Fuel Injection Selection Guide

What an average enthusiasts Need to know.

On the surface, fuel injectors may seem simple.  They are an electrical-mechanical device that opens and closes allowing fuel to enter the engine's combustion chamber.  The on-board computer controls the fuel injector by sending out a signal in the form of a pulse.  The length of time that the pulse occurs will deliver more or less fuel into the engine.  The pulse is measured in milliseconds open time.  In order to provide the correct air to fuel ratio, the pulse will vary based on the rpm and load the engine is seeing.  

As a performance automotive enthusiast, it's no surprise that we all want power.  But how does the fuel injector relate to horsepower and why does it really matter? 

To maintain good combustion and torque/power production from a four-stroke gasoline engine you need to achieve the proper parts fuel to air flow entering the engine.  This is known as the air fuel ratio or AFR.  If you double your airflow entering the engine you need to double the fuel delivery at the same rate to maintain the same AFR level.  What does this mean in terms of horsepower?  To translate the horsepower level of an engine to the fuel injector flow rate, we use a term called BSFC or brake specific fuel consumption.  The BSFC is the key to understanding an engine's efficiency of fuel consumption with respect to horsepower.  The lower the BSFC number, the greater the engine's efficiency.  I want to keep this tech article simple so the average person can generate their own opinion on what injectors they need for their project, budget, and power levels.  In lieu of showing a lot of math, there are several online fuel injector vs horsepower calculators that are fairly accurate.  You can check them out below: 

Injector vs horsepower calculator #1 

Injector vs horsepower calculator #2 

Injector vs horsepower calculator #3 

From any of those calculators, you will be able to very quickly determine what cc/min size injector you will need to fuel your desired horsepower level.  You should note that the fuel injector size vs horsepower will drastically change based on the fuel being used.  E85 will require approximately 30-40% higher fuel flow capacity to achieve the same AFR level as gasoline.  The BSFC in this case will jump around the same % in terms of fuel efficiency vs horsepower, so expect to use more fuel and size your fuel system accordingly!

Now that you have a good foundation to size your fuel injector based on your engine, application, and fuel, what is the next important part of selecting a fuel injector?  Injector Data!!!  What is injector data and why is it important?  Above, we learned that the on board computer sends a pulse to the injector to command it to be opened or closed.  What was not mentioned was the dead time that the injector will experience.  Dead time can be most simplistically defined as the time it takes the pintle to open and fuel to flow once the internal electrical circuit gets energized from the on-board computer.  Without providing the on-board computer the accurate data on how the injector responds, or its dead time, it will not know how to deal with the lag characteristic of the injector you are using.  This very brief period of lag has a huge impact on the fueling to the engine, especially at very low pulse widths (opening and closing), which are found at idle and light throttle operation.   Having the correct injector data is key to making an engine run correctly with an EFI system.  

So, you're probably thinking, "Why do I care, my tuner will have to take care of it."  While that is partially true, a tuner's job while tuning your car is working with the parts that you have installed on your car.  Accurate injector data is not easily obtained if the injector manufacturer does not supply it.  Why does this matter to you?  Without your tuner having accurate injector data, the on-board computer will not know how to properly control the fuel injector, specifically in the idle and light throttle driving situations.  Will it work with incorrect data?  Yes.  Will it run, idle, and drive better and more consistent while achieving better fuel economy with proper injector data?  Absolutely.  So, when selecting a fuel injector, make sure you confirm that the manufacturer provides accurate injector data.  Injector Dynamics are our go-to injector brand because they have excellent accurate injector data available in all kinds of formats for different platforms including GM, Subaru, Ford and universal standalone applications, to name a few.  This makes the job of a tuner much easier.  A tuner won't have to spend time developing the injector characterization data, which can range from hours in some cases to nearly impossible for newer ecus like Ford and GM.

The last piece to the puzzle is injector size vs linear fuel delivery.  You're probably thinking, "What the hell does that mean?"  Every injector has a point in its operation at low pulse widths that will start to make the fuel flow coming out of the injector nonlinear.  Nonlinear fuel delivery simply means that if we expect the injector to deliver 40 cc/min at 1.0 ms (millisecond) pulse width, it should theoretically deliver something like 35 cc/min at 0.8 ms.  In almost all cases this is untrue.  Every injector will have a point where it's "on the cliff."  This means that there will be linear fuel delivery with pulse width until a certain point ("the cliff").  Below that point, we can no longer accurately account for the injector flow by the commanded output from the on-board computer.  Why does this matter?  At idle, which will be the lowest pulse width operation, we want to achieve a 14.7:1 AFR ratio ideally.  If we go back to the example above, if we are operating at 1.0 ms at idle at 14.7:1 AFR and the air temperature increases after driving, the injector output reduces to 0.8 ms to keep the AFR at 14.7:1.  At this point, we are not flowing at a linear rate, which causes the fuel delivery to drastically drop off.  This would create a lean condition and you will experience either a hunting idle if the oxygen sensor tries to correct the massive fueling drop OR get a lean misfire that will cause the engine to run very choppy and unstable.  This nonlinear behavior can occur at light throttle driving as well, depending on the injector characteristics.  In general, the larger the fuel injector is the lower you will have to reduce the pulse width to achieve the same AFR as a smaller injector.  The lower you run the injector in pulse width the greater chance it has to operate either "on the cliff" or in a nonlinear condition.  This makes for a miserable driving experience from a lean misfire.  I have tuned some cars that were just plain horrible to idle and drive and the only way to correct the problem was to richen up the AFR mixture.  Over time, this will wash the cylinder walls out of oil and wear the piston rings quickly, which will lead to an engine rebuild.  Not so ideal now, huh?  

So what can be done?  We all want to make the most horsepower possible, but in doing so, the fuel system is a critical component in how the car will perform in real world conditions.  If you have a 4-cylinder turbo car that makes 800-900 whp you will need a large injector in the range of 1600- 2000cc/min injector to deliver ample fuel.  That same car will have a difficult time with idle quality and drivability compared to a car with a smaller injector like a 1000cc/min.  It should be noted that when selecting a fuel injector for an alcohol based application (e85), the injector will be open much longer to deliver the fuel which, in many cases, keeps the injector operating in a linear range.  Oftentimes, I see flex fuel cars run poorly at idle and light throttle driving on the 93 octane portion of the tune, but run great on e85 because of the longer opening times.  Another important detail is that even on a smaller injector size like 900-1000cc/min, the quality of the injector internals and spray pattern both play a role in the linear response of the injector.  Keep in mind that just because two different brand injectors are the same size, they won't always perform the same.

The takeaway.  We learned how to calculate the injector size for the horsepower goal that we have.  With that injector size we know that you should be looking at a quality injector manufacturer that can supply accurate and detailed injector data for you or your tuner when you tune.  Lastly, we learned that the larger the injector the lower the pulse width at the same engine rpm, load, and AFR.  This can be a double edge sword because at idle and light throttle the larger injector will operate at or close to the nonlinear zone.  As a result, idle and drivability can suffer.  Choose your fuel injectors wisely!  They can literally make or break how well your race project will run.