Don't understand what DFI (Digital Fuel Injection) is or what it's supposed to do? Take a number and get in line because you are not alone. We deal with a lot of questions on DFI, and I hear a lot of things said, even by some people that are supposed to know about DFI, that are just flat wrong.

First let me say that this series, like all of our tech articles, deals with modifying and or replacing the stock systems that came with your car. About 99% of all cars that came with fuel injection use an analog computer system to control the engine functions. Some of the later model sporty type cars and high end (expensive) cars used Digital injection, and some are even sequential fired systems.

Also I should explain that we are discussing multi point fuel injection here and not throttle body injection. The difference is that multi point has at least one injector per cylinder placed at the intake mounting flange. A throttle body injection system has the equivalent of a carburetor body with 1 to 4 injectors mounted on top. Although throttle body injection is still more accurate than a carburetor at controlling air/fuel ratios, they are not as efficient as multi point.

Fuel Injection systems, regardless of the data type (digital or analog) all try to do the same things. They are meant to fire the injectors at the right time, and to keep an eye on the air/fuel ratio and engine load to keep the engine running in peak efficiency to produce smooth power curves.

Injectors are available in many shapes, sizes, and types, but all of them perform the same task. They get a measured amount of fuel into the engine in the finest spray pattern possible. That's one of the big advantages of fuel injection over carburetors, the fuel atomization. The finer the fuel is atomized into the incoming air, the more explosive the mixture is and the more efficiently the mixture burns. Efficiency may sound like the wrong word, but it doesn't just apply to high gas mileage econo cars. It simply means that the higher the volumetric efficiency is, the more fuel in the fuel mixture is being burned (less waste). Now that can mean better emissions, fuel mileage and most importantly, power.

Most injectors have a spray pattern between 10 and 30 degrees, and are angled toward the intake valve in the head. All injectors have a flow rate associated with them that is measured in either lbs./hr (pounds per hour), or CC/Min (cubic centimeters per minute). Also, the more cylinders an engine has, the smaller the injector size it needs as a rule. A Mustang comes from the factory with 19 LB/hr injectors for a 302 V8 engine, but a Mitsubishi Eclipse comes with 34 LB/hr injector.

Smaller engines generally make more HP per cubic inch than the larger engines. Take those same 2 engines for example, the 302 makes 245 HP and the 2.0 liter (122 cid) makes 205 HP. Injector size depends on cylinder size, number of cylinders, and total HP output. The injector has to provide enough fuel to support the HP and still stay within its duty cycle. The duty cycle is how long the injector is open (injecting fuel) at max HP. Most injectors like to stay below 85% duty cycle or they overheat and can fail. The calculation for figuring the correct injector size for your car appears below.

Using the 302 engine above :

HP = total engine horsepower (not SAE net)
bsfc = brake specific fuel consumption. Use .48 for normal engine, .56 for turbo or blown
#inj = Total number of injectors

LB/hr =  ((HP * bsfc) / #inj) / .85

((245 * .48) / 8) / .85 = 17.29 LB/hr

The .85 above is the duty cycle of the injectors. You can change that to see how it effects the injector size.  If you change the duty cycle to .80 (which is 80%) the injector size required increases. That's because the injector is open for a shorter time per cycle using 80% instead of 85%. So the injector has to be larger to supply the same amount of fuel.

To convert from pounds per hour to cc/min multiply by 10.5.

17.29 * 10.5 = 182 cc/min (rounded up)
 

Playing pressure games

You can also effectively increase the size of the injectors in your engine by raising the fuel pressure that the pump supplies. That is confusing at first, so we will step through why it works.

Most fuel systems for injected engines run 36 to 40 PSI stock. Say we have those 19 LB/hr injectors from the Mustang above and want a bit more fuel to the engine. The cheapest solution is to add fuel pressure by getting an adjustable fuel pressure regulator. This approach is only a good choice if you don't need a lot more fuel. If you need more than 3 or 4 LB/hr do yourself a favor and buy bigger injectors.

One of the drawbacks of raising fuel pressure is that it richens up the fuel mixture everywhere including at idle and part throttle. That makes for a sloppier idle and less fuel mileage not to mention the watering eyes it causes if you are behind the car.

Turbo cars come with a rising rate regulator that raises fuel pressure by 1 LB pressure for each LB of boost.  That's a nice compromise since it only happens when the car is under acceleration (making boost). It also solves the rich idle problem for the manufacturer.

All of the DFI computers available to my knowledge allow you to map your fuel system in a way that allows the use of huge injectors while still having a good idle and quick crisp throttle response. Since the computer is controlling the time the injector stays open anyway, it doesn't pose any problem to shorten that time under light load or at idle.
 

Selecting injectors

After you determine what DFI system to use, decide what type of injector you need to run. Below are some basic descriptions of the different styles and types of injectors that are available. Each has its good and bad points, but all work very well. When we talk about fast or slow acting, you need to keep in mind that the times are measured in milliseconds and some reaction times are in the nanosecond range. Don't discount an injector based on our comments about being fast or slow acting. All injectors must work within a maximum window of under 15 milliseconds running wide open.

The differences between the types and styles only becomes important if you are searching for 1000ths of a second at the track and extreme RPM ranges. Any injector can handle the vast majority of street/strip cars.

Make certain that you follow some simple rules when you start gathering parts for your DFI system.

1. Don't mix injector types. Use all low or all high impedance types
2. Don't mix injector sizes. Use the same flow rated injectors everywhere. You can use a different size for staged injectors with some DFI computers, but it is much easier to use the same size for all.
3. Don't mix injector styles. Use ball, pintle or disc styles, but use the same style for all injectors.
4. It's better to have injectors that are too big than it is to have them too small. If you plan to do more modifications to your car in the near future, buy a bigger injector than you need now and map the system a bit leaner, then you can remap later to use the full injector size.
 

Injector styles

Injectors come in lots of sizes and shapes, and they also come in several styles. It makes a difference what type of injectors you choose to run in a DFI system.

Pintle type

The pintle type injectors are very common and are used by most manufacturers. They have a long pin (or pintle) inside that is raised each time the injector fires. The pin normally seals the hole in the bottom of the injector base until it is raised by a magnetic coil. These injectors are very reliable and are very self cleaning, but they tend to be slower to respond than some other types because of the weight of the pintle itself.

Disc type

Disc type injectors are very fast operating, but are more expensive than the pintle type, and can get clogged or obstructed easier. They use a thin disc to seal the injector base which is lighter and so responds faster.

Ball type

The ball type is a great compromise between the pintle and disc types above. It has a bit more tendency to get dirty (not as self cleaning) than the pintle type, but has fast response, and are very reasonable in price. These have become the choice of most racers because of a good cost Vs performance ratio.

Impedance types

Injector impedance is a fancy way of telling you the resistance value of the electrical coil inside. There are 2 styles of injectors, low impedance and high impedance.

Low impedance

Low impedance injectors are also called peak and hold type. They use a high amperage (4 to 20 amps) pulse to open the injector and then a lower amperage (.6 amps or so) to hold it open for the required time. These injectors are faster acting and can be really tortured, but they are more expensive than the high impedance styles. These injectors come in .9 ohm to 4 ohm and can be tested with any ohm meter.

High impedance

High impedance injectors are between 12 to 16 ohm. They require less current to open and are used in many factory injected cars. They are generally slower acting than the low impedance type, but are significantly less expensive.
 

Batch fired or sequential fired?

What the hell does that mean anyway?  Well, simply put, it describes how and when the computer tells the injectors to open and let fuel into the engine.

Batch fired

Batch fired systems fire all injectors at the same time multiple times during the engines 4 cycles. That means if you have an 8 cyl. engine, all 8 injectors fire at once each time a cylinder opens the intake valve(s). Generally the injectors fire before top dead center and bottom dead center. That ensures that all cylinders needing fuel get it in most engine designs.

About 99% of all factory systems are batch fired, and most of the aftermarket systems can do batch fire as well. What we are mainly concerned with here is performance, and if we can get better gas mileage at the same time, that's great. As wasteful as it sounds to fire all injectors at once, it is actually quite efficient, and offers some advantages in cooling as well.

First and foremost, batch fire systems are relatively inexpensive to manufacture, and they are much easier to program (map) initially and modify later if needed. You should also understand that even the worst fuel injection design is generally far more efficient than any carburetor system can be.

Before you write off even considering a batch fired computer for your DFI system, consider that many, many race cars around the world are producing over 1,000 HP using batch fired digital systems.
 

Sequential fired

A sequential injection system fires the injectors much like your ignition fires spark plugs, one at a time. These systems are incredibly flexible in that they can be very precisely mapped to get every last HP out of each cylinder. They are also much more complex to program initially, and tune. To get the added benefits of SFI, you must put the car on a dyno for the time it takes to get the map just right. This always runs into many hours of dyno tuning and it can get expensive pretty fast.

Sequential systems can produce more HP and conserve more precious fuel at the same time than a batch fire system can. The only real drawbacks are the added complexity of creating and maintaining the maps and the added dyno expense if you modify the car after the map is set up, and an initially more expensive DFI system to buy.

Remember that if you are running a street/strip car and just want more HP, better response, and potentially much better times at the track, you'll be better off with a batch fired system. Keep in mind that even the budget batch fire DFI systems are far more controllable and efficient than the factory system. You never have to worry about buying an EPROM for your computer again, you simply make adjustments where they are needed. If you are looking for every 1000th of a second at the drag strip in a serious race car, then a sequential system may be just what you are looking for.
 

Staged injectors

Many DFI systems support staged injectors. That simply means using 2 or more injectors per cylinder and turning on the second set under conditions that you specify when doing the fuel map.

What this does is allow you to run 16 40 LB/hr injectors in your turbo'd 315 cid Mustang instead of 8 80 LB/hr injectors. That gives you nice idle and throttle response, descent gas mileage, and all the fuel you need when you need it.

Some DFI systems use a primary set to run all the time and then add the second set according to driving conditions. Many DFI systems run all of the injectors at the same time, but only at 50% of the normal duty cycle in the map. As you feed more throttle and the load increases, the computer increases the pulse width to each injector until all are running at full mapped duty cycle.
 

Wide band or narrow band?

Many DFI units are available that can use a wide band O2 sensor. An O2 sensor is a device that is placed in the exhaust close to the head. It's purpose is to generate voltage based upon the temperature of the exhaust passing by it. A standard O2 sensor reacts to changes pretty slow considering the amount of exhaust that is flowing by it, and it is not an accurate tool to measure true air/fuel ratio. It can give you a rich /lean indication, but isn't accurate enough to estimate the air/fuel ratio.

A wide band O2 sensor (or Lambda sensor) is faster reacting and can, in theory estimate an air/fuel ratio. I say in theory because I have yet to see a wide band sensor that is as accurate as it claims.

Both types of sensors react to heat. The hotter the exhaust temp is, the leaner an engine is running. The richer it runs, the cooler the exhaust. The sensors have an operating range of between 1000 and 1700 deg. F. The DFI system, (and stock injection systems) read the sensor to find out if the fuel system is too rich or too lean and then make corrections as needed.

So which is better? Well, it kind of depends on your point of view. Wide band sensors have a relatively short life span and are much more expensive to replace. They are faster, and much more accurate than a standard sensor, but the average life span is about 250 - 300 hours of running time. If you drive your car an hour a day, you'll replace it in less than a year. Running leaded fuels also shortens the life of both types of sensors.

Again, for most street/strip/roadrace cars out there, the standard O2 sensor would probably be just fine. For more serious applications where you may be running close to the edge for maximum power, the wide band would be the way to go.
 

Operating basics

Ok, now that the definitions are out of the way, what really happens in the engine, and why?

Remember high school auto mechanics class? That's OK, I didn't take auto mechanics either. An engine is more or less an air compressor that is self powered. It needs 3 things to operate, air, fuel, and an ignition source. Since we are not talking about diesel engines here, the ignition source is the spark plug.

That leaves air and fuel. Most DFI computers control both the ignition system and fuel system, and completely take the stock computer out of the loop. That leaves all engine related tasks to the new DFI computer, and the stock computer is left with anything else (anti lock brakes, climate control, etc.).

The computer uses maps for both fuel and ignition that it refers to throughout the RPM band of the engine. It reads sensors and updates itself about 28 times per second or more. DFI doesn't change the way an engine works in any way, but it does change the way that the engine gets fuel and spark.

The fuel map usually has points for normal and heavy loads, as well as manifold pressure (vacuum and boost) for each RPM point. The RPM points are in 250 or 500 RPM increments, and each point gets a target air fuel ratio for the computer to maintain under each of the engine conditions (heavy load, sudden throttle open or close, etc).

We aren't going to try to explain the mapping process here, there are just too many system and software types available. The basic operation of the system is pretty simple though. The computer monitors the throttle position sensor, O2 sensor, water temp, and intake air temp. It also monitors the MAP sensor to determine the load of the engine. MAP stands for Manifold Absolute Pressure, and measures both vacuum and boost conditions.

Most DFI systems are of the speed density type. Instead of using an expensive and restrictive mass air flow sensor like most factory injected cars, DFI uses real time information supplied by its sensors. It can do that because digital systems are faster and more accurate than analog type injection.

The computer can decide how you are driving by watching the MAP sensor, and RPM. If for example you are driving at 2300 RPM at 15 inches of vacuum, that's light load (cruise). If you are at the same 2300 RPM and at 0 inches of vacuum, that's hard acceleration. In a turbo or blower car, under hard acceleration, the MAP would see boost pressure.

During all of those driving conditions, the computer is busy comparing sensor readings and adjusting the fuel and ignition curves to match the driving conditions. The whole purpose is to feed the engine enough fuel to keep it from leaning out, but not to run too rich and loose power. It may add or decrease ignition timing depending on the conditions also.
 

Ignition types

Your car has an ignition system now, but you might want to consider changing the ignition type now that you are converting to a DFI system. With a DFI system, you are no longer limited to the factory type of ignition system. You might change to a crank triggered  system using a distributor, or you might want to take this opportunity to change to a distributorless ignition using multi coil packs.

Standard distributor

The old tried and true distributor can still be used with most DFI systems as long as it is optical/hall effect, or magnetic type triggering. Many DFI systems do not support points type distributors. You may have to install a crank trigger pickup and modify the balancer to get the proper trigger signals. Many cars, especially imports use a hall effect type of distributor that should work fine for most DFI systems.

The only real drawback to distributors is that they are a serviced part. Meaning that they still need to be kept clean and dry, and caps and rotors still have to be inspected and replaced periodically. Also they do develop wear and are not as accurate as other ignition types. Overall though, the old distributor still delivers killer spark to the plugs and is reliable.

Distributorless ignition

These systems use no distributor at all. The place where the distributor used to live is plugged (adapters are available for many engines). The system is triggered by a camshaft or crankshaft sensor that can be optical or magnetic. Magnetic is the most popular, since optical triggers tend to be more bulky using a toothed or slotted wheel of some sort to measure crank angle.

The system works by receiving a top dead center and bottom dead center reference from the trigger, and additional signals for 30 or 90 degree positions. The ignition section of the DFI computer needs to "learn" how your ignition trigger works, and most are pretty straight forward in their setup.

For a magnetic trigger a series of magnets is imbedded into the balancer or crank pulley with one reference magnet reversed in polarity. The sensor is rigid mounted to the block and the spacing is set to the sensors specs. It is very important that the sensor mount be rigid. If you can move it with moderate thumb pressure, it isn't stable enough.

Ignition firing types

Yes there are different ways to fire the ignition also depending upon how the ignition is set up. Multi coil systems can be setup in different ways. You can for example use 2 MSD coils to fire all 4 cylinders using coil #1 to fire cyl 1 and 3 and coil #2 to fire cyl 2 and 4. This type of setup will fire each coil once per crankshaft revolution. It is sometimes called a wasted spark system because one plug is always firing at the wrong time (wasting the spark to that plug).

Sequential ignition is what happens with a distributor, and with multi coil systems that use a coil (or coil pack) for each cylinder. Coil packs contain 2 coils each in a compact coil unit and can handle 2 cylinders independently. In this arrangement, the DFI system triggers each coil to fire in firing order sequence. It is very accurate and gives you full control over timing advance/retard and dwell time (coil charge time)

regardless of the type of system that you decide to run, the DFI computers ignition section will be able to handle the timing and dwell cycles as needed. It also means that you no longer need that ugly and bulky vacuum advance pot on the side of the distributor since you can program whatever advance curve you wish under differing engine loads. Since the computer handles the timing totally, changes occur in thousanths of a second.

You can for example set the timing for 0 advance while cranking the starter and advance to 15 degrees at 750 RPM. Many cars idle and run great with 18 degrees of advance at idle but the system can back the timing off to 10 degrees as soon as engine load increases to avoid detonation. Since timing response occurs in milliseconds, all kinds of possibilities exist for mapping the ignition curve.
 

Forced induction and DFI

Does DFI work well with forced induction systems like blowers and turbos? You bet they do. Since they control both fuel and ignition curves, they are ideal for forced induction. They can add fuel under boost conditions and retard timing at the same time to avoid detonation. Most systems also have programmable outputs so you can map in an electronic boost control switch to let the computer completely control boost levels that you set. That means that you can actually map the system so that a drag car can leave the line under partial boost and switch to full boost at a given RPM to avoid excessive tire spin.

Most DFI computers have some sort of soft touch rev limiting built in as well as fuel pump control and safety shutoffs. Some systems have a 2 stage rev limiter built in, one that allows a low RPM limit to leave the line, and a second that is the high RPM safety limit. I set my low limit to 3000 RPM and trigger it with a hidden switch as a "valet" feature in case there is a chance that someone will be driving my car.

Several DFI systems are now including a full featured turbo timer function that allows the engine to run for a pre-set period of time after the ignition is turned off. You simply put it in park or neutral, set the brake, turn off the ignition then get out of the car, lock it and walk away. In this way the turbo gets to cool down at idle with oil flowing through it to avoid coking (oil baking onto the shaft and bearings). Then the system shuts itself off and arms any alarm system that may be installed.