All too often I have seen racers either underestimate what their injectors can support, or see racers over injector the death of their cars. Some people follow the path of what their friend is using, just like they do with other high performance parts. But what do you do if your friend is wrong?  After a while, every racer will jump on the bandwagon and an injector will become a flavor of the year. This goes back to the 5.0 days when 50 pound injectors were the largest possible injector you could get. Then 72’s came along, then 83’s and 96’s, and then 160’s. Now that just about any size is available, it’s easy to go overboard with injector size. Finding the appropriate injector size does not just apply to racers, it applies to anyone that modifies their vehicle.

The overall principle is simple, the more air you have moving through your engine the more fuel you will need to produce a specific amount of horsepower.  A lot of people think bigger is better, just like bigger cams, more compression, huge ports and so on. While you will almost always have better results running a larger injector than necessary, some people are just going nuts. I've seen 42 and 50 pound injectors on 4.6 Mod cars with stock boost pulleys that use a FMU.  That is just plain overkill.

This article is long, and I have tried to start out from basic concepts to advance. I have outlined the basic fundamentals first. As the article goes on, it will get more advanced. As you learn the basics you should have no trouble understanding the more advanced sections. The best thing to do is to take the article in bits and pieces.  Although the article is written in the simplest to understand form, don’t try to
swallow the entire article at once if you’re not familiar with the basics. Please note that this article is only a reference guide, not a bible. The numbers below represent ballpark figures. The intended purpose is to use this article as a reference guide for the purposes of learning and referencing to a real life application.

Fuel injector types and their flow ratings:
You’re probably familiar with the common fuel injector sizes. 24,30,36,42,50,72,83, 96,160.  What you might not know is that there are a few different types of injectors out there. There are 3 types of injectors out on the market today. Pintle, disc, and ball.  Most people that read this article are/will be using Pintle style injectors. Pintle injectors are the injectors you see the majority of companies selling such as 24,30,42,and the new 50 pound injector. A disc injector is a larger injector such as 72 and 83 and 96 pounders.

Pintle injectors are rated at a fuel pressure of 40 psi. Disc style injectors are rated at a fuel pressure of 44 psi. So a typical 24 pound injector is rated a 24 pounds at 40 psi. This is important to understand because we will be talking about what happens when the fuel pressure increases. A 24 pound injector rated at 24 pounds at 40 psi will flow 35 pounds at 85 psi. In turn it will support more horsepower.

Injector duty cycle
The duration that an injector stays open is called injector duty cycle. At idle, injector duty cycle is approximately 2% in duration. The higher the load and position of the TPS sensor (gas pedal) the longer the injector is going to stay open.  At WOT, it is ideal for an injector to stay open for 85% of the time.  The higher the injector duty cycle, the longer the injector is staying open. The longer it stays open the
hotter it gets. The hotter it gets the less efficient it will be.

How high is too high?
If an injector stays open longer than its operating efficiency range, the injector overheats, and the spray pattern can distort. When the injector overheats the gasoline going into the combustion chamber becomes much hotter. (Remember the cooler the gas, the better). There are several arguments and theories of the maximum operating duty cycle. It kind of falls in the same category of the maximum
limit stock fuel rails and lines can support. One argument is that short bursts like ¼ passes aren't as harmful as longer WOT blasts. Another argument is that the math doesn't lie. In my opinion it’s always better to be safer than sorry. With this said, in the rest of this tech article I will be using 2 injector duty cycle ratings. One is more conservative, the other is a little higher. I always recommend trying to shoot for 85% as a goal.

BSFC
BSFC is short for Brake Specific Fuel Consumption. This is how much fuel an engine consumes rated in pounds of fuel per hour to produce a given amount of horsepower. The smaller the displacement of the motor, the smaller the BSFC. A naturally aspirated 5.0 engine has a BSFC of 0.5, a naturally aspirated 4.6 has a BSFC of approximately (0.48). When you add a power adder into the equation the
BSFC goes up. For a 5.0 engine it jumps to 0.6 for a 4.6 engine it jumps up to 0.58

RETURN style fuel systems versus Returnless types.
1999 and up Ford Mustangs use a Returnless fuel system. This makes it harder for a higher horse powered 4.6 to use a smaller injector. A return type fuel system takes the excessive unused gasoline that don't get used by the fuel injectors and returns it to the fuel tank to be reused.  A return system FMU can use a FMU in its system to help maximize a smaller injector.

What is an FMU?
The word FMU stands for Fuel Management Unit. Essentially it is a fuel pressure regulator that will increase your fuel systems fuel pressure as it senses boost in a supercharged/turbocharged application. 1999 and up models use a returnless fuel system and can not
use an FMU (more on that later)

The FMU has a vacuum signal routed to it so that it can tell when boost is being applied via the supercharger to the motor.  This in return will cut off the path back to the return line, which in return raises fuel pressure. Increasing fuel pressure is one way that will make your fuel injectors bigger in supply. By increasing the fuel pressure in the fuel system, your overall flow rate will increase.

A small disc inside the FMU does the job of determining your pressure. The size and orifice of the disc will determine the rate of gain for the FMU. Essentially it is a multiplier. Lets use a 6:1 FMU for example. Your engine produces 10 pi, once the supercharger actually produces 10 pi, the vacuum signal going to the FMU tells the FMU its at 10 pi. The FMU at this point will increase fuel pressure 60
pounds. If the car were making 2 pounds of boost, it would increase it by 12 pounds, and so forth. So as you can see, by increasing fuel pressure in relation to boost, it can maintain an air to fuel ratio that is roughly close and safe.  For precision tuning, the injector duty cycle would need to be trimmed through the calibration of the computer via a computer chip.

FMU's are great for street cars that are making fairly decent horsepower, and allowing the vehicle to retain streetibility and gas economy by using a smaller injector. 500 and 600 horsepower cars can now run 36 or 42 pound injectors instead of 50 or 72 pound injectors.

The limitations of FMU’s come into play with higher boost applications. The higher the boost the harder it is to control fuel pressure. Your average Bosch 24,30,36,42 and 50-pound injector can handle fuel pressures in excessive of 100 or less. Once you start going over 100 psi, the injector will have a hard time doing its job (in the same way that it would if it was overheated and on all the time). The spray
pattern can distort, and cause unsafe conditions.

Why do some people say FMU’s don’t work?
Some tuners refuse to use FMU’s because they say they are not fine tunable enough. A large part of this theory started back when the Kenne Bell superchargers came out. Kenne Bell Superchargers aren't known for their stellar FMU’s. Their FMU’s made it difficult for the tuner to achieve the desired air to fuel ratio needed, especially at tip in conditions. At that point the tuner would just stick 36-pound
injectors in and the problem was solved. Some tuners whom aren’t the most knowledgeable would also blame the FMU because they couldn’t get the tune correct. Over the years, lack of knowledge and over exaggeration contributed to this myth.  You simply just have to know when FMU’s would work, and when they are less desirable.

Supercharged engines require more fuel
If you have a supercharger on your vehicle, it takes horsepower to drive the compressor.  The harder you spin the compressor the more horsepower it will take to drive it. So when you are figuring in total engine horsepower, remember that your motor is actually making more than you think because of the parasitic hp losses to drive the blower. Always add in extra fuel to the equation to be on the safe side.

Rear wheel horsepower versus flywheel (engine) horsepower.
The difference between the two is important to understand and convert from one to another.  Ford rates their mustang's power output at the flywheel. This number is used more so because it will be a higher number than the power you can actually use. The higher number will sound more impressive and more tempting for a potential customer.

Rear wheel horsepower is a number that you can actually use, its real life horsepower. It is a number that reflects the drivetrain power loss. If you ever have dynoed your car on a chassis dyno obviously you know that the numbers given are rear wheel horsepower.  If a stock 97 4.6 cobra produces 250 rwhp on a chassis dyno, you can figure out the drivetrain losses. The advertised engine horsepower is 300 and
the actual power at the tires is 250. Your car has a 50 horsepower loss through the drivetrain.

To convert rwhp horsepower to engine horsepower a very close ballpark formula is this.  RWHPX1.17=BHP (Brake horsepower) AKA engine horsepower.

Boost in relation to horsepower
It’s important to understand the relation of boost to horsepower. As you might know all to well, owners with supercharged or turbocharged cars often want to increase the total boost output.

As a rule of thumb figure a 12.5 bhp increase for every pound of boost increase.

Why do I say 12.5? The answer is simple. Some superchargered systems are more efficient than others. Whether it be the actual blower or turbo that is being used or if the speed at which the blower is being spun. 1 pound of boost increase can range anywhere from 10 to 15 hp. The best thing to do is take the average to get the best round about estimation. So if your planning on going from a 3.6 pulley to a 3.3 pulley and expect a 2.5 psi increase. Take the 2.5 psi and multiply it by 12.5. 31.25 bhp increase would be a safe estimation of increased power. This is important to know because now you can figure out if your fuel system can support the extra 31.25 bhp without any modifications.

The reason the conversion is necessary is because all injector horsepower ratings are rated at BHP.  This will help you design your fuel system around the power that you are making and or planning on making if big changes are the picture.

Applications and limitations

Here is an example of a 24-pound injectors limitation of the horsepower it can support.

24 pound injector (BSFC of 0.57) and at 90% duty cycle can support 303 hp.
24 pound injector (BSFC of 0.57) and at 95% duty cycle can support 320 hp.
A 24 pound injector at 85psi will now flow the same as a 35 pound injector
A 35 pound injector can support 453 bhp at 95% duty cycle.
The same injector will now support 429 hp at 90% duty or 455 hp at 95% duty.
The same injector can now support 125 BHP more by simply increasing fuel pressure.
**Note that these numbers apply to a 4.6 litre engine. A 5.0 has a slightly higher BSFC and will
lower the numbers it can support slightly.

Here are some examples of injector sizes and  their maximum horsepower applications. Two numbers will be giving for every spec. One is the optimum operating range (85-90% duty cycle) the other is a higher 95%. Which is safer? It is all a matter of opinion, it all depends upon how hard you want to push your injectors. The harder you push them, the greater the risk for potential problems. All horsepower is
measured in flywheel horsepower

30 pound injectors at an engine BSFC of 0.57 and at 90% duty cycle can support 378 hp.
30 pound injectors at an engine BSFC of 0.57 and at 95% duty cycle can support 400 hp.
A 30 pound injector at 85 psi will now flow the same as a 43 pound injector.
A 43-pound injector can support 573 bhp at 95% duty cycle.
**Note that these numbers apply to a 4.6 litre engine. A 5.0 has a slightly higher BSFC and will
lower the numbers it can support slightly.

36 pound injectors at an engine BSFC of 0.57 and at 90% duty cycle can support 454 hp.
36 pound injectors at an engine BSFC of 0.57 and at 95% duty cycle can support 480 hp.
A 36 pound injector at 85 psi will now flow the same as a 52 pound injector.
A 52-pound injector can support 693 bhp at 95% duty cycle.
**Note that these numbers apply to a 4.6 litre engine. A 5.0 has a slightly higher BSFC and will
lower the numbers it can support slightly.

42 pound injectors at an engine BSFC of 0.57 and at 90% duty cycle can support 530 hp.
42 pound injectors at an engine BSFC of 0.57 and at 95% duty cycle can support 560 hp.
A 42 pound injector at 85 psi will now flow the same as a 61 pound injector.
A 61 pound injector can support up to 813 bhp at 95 duty cycle.
**Note that these numbers apply to a 4.6 litre engine. A 5.0 has a slightly higher BSFC and will
lower the numbers it can support slightly.

50 pound injectors at an engine BSFC of 0.57 and at 90% duty cycle can support 631 hp.
50 pound injectors at an engine BSFC of 0.57 and at 95% duty cycle can support 666 hp.
A 50 pound injector at 60 psi will also act like a 61 pound injector
**Note that these numbers apply to a 4.6 litre engine. A 5.0 has a slightly higher BSFC and will
lower the numbers it can support slightly.

Lets now go over some examples of real life engine combinations.

Heres an example.

How do I achieve 80 psi so that my injectors can support more power?
You must have a return style fuel system. Secondly you must determine how much boost your car is making. Then determine what FMU calibration you will need to achieve your desired fuel pressure rating. Most supercharger kits will come with the appropriate calibration for a stock pulley application. The problem lies when you want to add boost.

Actual FP=FMU Cal X Actual boost (in real time)
Base FP should always =40 psi without vacuum at idle (no load)

Example: A car makes a maximum of 8 psi. Your FMU has a rate of gain of 10:1
8 psi (x) 10 rate of gain = 80 psi
24 pound injectors at 80 psi flows like a 34 pound injector
A 34 pound injector is enough to support 422 bhp at 90% duty cycle at a BSFC of 0.58 (4.6)

Lets take another example.

Lets use the same car, the owner has 8 pounds of boost but now wants 15 psi. How much injector and
or FP will he need?

First thing to do is approximate how much power the car will add.
Assume the extra boost are the only changes.
1 pound in boost increase=10-15 hp (minimum)
Boost in now increased by 7 psi or approximately 70 to 105 hp.(use the higher number)
Vehicle dynoed at 380 rwhp with 8 psi.
Engine hp=(RWHP)x1.17
Engine hp = 444
New engine hp = 444 + 105 hp (take the greater of the new estimated hp)
New engine hp =549
Your fuel system should now support at least 549 hp.
There are several different ways for a fuel system to support 549 hp.
Looking at the injector guide above, a 42 pound injector could support it if you pushed the injector hard
enough (95% duty). You could also go with a 50 pound injector (no FMU) and it could support that
horsepower level easy.

But what would you do if there were no 42 or 50 pound injectors. Up to 1999, this was the
case.
What if you wanted to retain your FMU? What would you do?

The first thing I would is plug an injector in and see if it works. Let’s try 30 pounders.

30 pound injectors at an engine BSFC of 0.57 and at 90% duty cycle can support 378 hp.
30 pound injectors at an engine BSFC of 0.57 and at 95% duty cycle can support 400 hp.
A 30 pound injector at 85 psi will now flow the same as a 43 pound injector.
A 43 pound injector can support 543 bhp at 90% duty cycle.
A 43 pound injector can support 573 bhp at 95% duty cycle.
**Note that these numbers apply to a 4.6 litre engine. A 5.0 has a slightly higher BSFC and will
lower the numbers it can support slightly.

So as you can see a 30 pound injector could get the job done if needed. Lets take a look at how we
would obtain 85 psi from the FMU.

Actual boost = 15 psi
Desired FP=85
6:1 FMU cal =(15x6)=90 psi

To achieve 85 psi use a 6:1 cal FMU and use a bleeder valve to decrease fuel pressure 5 psi.

So theoretically a 30, 42, and 50 pound injector will all get the job done. It all depends on how hard you
want to push the injectors, and if you want to use an FMU or not.

I have a 99 4.6 with a returnless fuel system. What do I do since I can’t use an FMU?
You will most likely find a fuel injector that will support horsepower requirements up to 660 bhp. Once you surpass this level of performance, you more than likely will need to go to a better engine management system such as MOTECH, and SPEEDPRO. Larger injectors such as 72’s and 83’s have low impedance. Another alternative would be to ditch the returnless style with an older 96-98 fuel return style. That way you could use an FMU.

Ok, I think 72 or 83 pound injectors are better suited for my application, what are the pitfalls?
Other issues besides fuel economy and derivability come into play here.  All disk style injectors such as 72 and 83 pound injectors are low impedance injectors. All pintle style injectors such as the 24,30,36,and 42’s are high impedance. The new 50’s are high impedance as well. Your computer has drivers setup internally and designed for the high impedance injectors. If you run low impedance injectors with a factory computer you must do one of two things.
1. Obtain an injector driver box that will match the two styles so that your computer doesn’t burn up.
2. Have your factory computer modified so that it can accept low impedance injectors. Places like
Alternative auto and JMS can now successfully do this procedure.

Great Reading and tech resources
http://www.pro-flow.com/mustmath.htm - All the necessary formulas you will ever need.
http://www.pro-flow.com/t-car.html - Great EFI and fuel system tech
http://www.pro-flow.com/faq.htm -Frequently asked questions.
http://www.pro-flow.com/how.html - Great Mass air tech

Dave King