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
brakespecific 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
nonintercooled 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
roadracing 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 GT40 type
components that will lower the BSFC and use a more realistic
0.85 duty cycle limit. Ford says this combination of GT40 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 GT40 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
M6007A392. 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 GT40X heads. To support this newfound 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.
