EFI questions answered

Understanding the basics of EFI and your options

Chad Westfall - February 09, 2012 10:00 AM


Throttle bodies come in a variety of shapes and sizes. Make sure you know what intake manifold you are using before purchasing one.

Tom Dufur

TBI systems are the simplest and easiest to bolt on and use. They are basically a self-contained system like a carburetor, just with a few extra wires.


The oxygen sensor provides a critical link in communications with the EFI system. Without an oxygen sensor, there is no way for the computer to know if it is measuring everything correctly and the air/fuel ratio could get out of balance quickly.


coolant temp sensor is a basic sensor. It allows the computer to know when the engine is up to operating temperature and can handle higher power levels.


fuel injector Most SFI systems use one per cylinder but in some extreme racing engines, there may actually be two of these per cylinder.


MANIFOLD In recent years, companies like Edelbrock have been casting intake manifolds with injector bosses. These make bolting on sequential fuel injection systems much easier. You used to have to weld injector bosses to the intake manifold.


EFI DISTRIBUTOR To really take full advantage of an EFI system, having the electronics control the spark is important. Here is an example of a distributor that has the proper leads and wires for an EFI system.


After the engine is up and running, it is a really good idea to have an expert like Modern Muscle tune the system on a chassis dyno, or in real-world driving on the street, to get the most out of it.

Image 1 of 9

Growing up in the heyday of the muscle car era, we all learned early on which of us knew how to tune a carburetor and who knew how to make power.

Whether we were lucky enough to build the high horsepower motor or whether our buddies built it, we loved and enjoyed every minute. As life got more involved and the weekends of cruising the strip and laying rubber happened less often, we found ourselves under the hood of the car with less frequency.

Most of us enjoy going back to our youthful hobbies; we pop the hood and tune and tweak our cars as often as possible. But over the years, many new technologies and inventions have come along, some good, some not so good. One of the good ones is Electronic Fuel Injection (EFI). EFI has changed the landscape under our hoods and how our engines run. But growing up with carburetors, EFI is a completely foreign concept to many of us. So, we thought we would get with the experts at Modern Muscle and get an explanation of what EFI is and what types of modification options are out there.

“EFI is a great way to control your engine and can reduce a lot of headaches,” said Justin Meyers, vice president of Modern Muscle. “But like anything, something easy can also be a lot of headache and aggravation if it isn’t done right or is a poor system,” Meyers continued.

EFI in its simplest form is Electronic Fuel Injection. By that I mean that the fuel is metered and injected by a computer. There are a few different ways this is done, but the fuel is 100-percent controlled by the computer. No longer are jets involved.

In order for the computer to regulate the fuel, it has to know what is going on. So, there are a few key components involved with EFI systems. If the computer is regulating the amount of fuel injected in the engine, it needs to know how much air the driver is letting in. So, the air is metered with one or two sensors. The driver regulates the amount of air going into the engine with the use of a throttle body. The throttle body is basically the same as the butterflies in a carburetor. The throttle body is generally connected to the gas pedal via a cable (some newer vehicles use what is called a Fly-By-Wire system where the throttle body is moved electronically instead of mechanically, but that is a whole other article). The cable operates a lever that moves a large plate in the inlet that regulates how much air is allowed into your engine. As you press the gas pedal down, it opens, thus letting air into the engine and the computer senses this extra air and commands that additional fuel is injected, which equals more power.

“To maximize your EFI system, you really need to have some sort of communication between the fuel system and the ignition system,” continued Meyers. “Otherwise, there are only a few benefits to EFI and most people who are comfortable with carburetors should probably stay with a carburetor.”

There are really three types of EFI systems out there that people should be considering. The first system, and probably the simplest system out there, is Throttle Body Injection (TBI) kits. These kits directly replace the carburetor. Usually the only modifications or upgrades required when installing a TBI system is to upgrade the fuel pump (higher pressure) and add an oxygen sensor in the exhaust. Outside of that, an engine coolant temperature sensor is used. No laptops or external computers are needed. Most systems come with a little hand-held interface that asks a few questions about the engine (cubic inch, number of cylinders, etc.). Almost no computer experience is needed and very little tuning knowledge is required. These systems are designed to be self-tuning and adjusting.

“The TBI kits are really designed for the do-it-yourself guy who is looking for something simple to replace their carburetor,” says Meyers. “There isn’t much gain (hp) in these type of systems outside of improved cold starting and some reliability,” adds Meyers.

The second system is an aftermarket Sequential Fuel Injection system (commonly called SFI). These systems offer individual injectors for each cylinder, which helps efficiency and creates more power and better economy, can tie into the ignition system, and offer a lot of advantages.

Generally, there is a decent amount of work that goes into changing over to an SFI system. Due to there being an injector for each cylinder, the carburetor intake manifold either needs to be modified or replaced with an SFI manifold, the fuel system needs to be upgraded, oxygen sensors need to be added, the distributor replaced, throttle body added, wiring run, etc.

The big downside to these systems is the tuning. While there are a lot of advantages, it is usually best left to the pros to tune the car, unless you have a lot of experience with electronic tuning.

“Fuel economy can really be optimized with an SFI system, along with reliability and horsepower increases,” says Meyers. “These systems are quite a bit more money, but if you are planning on keeping the car and driving it, they are well worth it.”

The third option is much like the second (SFI) but it is an OEM system. A lot of today’s engines have roots dating back to previous model years. If you are fortunate enough to have an engine that has lasted the test of time, then updating the electronics on your engine may be possible. Some companies make parts to help make the conversion easier. These systems can be done relatively cost effective, assuming your engine hasn’t changed much.

“This option is only a good option for a few types of engines,” Meyers said. “For the vast majority of people out there, trying to adapt an OEM system from a newer engine to an older carbureted engine is going to be quite expensive and require a lot of one-off parts. Aftermarket EFI systems have come a long way over the years and offer many of the same advantages as OEM systems. So, the additional cost of the OEM system usually isn’t even close to being worth it,” Meyers concluded.