use Ford's new 392 crate motor p/n M-6007-A392. Out of the crate, using a 750cfm carburetor, this engine dyno tested at 453 hp with a .454 BSFC. Let's calculate the injector size you would need if the 392 were to be fuel injected.

     Fuel injector size = (453 hp x 0.454) / (8 x 0.85) = 30.2 lbs./hr units
     or the ACCEL/DFI p/n 150130 injector


As a point of reference, this same 392 crate engine has made over 530 hp on a dyno with Air Flow Research 185cc heads vs. stock GT-40X heads. To support this new-found power, using the same equation, larger 35.2 lbs./hr units or the ACCEL/DFI p/n 150136 would be needed. So when calculating injector size, if you are planning on large power adders in the future, keep in mind that you may have to upgrade your injector size. Just like if you might have had to put a bigger carburetor on a modified motor in the past.

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Copyright February 2011 Eugene Blanchard