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Carburetor Versus Fuel Injection

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by Huw Evans  More from Author

Making the choice that’s right for your project.

For the last few decades, fuel injection has morphed from a curiosity into an essential part of our daily transportation. Driven largely by the need to maximize fuel economy as well as the need to minimize emissions, EFI has also given us levels of reliability unfathomable with carbureted vehicles of yore. On the hobby side of things, fuel injection has become increasingly popular as well. This has allowed us to pursue even greater levels of performance without the need for ultra-aggressive cam profiles and monster carburetors. It’s not surprising that more and more project builds center around electronically fuel-injected engines. But, in hot rod circles, the decision to choose EFI over traditional carburetors isn’t always so clear-cut.


Tradition

In the last decade or so, we’ve seen a healthy growth in the traditional hot rod movement. This focuses on the back to basics approach, building cars that generally adopt the look and feel of rods built during the genre’s golden age, the late 1940s through the early 1960s. Although some cars adopt fairly radical profiles (extreme channeling and chopped tops), the emphasis here is using period parts or faithful reproductions to achieve the classic look. When it comes to engines and fuel systems, that pretty much means sticking with carburetors.


Simplicity

Besides tradition, another reason for sticking with carburetors is simplicity. Even the most basic of fuel injection systems are fairly complex, requiring specific intake manifolds, high pressure fuel lines, injectors and a high-volume pump to feed the system. With a carburetor, you don’t have these issues to contend with. Look at any small engine and, chances are, it will use a carburetor. From leaf blowers to lawn mowers to chain saws, carburetors are used because all they essentially need to do is provide enough air and fuel to start the engine, allow it idle and permit it to operate at wide open throttle. In automobiles, carbs are required to cope with more variables, such as change in engine speed, acceleration, and deceleration as well as extended idling, but the basic principles are the same.

In automobiles, different sized jets at different locations on the venturi are employed to handle different operating conditions.  Most automotive carburetors also employ an emulsion tube, a length of pipe with small holes which is designed to pre-mix the air/fuel mixture before it enters the main venturi to achieve optimum combustion. By changing the size of these jets, specifically the main and air-bleed jets, along with the diameter of the emulsion tube and the number of holes, it is fairly straightforward to improve the engine’s performance. In order to reduce the risk of detonation but prevent flooding or fouling, many carb tuners aim for an optimal air/fuel ratio of 13:1. Airflow in carburetors is rated at CFM (cubic-feet per minute) – the greater the airflow, the more fuel required (via larger jet sizes).


Cost

Cost is another major reason for choosing carburetors over fuel injection. Even though most cars today use fuel injection, carbs are still plentiful and relatively cheap. Used ones can be picked up at swap meets, via other car enthusiasts, speed shops or on the web. In addition, aftermarket manufacturers, including Barry Grant, Edelbrock and Holley, sell a huge range of brand new carburetors, designed for everything from mild street driving to all out drag and road racing. In addition, there are numerous specialists out there to assist with tuning and rebuilding carburetors.


How Carburetors Work

In very basic terms, think of a carburetor as a length of pipe. At one end of the pipe is moveable flap that opens and closes – the throttle plate. This controls the amount of air entering the pipe. Further down, the pipe narrows inward (the venturi). On this section of the pipe there’s a small hole, housing a jet, which squirts fuel into the venturi. Automotive carburetors use three cycles or circuits to deliver fuel: idle, part and full throttle. Depending on how much the throttle plate is open, a certain amount of fuel is needed in proportion to the air rushing in order to allow the spark to ignite the mixture and fire the engine. The theoretical ideal or Stochimetric ratio for complete combustion is 14 parts air to 1 part of fuel (14:1); however, in practice, that is rarely achieved with carburetors.

A cold engine requires more fuel to operate than one that’s warmed up, since fuel is the more combustible of the two elements that need to be ignited by the spark. In the simplest of terms, the throttle plate, besides controlling the amount of air flowing into the carb, also controls the amount of vacuum in the venturi on both the top and bottom sides of the carb. On the bottom side, when the plate is closed, there’s a buildup of vacuum. By placing a secondary jet right below the throttle plate (the idle jet), the fuel can be drawn down the venturi more quickly and into the manifold and combustion chamber. This results in a richer air/fuel mixture, allowing the engine to start more easily when cold. (In addition, a manually-adjustable mixture screw can also alter the air/fuel ratio under certain operating conditions). In most automobiles, a secondary, moveable plate, called the choke plate, is mounted below the throttle plate. When the engine is cold, it fully blocks the flow of air through the venturi from the top side of the carb. As the engine warms up, the choke plate is pulled back, increasing airflow and reducing the flow of fuel from the idle jet. Chokes on automotive carburetors are either operated manually (often via a lever), or automatically, via an electrical wire and heating element that’s linked to the ignition system and fuel cut-off valve. As the electrical current warms up, the heating element expands, gradually opening the choke plate. The level of heat passing through the electrical current determines how much the choke is opened (more under warmer conditions, less under colder ones). Once the engine warms up, the carb switches from the idle circuit to the part throttle circuit, relying on the main jet to deliver the fuel. At full throttle, the main jet and air bleed jet (mounted at the top of an emulsion tube) provide fuel delivery. (The emulsion tube is a bit like a straw, which pre-mixes the air/fuel before it reaches the venturi and combustion chamber, to reduce the risk of detonation.)


The Case for Fuel Injection

If you’re not so concerned about tradition when building your next hot rod or custom project and the thought of tuning multiple Strombergs every other weekend doesn’t whet your appetite, then going with electronic fuel injection may be for you. Although cost and complexity are greater than with carburetors, even on a car like 1928-34 Ford, there are many benefits of going with EFI. For starters, once the engine and fuel system have been fitted and tested, they require minimal maintenance. Problems associated with carburetors, including rough idling, cold weather starting and excessively rich mixtures are virtually eliminated, because EFI systems allow for a much more precise fuel delivery which is exactly tailored to the amount of air entering the engine.

The reasons why fuel injection has become so widespread, at least from an OE standpoint, go back to emissions requirements instigated in the 1970s. In order to reduce the output of harmful exhaust gases, more precise control and metering of air/fuel mixtures were required. In order to successfully achieve this, oxygen sensors, placed in the top of the exhaust stream, relay the level of O2 passing through back to an electronic engine control module, which then precisely adjusts the air/fuel ratio in real time. Also known as closed loop control, this requires more precise fuel delivery, simply not possible with carburetors and their relatively crude fuel jets and air flow control. As a result, fuel injection came to the forefront.

Early systems used injectors placed atop a central hole in the intake manifold (much like a carburetor), but have been supplanted by sequential injection which completely separates the air and fuel paths, drawing the former in through the top of the engine via a throttle body and a manifold with individual intake runners and then squirting the fuel in via a rail linked to individual injectors for each cylinder at the bottom of the manifold. Because of the need for more precise fuel delivery, much greater fuel pressure is required (usually between 40-45 psi – versus 8-9 psi for carbed systems); hence EFI systems use high pressure lines and a high volume, electrically-operated pump, often with a main and return fuel line (that draws excess fuel back to the gas tank). The pump can be externally or internally mounted, but for quieter operation and reduced maintenance, pumps mounted in the tank itself are becoming increasingly popular (the fuel also cools the pump, extending operating life).


Getting the Right Components

When it comes to building a hot rod or custom with electronic fuel injection, choosing the right donor car is critical. Central to the EFI system is the engine’s electronic control module which, via air intake temperature sensors, throttle position sensor and manifold absolute pressure sensor, precisely monitors the flow rate of air into the engine. Using this data, the ECM consults its internal look up tables to match the flow rate of fuel to achieve the Stochimetric ideal of 14 parts air to 1 part of fuel. When you’re considering EFI, you need to get your hands on a complete system, ECU, wiring harness, fuel rails and injectors, plus determine if you’ll need new fuel lines, fittings and an electric fuel pump.


Crucial Tips

It’s surprising how often this is overlooked, but it is utterly crucial to make sure that you choose the correct wiring harness and computer designed to work for a specific vehicle application. If you don’t, you’ll be forever chasing electrical gremlins and suffering with a car that just won’t run properly.

Another aspect when choosing fuel injection is to select the right camshaft. Cams designed for fuel injection applications feature higher lift profiles and wider lobe separation. Because huge amounts of air aren’t required to be drawn down from the top of the manifold, the heads and intakes on EFI engines are designed to promote greater airflow inside the intake runners and ports, and the low lift and wide separation on the camshaft allows greater promotion of power and torque. Unless you’re lifting the entire engine and EFI system from the donor vehicle and are not too concerned about individual performance, you’ll want a custom ground camshaft designed to work specifically with your engine and fuel injection system to achieve a balance of good drivability, as well as ample power and torque delivery across the rpm spectrum.


Calculating Injector Sizes

When it comes to fuel injection, figuring out the right size injectors to use is critical, but it is also one of the most misunderstood aspects of EFI.

In order to determine the size (lb/hr or flow rate) required, you need to find out what your engine’s Brake Specific Fuel Consumption (BFSC) actually is. This term refers to its efficiency, i.e. how much power it makes in relation to the amount of fuel it uses. For most mild-street engines a BFSC of .50 is the accepted rule of thumb. More aggressively modified, but normally-aspirated, engines are rated around .55, while those using power adders, superchargers, turbochargers or nitrous are rated at .65.

When calculating injector size, there are some general terms that are widely accepted, one being that optimal injector efficiency is around 80 percent. More than that, and the injectors tend to be overworked, causing heat to build up and efficiency to rapidly tail off.

Taking that into account, we can use the following formula to work out our required injector size based on rear wheel horsepower (H), BSFC, C (the number of cylinders) and D (injector efficiency):

H x BSFC / C x D

So, if our normally aspirated, mild street V-8 engine is rated at 300 horsepower, this will give us:

300 (hp) x .50 / 8 x .80  = 20.9

Based on that calculation, we would need an injector that’s closest to this flow rate for optimal performance. In this case, it would be 24 lb/hr units, as found on many OE performance applications and through aftermarket suppliers.






One of the most important elements when planning your next hot rod project is the engine, and choosing the right fuel system for that engine.




Bigger, heavier cars, like this custom 1950 Mercury, often require torquier, larger-displacement engines to move them. Multiple carb setups or, if you’re not too concerned about tradition, tuned port fuel injection, can really add benefits.




When it comes to engines and fuel systems, donor cars are a good place to start. This well-used Lincoln Continental Mk IV was found at a swap meet. Its big 460 V-8 and four-barrel carb are the perfect foundation for a custom sled.




On the fuel-injected front, one of the easiest and cheapest ways is to purchase a well-used pickup like this 1990 GMC Sierra. Make sure that you get the computer, wiring harness, injectors, relays and the complete system from the donor vehicle to avoid endless troubleshooting problems.




Even today, small block 350 Chevy engines like this are cheap and plentiful. This one sports throttle body EFI, but is easily converted to either sequential injection or carbureted form.




If you’re building a 1932-34 Ford and want modern drivability but a classic blue oval small block V-8, there is no better way than using a 1987-93 Mustang 5.0 as a donor car, since the parts are cheap and plentiful.




The sequentially-injected 302 Ford V-8 almost rivals the small-block Chevy in popularity and is backed by a ton of companies that supply speed parts for it. It is extremely durable, and the fuel injection system is easily accepting of power adders like super and turbochargers.




If you would like something really different and more traditional, you can always opt for an old carbureted straight six, like this Studebaker 170 cubic inch, though parts are often very hard to come by, especially when it comes to improving air and fuel delivery.




Once you’ve found a suitable donor car, you’ll want to remove the engine, drain the fluids and inspect it. This is a sequential fuel injected 302 that’s destined for a supercharger and duty in a lightweight coupe. The fuel system will be upgraded with high flow 42 lb fuel injectors and a high flow 255 liters per hour electric pump, to cope with the increased level of air delivery via supercharger boost.




Choosing the right carburetor hinges on many factors, namely your goals with the car. Besides OE used carburetors, companies like Barry Grant, Edelbrock and Holley can supply brand new ones, many of which are based on classic designs.




The right intake manifold is just as crucial as carburetor selection. This cross-ram unit is a fairly exotic piece, designed to maximize mid-range power, making it ideal for passing or road racing.




Sequential fuel injected engines totally separate the air and fuel parts. The air is channeled from an inlet tract and throttle body through individual runners to the combustion chamber. The unique elbow seen here is unique to the 1994-95 Mustang V-8 engine and often requires a special inlet elbow.




Used throttle bodies can be picked up very cheaply these days. The size of throttle body can have a major impact in airflow and the amount of power you can make. Most run 60 to 80 mm in diameter.




This lower intake manifold for a sequential fuel injected Ford application features staggered runners to improve air flow efficiency, ultimately resulting in better combustion and increased power and torque.



Sequential fuel injected engines often have the airflow metered by Speed Density or Mass Air. On Mass Air systems like this, a small sensor mounted in the inlet tract precisely measures the rate of air flowing into the throttle body and relays it to the engine’s computer, which adjusts the fuel flow to compensate.




A set of good quality injector rails are crucial when planning an EFI install or conversion. These units, available from Professional Products, are designed for 302 Ford engines and support 24 and 36 lb injectors, making them ideal for most mild street and performance applications.




Choosing the right injector size is critical to getting the best out of your engine. Many enthusiasts use too large an injector, which actually robs them of power.



O
ther components to consider with fuel injected systems are the lines, pump and fuel tank. Shown is a 1986-89 Ford system with a 15.5-gallon tank and pump housing. The high flow pump fits inside the housing, which is then installed inside the tank via a lock ring and gasket. This not only makes pump operation quiet, but the fuel in the tank also cools the pump, which improves operation and extends life.




Even with a small block Chevy crate engine, aluminum intake manifold and Edelbrock carb, you can still have a great looking traditional style rod.




At the other extreme, a trick fuel injection system can add the finishing touches to an all-out show car like this gorgeous 1956 Chevy.


USEFUL SOURCES

Barry Grant
706- 864 8544
www.barrygrant.com

BBK
www.bbkperformance.com

Edelbrock
310- 781-2222
www.edelbrock.com

Fuel Air Spark Technology
877-334-8355
www.fuelairspark.com

Holley
270-781-9741
www.holley.com

Mr. Gasket
216-688-8300
www.mrgasket.com



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