Classic car followers, like much of society, can be slow to embrace change. In the automotive world, advances in technology often mean considerable improvements in performance, and every gearhead can agree that is an admirable pursuit. But we still resist the fuel differences.
Throughout much of the history of the internal combustion engine, a carburetor has been tasked with introducing a combustible mixture of air and fuel through an intake tract, and finally to the combustion chamber, where a spark ignites the incoming charge and converts that energy into work through the engine’s pistons, connecting rods, and crankshaft. As engines evolved, so did the way they were fed fuel. But even with design improvements that allowed the carburetor to function in a wide range of conditions, it remained (as many would refer to it) a calibrated fuel leak.
The throttle body electronic fuel injection (EFI) is a bolt-on replacement for a carburetor as we have previously discussed in carburetors vs fuel injection. It cinches down to either a square- or spread bore-flanged intake manifold, and utilizes the original throttle linkage and vacuum connections, so it is compatible with even factory-stock hardware. There are four 100 lb./h injectors in the throttle body and 800 cfm of airflow.
Carburetors
Carburetors are mechanical metering devices that use atmospheric air pressure and intake vacuum to deliver gas to the engine. Intake vacuum siphons fuel from the fuel bowl through the main metering circuit and idle circuit so it can mix with air as the air/fuel mixture enters the intake manifold. Cold starting requires a fairly rich air/fuel mixture, so a manual or automatic choke is needed to restrict airflow into the carburetor and to increase the throttle opening for a faster cold idle speed while the engine warms up. Chokes require adjusting and can be troublesome. Race carburetors have no chokes because they get in the way which can make starting difficult in cooler weather.
Carburetors also have adjustment screws for the idle mixture and idle speed. The idle mixture affects idle speed, so the two must be adjusted more or less simultaneously so the idle will not be too fast or too slow.
Opening the carburetor throttle to accelerate requires an accelerator pump circuit and discharge nozzles so the engine does not bog when it ingests a big gulp of air. Setting up the accelerator pump can also be tricky because too much gas is just as bad as too little fuel. And if somebody pumps the throttle linkage a little too much when cranking the engine, they can flood the engine and wet foul the spark plugs.
Carburetors also have a power valve circuit to add extra fuel when the engine is lugging under load and intake vacuum drops.
Carburetors also require constant adjustments. As soon as the air temperature or atmospheric pressure changes, it is out of tune. The float level setting inside the fuel bowl affects the air/fuel mixture, which is determined mostly by the size of the main metering jets or needles. Adjusting the air/fuel mixture requires manually changing the jets to richen or lean the fuel mixture. On a hot summer day, warm air is less dense, so smaller main jets are needed to maintain the same air/fuel ratio. Conversely, on a colder day air is denser so larger jets are needed to prevent the air/fuel mixture from leaning out.
This constant back and forth fiddling have long been a ritual of race day carburetor tuning. But with fuel injection, you do not need a screwdriver or wrenches to change jets, accelerator pumps and power valves because Electronic Fuel Injection (EFI) is self-tuning.
How EFI Works
With fuel injection, the fuel mixture is monitored by a wideband oxygen sensor mounted in the exhaust. Throttle position, air temperature and manifold absolute pressure (MAP) are also monitored by separate sensors. Feedback from these sensors allow the EFI control unit to constantly fine tune the fuel/air mixture to changing operating conditions.
EFI is less dependent on intake vacuum for fuel delivery because fuel is sprayed into the engine through injectors. A fuel pressure regulator maintains steady line pressure to the injectors, while the EFI control unit determines the duration (on time) of each injector when fuel is sprayed into the engine. Increasing injector duration adds more fuel while decreasing injector duration leans the air/fuel mixture.
With throttle body EFI systems, fuel is sprayed through the throttle body into the intake manifold like a carburetor. With multi-port EFI, each cylinder has its own separate injector and fuel is sprayed directly into the intake ports in the cylinder head. If the multi-port EFI system has “sequential” injection, each injector is triggered individually rather than gang fired, which allows for faster, real-time air/fuel adjustments.
Many late model engines with Gasoline Direct Injection (GDI) take it a step further and spray fuel directly into the combustion chamber under extremely high pressure, like a diesel. Currently, there are no aftermarket GDI conversion kits available. Such a kit would require different cylinder heads, different pistons (because GDI pistons usually have depressions, pockets and/or ramps to direct airflow within the cylinder), and a high-pressure mechanical fuel pump. However, there are numerous aftermarket throttle body and multi-port EFI conversion kits that allow almost any carbureted engine to be converted to EFI.
EFI Advantages
EFI offers a number of important advantages over a mechanical carburetor:
- Because fuel is sprayed under pressure into the engine, there is much better fuel atomization. A more homogeneous air/fuel mixture improves cold starting, idle smoothness, throttle response, fuel economy and emissions.
- Throttle body EFI systems can be easily installed on almost any engine with a square bore 4150 or 4500 style intake manifold, including dual quad carburetor setups on higher horsepower engines. It is a bolt-on installation that uses the same throttle linkage and air cleaner as the original carburetor. It does not alter the basic look of the engine or intake plumbing.
- EFI provides consistent all-year drivability for street driven vehicles. It is an ideal upgrade for a vintage muscle car, sports car, classic car, street rod, resto rod or race car (if rules allow it). Many of today’s resto rods are fueled by some type of EFI system (OEM or aftermarket). With EFI, you do not have to wait five minutes for the engine to warm up before you can drive away without the engine bogging or backfiring. An EFI conversion allows the engine to fire immediately with the first crank, to idle smoothly whether the engine is hot or cold, and to accelerate without any embarrassing pops, sputters, or bogs.
- On many applications, EFI can add horsepower (although some would argue that a big carburetor always makes more power). An engine can usually handle a throttle body with larger bores (and thus more CFM airflow) with EFI. Some throttle body EFI systems are rated at 1100 CFM, which is more than your typical 650 or 800 CFM carburetor that is often used on a small block or big block motor. A carburetor that is too big for a given engine will often have poor throttle response and torque at low to mid-range RPM but work well at high RPM. With EFI, a larger throttle body works well at all engine speeds.
A multi-port EFI system with a front-mounted throttle can sometimes provide better airflow into the engine than a single plane intake manifold with a carburetor. Some EFI suppliers claim the change is good for twenty more horsepower over a carburetor.
- Multi-port EFI provides better cylinder-to-cylinder fuel distribution than a carburetor because only air flows through the intake manifold. With a carburetor, droplets of fuel can separate and puddle when the air/fuel mixture turns corners. The end cylinders also tend to run leaner than the center cylinders in V8 engines with a carburetor. Some intake manifold designs do a better job than others of equalizing airflow and fuel distribution, but with multi-port EFI it is less of an issue.
- Most bolt-on throttle body EFI kits can handle engines from 450 to 650 horsepower depending on the flow rate of the injectors and fuel pump that are used. With dual EFI throttle bodies, up to 1,200 horsepower is possible depending on the application.
- EFI can also be used with turbochargers, superchargers and/or nitrous oxide. Programming allows the EFI control unit to adjust the fuel mixture and timing under boost pressure or when the engine receives a shot of nitrous.
- Most throttle body EFI systems as well as some of the multi-port EFI systems are self-tuning and do not require a laptop or advanced computer skills to tune and adjust. For serious racing, the more advanced professional high horsepower port EFI systems do require a laptop and advanced tuning skills to dial in. The fuel mixture and ignition timing maps can be worked out on a dyno or with the engine in the vehicle – the latter typically results in a better tune because it considers the effects the transmission, drivetrain and exhaust system have on engine performance. The more sophisticated the control unit, the more data points it allows in the fuel and ignition maps.
Finally
Most EFI conversion kits are “speed-density” systems that use engine speed, throttle position, air temperature and intake manifold pressure to estimate airflow into the engine. Some original equipment EFI systems are also speed-density systems (Chrysler and older GM Tuned Port Injection), but most are “mass airflow” systems that use an expensive MAF sensor mounted ahead of the throttle body to read airflow into the engine. MAF systems are more accurate.
