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CHOOSING THE CORRECT FUEL INJECTOR FOR YOUR APPLICATION
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. Itís smart to be slightly on the rich or safe side.
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 itís 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 injectorís
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 isnít always better.
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