Taking The Mystery Out Of Fuel Injection

What fuel injection does makes what carburetors do seem like little more than guesswork

Let's take a moment to remember the carburetor.Hector Cademartori

This article was originally published in the 2001 issue of Cycle World’s Power & Performance magazine.

A moment of silence, please, for the carburetor. It is dead, you know; we motorcyclists are just hanging onto the remains until the official death certificate is issued. There is no question that those traditional fuel-mixers are ancient history, beloved but outdated instruments of another era made obsolete by electronic fuel injection. After all, there hasn't been a carbureted automobile produced in more than a decade; and with an increasing number of new bikes being equipped with EFI each model year, it won't be long before the carburetor is discussed only in the past tense among motorcyclists, as well.

That transition, from carburetion to fuel injection, scares the hell out of some people, and not just the ones who like to tinker with their bikes on weekends. Even a lot of experienced tuners consider fuel injection a new-age intrusion that has unnecessarily complicated what formerly was a simple procedure.

But that's to be expected: Any time a new technology begins to replace an older one, the high priests of the old way feel threatened. Those of us who learned the tricks of carburetion don't like to see our hard-earned knowledge become irrelevant. We balk at the notion of software hotshots, using a whole new language we don't understand, taking over the task of fuel delivery. Many hot-rodders also fear that once fuel injection becomes universal, engine modification will be impossible.

Not true!

fuel injection
Class is in session, so pay attention.Hector Cademartori

Although it seems that fuel injection threatens to take control of fuel mixture out of the user's hands and put it into those of government emissions regulators, this is not the case. Yes, EFI is becoming the preferred method by which manufacturers can comply with ever-more-stringent exhaust-emissions regulations. But new developments that we'll discuss in a few minutes are making it simpler to tune fuel injection, and those advances will soon make adjusting the mixture an easier process than it ever was with carburetors.

Actually, when you make a point-by-point comparison between carburetors and fuel injection, EFI seems a much more rational way to deliver fuel into an engine. To understand why, let's take a quick refresher course in carburetion. Carburetors are analog devices that deliver fuel in response to variations in intake vacuum. This vacuum arises, as noted by Bernoulli's famous principle,from the fact that as air is set into motion, it trades away some of its pressure to create velocity. The resulting partial vacuum in the venturi of a carburetor is powerful enough to draw up fuel from the float bowl and spray it into the airstream, thereby forming an air-fuel mixture. This low pressure—referred to as the "metering signal"—is the analog of the engine's fuel needs.

The art in carburetion consists of ensuring that fuel is delivered in proper proportion, accomplished by means of fuel jets and airbleeds. As engine suction draws air through the carburetor, the low pressure in the venturi, acting on the jets and airbleeds, delivers an approximately correct flow of fuel to the engine. And as rpm and throttle position change, correct fuel flow is approximately maintained.

We use the word "approximately" here because carburetors thrive on consistent intake signal. If the engine's in-take process is much the same at 1200 rpm as it is at 6000, it's easy to get good mixture from a carburetor. But anything that changes or upsets this consistent signal makes carburetion more difficult.

A bigger carburetor is the classic example. We put on a big carb to get more airflow to make more power on the top-end; but the bigger the carb, the slower the velocity of the air passing through it at any given rpm and throttle opening, and so the weaker its vacuum signal becomes. By giving the metering circuits weaker signals to work with, the big carburetor makes fuel delivery sluggish at lower rpm; the rule is, the bigger the carb, the bigger the tuning problems. But with electronic fuel injection, which does not depend upon intake vacuum signal, this problem simply goes away.

What fuel injection does makes what carburetors do seem like little more than guesswork

When an engine is modified—for example, by installing longer-duration cams and a less-restrictive exhaust system—the nature of the intake suction pulses is changed. Tuned exhaust pipes may make intake suction stronger in certain speed ranges but weaker in others. And longer-duration cams weaken suction pulsing at lower rpm. Because the suction signal received by the carb has changed, so does the fuel mixture, thus the engine runs poorly.

Carburetion experts develop a feel for these kinds of situations. With a few running tests, a carb wizard can determine which way to go with changes to which systems. So, after installing different jets or other metering hardware, he has the engine again running nicely—although there still may be very little he can do to fully compensate for the weaker intake signal.

On a modified fuel-injected engine, however, none of this wizardry is useful. Injection systems operate on an entirely different set of principles that allow the delivery of fuel into the intake stream to be based on an engine's actual needs rather than simply on the flow of air over an orifice. By comparison, what fuel injection does makes what carburetors do seem like little more than guesswork. Here are the key basics of EFI:

Time Injection

Carburetors spray fuel continuously at a rate that varies with airflow. Fuel injection uses one electromagnetically controlled nozzle per cylinder to spray fuel in timed squirts—one spray per cylinder for every two revolutions of the crank. The fuel pressure available at the nozzle is always constant, maintained at a regulated value in a fuel manifold by an electric pump. Therefore, fuel quantity—and thus, the air-fuel mixture—is determined only by how long the nozzle is held open. That duration, called the "on-time," is decided by a control computer.

Stored Fueling Information

The control computer—usually called an Engine Control Module (or "ECM" for short)—contains a kind of library in which is encoded the correct fuel quantity for the nozzle to inject at every engine rpm and throttle position. This information is determined through extensive dyno testing at the factory during the engine's development stages. Fuel delivery is varied at many rpm points and throttle positions to find the on-time that gives maximum torque. The complete list of those variables, which relate max-torque on-time to rpm and throttle position, is what is called the "fuel map." The map for street-legal motorcycles must also be configured to let the engine meet the prevailing emissions requirements; but on most modified or racing engines, the only criterion is maximum torque at all throttle settings and rpm points.

Because of the way in which fuel injection works, its accuracy does not depend upon the tricky correspondence between the engine's actual fuel needs and the intake suction signal. EFI gives the engine what it needs because it knows what those needs are in all conditions by referring to the fuel map. Of course, if those fuel needs are changed because of modifications to the engine, the fuel map must be changed to suit.

Rapid Response

The control computer nows the engine rpm and the throttle opening at all times because it receives signals from a tachometer and a throttle-position sensor. Before each fuel-injection spray is delivered, the computer effectively looks up the correct on-time for the current rpm and throttle position, then holds the nozzle open for that prescribed amount of time.

This is easy work for the computer because a typical ECM operates at between 1 and 10 million information processing steps per second—or 1 to 10 Megahertz (MHz), to use popular computer terminology. This means that at, say, 5000 engine rpm, a 10 MHz computer can perform 120,000 such steps during each rotation of the crank-shaft, or 333 steps for every degree of crank rotation. The computer can operate this quickly because it is of fingernail size, so electrical signals within it travel only tiny distances. This nearly instant response of EFI to changing conditions makes the engine much less likely to bog or hesitate when the throttle is opened suddenly.

Automatic Correction

Because fuel evaporates incompletely in a cold engine, carburetors use either a choke or an enrichening system to richen the mixture for cold-starting. Carburetors run richer as the weather warms and leaner as it grows colder, thereby changing the power available. And carburetors also allow the mixture to be too rich as we ride to the top of Pike's Peak, where the air is thinner, and too lean as we travel into Death Valley, where the air is more dense. Racers get peak performance in all conditions by changing carburetor jetting to maintain a best-power mixture—not a viable technique for general street riding. In production streetbike engines, carburetion is set rich enough to safely cover anticipated running conditions while also meeting requisite emissions standards, and we tolerate any power losses these trade-offs might produce. With the processing power of a computer, however, these compromises disappear, and all this tuning work can be done automatically. Sensors located around the motorcycle report engine temperature, atmospheric pressure and air temperature to the computer, which then uses simple arithmetic (you can do a lot of figuring in 120,000 steps) to modify the engine's fuel delivery according to all these variables. This is why fuel-injected engines start so well when cold, why they can be immediately ridden off without stalling or hesitation, and why they don't blubber with excess fuel at very high altitudes or starve for fuel at very low altitudes. The fuel map provides the basic data for fuel delivery, and the sensors modify this information according to current engine temperature and air density.

Complete Adjustability

There are things you just can't do with carburetors, such as vary the time at which fuel reaches a given cylinder, or change the mixture only in one particular part of the rpm band, or vary the strength of the mixture from one cylinder to another, or even from one gear to another. With fuel injection, all of these options—and many more—are not only possible but easy to accomplish.

What's more, fuel injection is obedient. We've all experienced carburetion "flat-spots," which typically occur on modified engines. A carburetor needs a venturi vacuum signal strong enough to make it spray fuel, but some combinations of exhaust pipes and cams, at certain speeds, can kill or even reverse that signal, causing a flat spot that often is impossible to eliminate. Fuel injection, however, is immune to such problems; it merely does what it is told by the fuel map and sensors.

Okay, you're thinking, this is wonderful stuff if you happen to be a computer hacker or an engine developer working for a motorcycle manufacturer. But what about the rest of us? How do riders who want to modify their fuel-injected engines manage to get their fuel mixtures correctly adjusted?

Well, more easily than you might think. Every day of the year, thousands of people who aren't computer geeks or software programmers tune fuel-injection systems. No programming is necessary. All we need is a way to tell the computer what we want. Several such ways exist.

Special Chips

In some ECMs, the fuel map is stored on a replaceable computer chip. Engines modified with certain cams, pipes and other parts have already been tested and mapped by some OEM or aftermarket manufacturers, and chips with such maps are available for users running those specific combinations. This is a plug-in replacement.

Addressable Fuel Curves

Some injection systems can be adjusted either by a special hand-held recalibration unit or by a laptop computer. The existing fuel curve—which relates mixture strength to rpm—can be downloaded and displayed on the computer's screen. Then you can enrich or lean out the whole curve, or any section of it, by any amount. Once you've made the changes you want, you upload the new fuel curve back into your ECM.

The net effect is analogous to that of changing jets, but it can be much more specific. Plus, you remove no parts from your bike. No jets will roll into inaccessible places, and there won't be a sticky place in your driveway from draining a carburetor float bowl. Mechanics at racetracks do this job every day: plug in, download, adjust, upload, road test.

Full Reprogramming

For some time, Dynojet Corporation has marketed devices for fuel-injection tuning under the name Power Commander. These devices change fuel delivery either by supplying adjusted sensor data to the ECM, or by directly changing the on-time of the pulses the ECM sends to the injector nozzles.

Now Dynojet even has begun to offer a service, at some of its associated dyno centers, by which the fuel injection on a motorcycle with any combination of modifications can be reprogrammed quickly and automatically. While running on the dyno, and with an oxygen sensor in the exhaust to measure fuel into the bike's ECM. As the dyno run progresses, all the many data points are automatically gathered and stored. Essentially, this system "learns" the correct fuel mixture curve as it runs, and this data is automatically stored in the Power Commander system on the bike as its new fuel map.

Time consumed? Such a dyno run takes typically 20-40 seconds. No muss, no fuss, no trial-and-error involving the repeated threading of jets into and out of a hard-to-reach float bowl located on the bottom of a carburetor inconveniently tucked behind an air cleaner. An engine tuned in this fashion—whether nearly stock or highly modified—will start easier, run smoother, respond more quickly. accelerate harder.

Electronic fuel injection is nothing to fear; it's merely a newer, better way to fuel an engine. And don't think of injection as the stealthy hand of Big Brother, either, because convenient tools are available by which to control it. Yes, EFI exists in the first place because it does the best, most flexible, most accurate job of controlling engine exhaust emissions, but it is also a superior high-performance fuel system in its own right.

This is why you should learn to work with fuel injection. Its principles are not that difficult to grasp; and once you do, you'll soon find that you have more and better control over engine performance than you ever had with carburetors.

Whether you like it or not.