Just thought I would take a couple minutes and maybe inadvertently answer a few questions about EFI systems. Several years ago I began the process of learning about the system that would one day completely replace the ever beloved carb. I am not claiming any type of expertise on this subject, but I though I would make an attempt to possibly answer some of the basic questions that many riders are curious about. Just keep in mind, as you read, that an EFI system is not entirely different than that of a traditional carburetor. Both ultimately perform the same function, just not with the same precision. Each adjustment to your base fuel program can be related to a change in jets on a carb. The main difference being a carb is a piece of hardware that uses these jets to make general changes to the air fuel mixture, and an EFI system uses software to make very minute changes to very specific areas of the air fuel mixture. First off, EFI in my opinion is much more efficient than any carb could ever be. Please let me explain. I know that there are many good carb tuners out there that are, or have been, unwilling to accept the “new” transition to computer based systems. I know that these same individuals can tune a carb to run very smoothly. I’m not comparing a finely tuned carb to an un-tuned EFI system. Instead, I am speaking of a finely tuned carb side by side with a professionally tuned EFI system. When tuning a carb, in order to resolve a lean or rich fuel mixture in a particular “area” of the AFR (air fuel ratio), it is predominantly done so by changing the pilot jet, main jet, or even both. These jets come in various sizes (openings) to allow different volumes of gravity fed fuel to enter the carb. Changing of a jet may indeed correct the AFR in the desired area, but it also changes the AFR in that entire “range”. Meaning, if you are at 10% throttle and 4000 RPMs, and you need to increase the AFR (which leans the fuel) due to a hesitation in throttle response, then you will inadvertently change the AFR at say 15% throttle and 4000 RPMs. But what if the bike wasn’t hesitating at the 15% throttle position? Well, there is no way to separate those two conditions by use of a carb. So the correction of one issue in one area may introduce an issue in another area. Now lets look at the same situation using EFI. The fuel delivery of EFI is done by means of a pressurized system that consistently feeds fuel to the injector. By use of a Fuel Tuner/Programmer (FMF Power Programmer or Power Commander III etc) you can easily fine tune for specific areas. Granted the FMF is not as adjustable as the PCIII, but is still much more specific than the adjustments on a carb. The PCIII will allow AFRs to be adjusted in intervals of 250 RPMs and (if memory serves me correctly) Throttle Positions of 0%, 2%, 5%, 10%, 15%, 20%, 40%, 60%, 80%, and 100% (or WOT). So if you begin a spread sheet from 750 RPMs and continue in 250 RPM increments to say 11,000 RPM, and you do so for each of the Throttle Positions, you can see the amount of fine tuning that takes place. So I ask, how can a carb ever be expected to compete with such a system, given each are tuned correctly? It just can’t. There will still be those that argue the subject at hand, but it just isn’t possible. Now lets discuss the basic types of EFI systems. There are many different types of EFI systems available, but most are based off of one of two generally organized systems. The first is what is referred to as an “open loop” system. This is a system that sends a signal to the injector based on a predetermined program, also known as a Fuel Map. This program may still include each of the TPS/RPM settings as discussed previously, however, the system does not “check” to see how a given signal/instruction affected the outcome. This signal is simply a “slave” to the program from which the instruction came from. This “check” has to be done by an experienced tuner by means of a dyno and additional analyzing equipment. Usually an EGA (Exhaust Gas Analyzer) is used in this process. This will tell the tuner what effect each signal change has on the final exhausted AFR mixture. If the EGA shows specific readings at a given RPM/TPS, for instance, then the tuner can make very specific adjustments to “just” that area of the program. That adjustment will have NO effect on any other area of the program. The down side to this system is that the tuning adjustments are made during a specific time period with a specific set of conditions, such as air temp, elevation etc. Once those conditions are changed, either by general temperature change or an elevation change during a ride, the ECU has no way of receiving input for those changes. The second basic type of EFI system is referred to as a “closed loop” system. This system is much like the first, with the exception of using onboard sensors to help determine the amount of time the signal is sent to the injector. The ECU will gather data from any available sensors and make changes to the preprogrammed fuel map based on this input. This “closed loop” system is much more effective than the “open loop” system due to its ability to receive real time information about the engine and its current condition. During a ride that includes elevation change, temperature change, and even load on the engine due to inclines etc, the ECU will make the necessary adjustments to the Map in order to maintain optimum AFRs for that motor and within its set perimeters. The level, or percentage, of change will be determined by how advanced the system is. These changes may be as small as a 3% or as much as 25% (possibly even higher) difference from the preprogrammed fuel map. Next lets look at some of the sensors used by the ECU to help determine the pulse width of the injector. EFI systems have many sensors that feed input to the ECU (Electronic Control Unit or Computer). These sensors, depending on which type EFI system your bike has, will send data about outside air temp, engine temp, engine RPM, AFR (Determined by an O2 sensor in the exhaust), Throttle Position (TPS – Throttle Position Sensor), Manifold Pressure (MAP – Manifold Absolute Pressure), and crank shaft position. Not every system has, or utilizes, all of these sensors. That depends a lot on whether the system is Closed Loop or Open Loop. Despite the belief of many, the computer does not control the amount of fuel pressure to the injector. The injector is a piece of hardware that will only release fuel at a specific rate. There are different injectors that will increase or decrease this given rate. Kind of like the difference between a garden hose and a fire hose. Obviously one allows much more water to flow when compared to the other. What the ECU does is increases, or decreases, the signal length to the injector (also known as pulse width). This allows more or less fuel to pass through the injector based on the length of signal. This pulse width is very precise and is adjusted by the ECU in milliseconds. Once again, I do not claim to be an expert on this subject. I’m sure someone here will make an attempt to prove that point. My objective here was to “simply” explain EFI systems and how they “generally” operate. This is very basic explanation that may help in someone’s understanding of our machines and how they work. I just thought I would share in my many hours of working on, reading about, and attempting to understand more of what our future with EFI consists of. I hope this will be helpful to someone. If anyone would like to share any information that will add to my basic understanding, I would be more than grateful.