For decades, carburetion served as the prime form of fuel delivery across the automotive landscape. However, subsequent technological advancements eventually led to the advent of new, highly innovative forms of fuel delivery, which revolutionized the way that we think of the internal combustion engine.
In response to tightening emissions standards, engineers set out to reinvent the wheel by developing a more efficient fuel system. This first involved equipping carbureted engines with primitive electronic control modules to optimize fuel mixtures. This was in response to various environmental factors. While this served as a step in the right direction, additional technological refinement was yet to come.
During the 1980s, an all-new fuel delivery system would make its way into production, thereby signaling the beginning of the end for carb-enabled fuel delivery systems. Many automotive manufacturers such as Chevrolet, Ford, and Chrysler, began fitting their vehicles with TBI (Throttle Body Injection) systems. This served as the beginning of a new era dominated by the rise of modern fuel injection.
The Anatomy of Fuel Delivery
Almost immediately upon the release of modern EFI fuel delivery systems, gearheads began toying with the idea of carb-to-fuel injection conversions. In theory, this was an idea that seemed viable, as most injection throttle bodies were designed to be fitted atop a preexisting engine’s intake manifold in place of a previously-used carburetor.
Many early attempts at such conversions proved successful, eventually spawning the development of numerous direct-fit EFI conversion kits. Today, such kits are plentiful and can be sourced for almost any carbureted engine. Carb to TBI conversions are among the most popular engine modifications of the current era and have garnered an enormous following for multiple reasons.
When deciding if such a conversion is right for you and your vehicle, it is important to understand the operating characteristics associated with each fuel delivery system.
Carbureted Fuel Delivery
Long before modern fuel injection came to pass, gasoline engines relied upon the use of one or more carburetors. A carburetor is perhaps best described as a vacuum-operated fuel/air metering device. Air is channeled into a carburetor through an inlet pipe that is connected to the engine’s air cleaner.
Upon reaching the carb, this air encounters a throttle plate, which meters its passage. This throttle plate is controlled by a mechanical linkage, which is manipulated through the actuation of the accelerator pedal. As the accelerator pedal is depressed, the carburetor’s throttle plate opens to a greater extent, allowing increased airflow volume to enter the engine’s intake manifold. Carburetors, in general, are rated by volume, which is denoted by a specific CFM rating.
Fuel enters the carburetor through a supply fuel line, where it is distributed to the unit’s reservoir, or bowl. As the carburetor’s bowl continues to fill with fuel, a buoyant float rises upward. This float is attached to a needle valve that seats within an orifice as the carb’s fuel level reaches its peak level. In its seated position, the needle valve prevents any additional fuel from flowing into the carburetor until necessitated through fuel consumption.
As air passes through the carburetor, a vacuum is intact upon fuel contained within the unit’s bowl. As a result, fuel is pulled through a set of additional orifices, known as “jets''. Fuel then flows through these orifices and is dispersed into the carb’s throat, where it mixes with the incoming air before being distributed into the engine's intake. This fuel-enriched air then makes its way into each respective combustion chamber via open intake valves.
TBI Fuel Delivery
An injection throttle body is a unitized fuel/air delivery device that mounts to a centralized point along an engine’s intake manifold. A throttle body of this style features 1-2 fuel injectors, which dose fuel through an open throttle plate. Injection throttle bodies look similar in appearance to a traditional carburetor but deliver fuel under high-pressure rather than solely by vacuum.
TBI fuel systems create fuel pressure through the use of a fuel pump, which is commonly mounted within a vehicle’s fuel tank. Fuel delivery is controlled by a throttle body mounted fuel pressure regulator. Meanwhile, intake airflow is metered by a throttle plate, which opens and closes to meet throttle demand in real-time.
Fuel injection systems of this nature also utilize a series of temperature sensors and pressure sensors to provide feedback to an integrated control unit. This unit then calculates injection rates based upon this feedback to optimize performance and achieve the degree of fuel efficiency.
Numerous OEMs adopted the use of TBI technology throughout the 1980s and 1990s, including Chevrolet, Nissan, Ford, and Honda. At the time of its introduction, throttle body injection served as one of the first EFI systems (electronic fuel injection) to be used substantially in automotive production.
Several parts manufacturers, such as Holley and Edelbrock, now sell aftermarket TBI conversion kits. These kits utilize specialty adapters/gaskets to make carburetor-to-TBI conversion possible. This has become a popular modification with those who wish to leave carburetor-related reliability issues in the past.
Where Carburetors Fall Short
Carburetion certainly has its place and played an integral role in automotive production for numerous decades. However, these systems are only so efficient, especially in the face of demands for increased performance and fuel efficiency. This is a fact that became painfully obvious during the early 1970s when concerns over exhaust-related smog output drove the federal government to act.
Beginning with the Clean Air Act of 1970, government oversight began to force change within the automotive industry. Numerous mandates limited the volume of pollutants that were deemed permissible within the exhaust output of any vehicle. The sudden onset of these newly implemented regulations dealt a swift blow to the automotive industry, even dealing a death blow of sorts to the muscle car wars. Manufacturers scrambled to reach compliance, and carbureted fuel delivery quickly came under fire for its relative inefficiency.
Carburetors, while easily tuned to produce extensive horsepower, were largely incapable of providing favorable fuel economy. Air/fuel ratios were often skewed toward the rich side in a bid to produce increased output, especially when coupled with both big and small-block V8 engines. Since carburetors deliver fuel as a product of engine vacuum, precise metering to each cylinder is not possible. Instead, the amount of fuel that is introduced to each cylinder is directly dependent upon the volume of fuel pulled through the unit’s jetting.
While fuel-injected systems utilize O2 sensors to monitor combustion efficiency and make necessary fuel trim adjustments, carburetors are incapable of such oxygen sensor/ECU guided adjustments. This provides less leeway when compensating for cold starts or periods of high throttle demand. In many cases, throttle response also tends to vary across an engine’s RPM range instead of remaining uniform across the powerband.
The Virtues of Fuel Injection
The vast majority of automotive manufacturers were quick to adopt fuel injection upon its coming of age and for good reason. With time, continued refinement has yielded new and innovative tech across nearly every facet of the automotive landscape. In the case of fuel delivery, TBI technology served as the next logical step in a progressive trend of development.
In almost every measurable area, fuel injection outperforms carbureted fuel delivery. If you are currently contemplating the switch from carb-enabled fuel delivery to fuel injection, you are certainly not alone, as an ever-growing number of automotive enthusiasts delve headlong into such modifications on an annual basis.
The following are some of the most prominent reasons for Carburetor-to-EFI (electronic fuel injection) conversion.
The installation of an EFI kit is known to bolster engine performance, with such modifications garnering a significant amount of attention from those within many different performance-orientated circles. Many consumers brag of impressive gains in horsepower with the installation of an EFI unit, though exact figures often vary from one particular vehicle model to the next.
It is not uncommon to hear of EFI conversions netting gains of 10-20 horsepower and 20-plus lb/ft increases in torque. While this might not seem monumental in many regards, such gains are sizable enough to be worthy of mention.
Simpler to Tune
Throughout the years, gearheads and performance junkies have whittled carb tuning down to a science. Without dispute, a properly tuned carburetor can be a wonder to behold. However, there is much more to complete this task than haphazardly twisting an adjustment screw. For many, tuning a carb to capitalize upon an engine’s full potential is easier said than done.
On the other hand, EFI systems can be tuned in a relatively simple fashion in comparison to many carburetors. In fact, the vast majority of such duties are handled by an EFI system’s control unit without manual intervention. Many units of this nature are self-learning and feature adaptive learning capabilities. Many fuel injection systems allow for additional adjustments to be made, such as those facilitated by adjusting fuel pressure on TBI units.
Increased Fuel Economy
EFI systems also tend to be far more fuel-efficient, thereby saving motorists money at the pump. This increased fuel efficiency comes as a byproduct of such a system’s ability to make “on-the-fly” adjustments to fuel delivery volume based upon feedback provided by numerous sensors. Each sensor utilized within this process plays a key role in ensuring that an engine’s fuel system is operating at optimal capacity.
The following sensors all are key in delivering vital feedback for fuel delivery optimization.
Manifold Absolute Pressure Sensor (MAP)
A manifold absolute pressure sensor monitors the air density within an engine’s intake manifold, which often fluctuates with changes in altitude. An EFI fuel system relies upon data from an engine’s MAP sensor to determine the exact amount of fuel that should be dosed at any given time. In the absence of a properly operating MAP sensor, numerous drivability issues can result.
Mass Air Flow Sensor (MAF)
An EFI system uses the data which is presented by an engine’s mass air flow sensor to determine proper dosing to maintain an optimal air/fuel ratio. If intake air volume were to increase, fuel dosing would also be elevated to prevent the onset of a lean condition. Along the same lines, a drop in air delivery would lead to a reduction in fuel dosage as a means of preventing a rich condition from developing.
Engine Coolant Temperature Sensor (ECT)
EFI systems utilize data conveyed by an engine coolant temperature sensor to make minor adjustments in fuel delivery volume. More specifically, a cold engine requires additional fuel to run efficiently. A vehicle’s ECU is programmed with this logic and will automatically provide more fuel to each cylinder when the engine’s ECT presents a low temperature reading. The engine coolant temperature also controls additional engine functions such as fan operation.
Throttle Position Sensor (TPS)
An EFI fuel system calculates the value provided by the TPS into an equation, which determines the exact amount of fuel that will be required while an engine is under a specific load. As the throttle plate opens, an engine’s ECU calls for the introduction of additional air into each cylinder. This prevents a lean condition from presenting itself as a significant volume of air is provided for combustion.
During the earliest day of EFI technology, when TBI fuel delivery was at the height of its popularity, O2 sensors served as the primary source of feedback within most fuel systems. The data relayed by an O2 sensor allows engine management software to differentiate between “lean” and “rich” conditions, thereby determining whether fuel trims should be increased or decreased in response to situational demands.
Engine Speed Sensors (Crank/Camshaft Position Sensors)
EFI systems utilize engine speed sensor data to match fuel delivery rates to varying engine load demands. Data relayed by the throttle position sensor can be compared to engine speed sensor feedback as a means of determining whether or not current fuel trims are sufficient to meet any load placed upon the engine. Fuel delivery can then be tweaked to enhance engine performance.
Is It Worth It?
Facts aside, one is still left to determine whether a fuel injection conversion is right for them and their vehicle. If you still find yourself mulling over such a decision, it is likely best to determine why you are considering such a conversion in the first place. Doing so will assist you in concluding whether or not a fuel injection conversion carries any merit for your particular situation.
Are you attempting to bolster your engine’s output? If so, ditching your carburetor for a new TBI unit might be your answer. However, it is important to keep gain-related expectations in check. Will you double your engine’s horsepower upon converting to fuel injection? Absolutely not. Nonetheless, such a conversion often yields modest improvements in both torque and horsepower.
Maybe frequent trips to the pump have led you to ponder the virtues of fuel injection conversion. If your car or truck currently passes everything except a gas station, then fuel injection might serve as the answer that you have been looking for. EFI systems tend to be significantly more fuel efficient than carbureted fuel delivery, saving drivers money in the long run.
Reliability also tends to be high atop the list of reasons for making the switch to fuel injection. Carburetors utilize a host of components that can, at times, prove problematic. Floats stick, jets become clogged, and shaft-related bushings begin to degrade. Additionally, vacuum-related issues can present headaches of their own. While EFI units are not void of their own shortcomings, such units typically prove to be quite reliable in the long run.
In any event, it is difficult to imagine a situation in which a carbureted vehicle would not benefit in more than one regard from undergoing an EFI conversion. The passage of time has yielded substantial advancements in the world of fuel delivery, effectively antiquating carburetion in the process. EFI conversions can be completed with minimal difficulty in most applications and typically require no permanent engine modification. This presents a win-win proposition of sorts, as a vehicle's engine can always be restored to its stock format should one desire.
As with any engine upgrade or modification, the true worth of fuel injection conversion is in the eye of the beholder. However, few revert back to the use of their exiled carburetor once it has been stored away on the shelf.