Injector Selection
As you add larger heads, cams and other poweradders, it's also
necessary to increase your fuel output. Choosing an injector for the
proper application is relatively easy, however, it's different for a naturally
aspirated engine versus a supercharged engine. To calculate the proper
injector size you need to know the Brake Specific Fuel Consumption
(BSFC). The BSFC is how much fuel is used per horsepower per hour. For
a naturally aspirated engine, that figure falls between .45  .5 BSFC.
Supercharged engines need more fuel, so the figure is slightly higher
at .55  .60. When you're doing your calculations, remember that you
don't want the injector to run at 100%. A safe buffer zone and recognized
standard is to let the injector run at no more than 80%. The lower the
BSFC number, the more efficient your assuming the engine is. So, unless
you've spent time on the dyno and know what your BSFC is, use the
higher BSFC number for your application and calculations to be safe.
Injector size = (HP x BSFC) / (number in injectors x duty cycle)
Example: Choose an injector for a 350 horsepower naturally aspirated V8 engine.
(350 hp x .5 BSFC) / (8 inj x .8 duty cycle) = 175 / 6.4 = 27.34
In this example, the answer was 27.34 for an injector size. If you've been researching injectors you'll
know that this falls between our 24 lb and 30 lb injector selection. The 30 lb injectors would be the better
choice, versus buying the 24's and cranking up the fuel pressure. If you already have 24 lb injectors, you
can always try increasing the fuel pressure to attempt to feed the engine. Most of us are always looking
for the next gadget to add horsepower and the 30 lb injectors would give you a little play room while
meeting the needs of the current engine. If you put in an injector that is too large for the combination,
the vehicle will have a hard time running correctly. Once you get as high as a 42 lb injector, you may find
you need a custom chip burned or an engine management system. Don't forget to match your Mass Air
sensor calibration with the proper injector.
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Injector Horsepower Limits
To find the approximate horsepower limit of a particular set of injectors we can use the following formula:
Horsepower limit = (Injector size x number of cylinders x duty cycle) / BSFC)
Example: What is the approximate horsepower limit of a 24 lb injector in a supercharged V8 engine?
(24 x 8 x .8) / .6 = 153.6 / .6 = 256
So, 256 horsepower is a safe limit for the 24 lb injectors if you have a supercharged engine, assuming a
maximum 80% duty cycle. A similar naturally aspirated engine with 24 lb injectors would have a safe limit
of 307 horsepower (BSFC of .5). Remember that these figures are simple mathematical equations. What
you or the next person can get away with all depends on your engine design, camshaft, timing, etc...
Most of us will never get an exact calculation for the combination we have, but the formulas will get us
into the ball park we need to be in.
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Fuel Pressures
Some people think that cranking the fuel pressure on an injector will get them the extra fuel they need.
However, it's not really that simple. For starters, the engines computer will make every effort to correct
any fuel that you add or subtract that differs from it's lookup tables while running in closed loop. When
running in closed loop, the computer is constantly monitoring the oxygen sensors (O2's) and adapting to
the O2 readings. The computer will only compensate to a certain percentage. The computer is designed
to adapt, learn, and create new tables based on what it learns during closed loop. This also allows the
computer to adjust for sensor wear over the years, small engine problems, etc.. At wide open throttle
(WOT), the computer is in open loop and the O2 readings are not part of the fuel ratio calculations. At
WOT, the computer uses the learned adaptive tables from closed loop to kind of guess at the proper air
fuel ratio. This is one reason why tuning is very important.
The Mustang injectors are a highimpedance (13.519 ohm) saturation type injector that produces a
coneshaped spray pattern. Increasing the fuel pressure will slow the injector's response time and alter
its calibration to the mass air meter.
Manifold pressures also effect the nozzle outlet pressure of the injector. (Nozzle outlet pressure is also
referred to as the Delta Pressure). The outlet pressure of the injector must remain constant. This
includes while your idling with 18" vacuum or under 12 lbs of boost from your supercharger. These forces
are constantly pushing and pulling at the injector. So under vacuum, that vacuum pull is actually helping
pull fuel from the injector. Under boost, the force is trying to prevent the fuel from being released from
the injector. Most of Fords injectors are rated at approximately 39 psi operating pressure. If you've ever
hooked up a fuel pressure regulator to the rail of a stock Mustang you should see approximately 30  32
psi.
Nozzle outlet pressure = fuel rail pressure  manifold pressure
To calculate nozzle outlet pressures with vacuum you need convert your vacuum reading to a
pressure (psi) figure. To do this we use the following conversion:
1 psi of atmospheric pressure = 2.036 inches of mercury (HG)
Naturally aspirated example: How do you get 39 psi at the injector if you're only seeing 30 psi at the
rail? Naturally aspirated engine with roughly 18" of vacuum at idle.
First, convert the 18" of vacuum to a pressure reading (psi).
18 / 2.036 = 8.84 psi
However, this is vacuum, so your 8.84 psi is a negative draw from the injector which is pulling more
fuel from the injector. Your figure should be:  8.84 psi
30 psi at the fuel rail  8.84 manifold pressure = 30 psi + 8.84 psi
= 38.84 psi (39 psi) at the injector
To calculate nozzle outlet pressures while under boost conditions, the same formula applies. However,
since you're not under vacuum, there is no need to change the manifold pressure to a pressure reading.
The assumption is made that your fuel pressure regulator will increase fuel at a rate of 1 psi per 1psi of
manifold pressure.
Supercharged example: You have a supercharger pushing 12 psi of boost into the manifold. What
fuel pressure do you need to overcome the boost pressure pushing against the injector? Your
injector is rated at 39 psi.
To keep the 39 psi rating of the injector we plug the numbers into the calculation:
Nozzle outlet pressure (39psi) = X (fuel rail pressure)  12 (psi boost)
X = 39 + 12
X = 51 psi at the fuel rail
So, to keep the nozzle outlet pressure constant (Delta pressure), your fuel rail setting should be set at 51
psi while under 12 psi of boost.
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Fuel Pressures and How the Fuel Injector Ratings Change
The last factor in this is how increasing the fuel pressure effects the theoretical size of the injector. For
instance, if we have a 24 lb injector, rated at 39 psi, and we increase the fuel pressure to 50 psi, we can
determine the approximate size injector our 24 lb injector is now simulating.
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New injector flow rate = _/ new pressure / pressure at the rated flow x rated flow
_______
= _/ 50 / 39 x 24
____
= _/ 1.28 x 24
= 1.13 x 24
= 27
Our 24 lb injector, at 50 psi, is now acting like a 27 lb injector. The same math applies if you drop the fuel
pressure from the rated flow of 39 psi. Your injector would act like a smaller injector.
Injector Horsepower Limits (@ 80% Duty Cycle)




NA Horsepower Limit (.5 BSFC)


SC Horsepower Limit (.6 BSFC)






































Topics For Discussion:
Injector Selection
Injector Horsepower Limits
Fuel Pressures
How Fuel Pressures Can
Change the Injector Rate