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For the Last Time, Throttle-Body Fuel Injection Isn't a Carburetor!

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author photo by Aaron Gold May 2017

Instead of digging up some obscure bit of automotive history, today I'm going to talk about one of my pet peeves: Confusion between carburetors and throttle-body fuel injection, otherwise known as TBI. It's an easy mistake to make -- I'll talk about why in a second -- but it's my sincere hope that by the time you get to the end of this article -- if you get to the end of this article* -- you'll understand that TBI is not a carburetor.

* When I say "if," I am making an attempt at self-deprecating humor, i.e. that you'll get tremendously bored and click over to the latest installment of Ask Doug. I am not implying that any misfortune will befall you sometime in the next few paragraphs, though on the odd chance it does -- anything's possible, right? -- I'm sure our lawyers would like me to say that it's not my fault.

Let's start with some basic definitions.

First, the carburetor: As some of you no doubt know, the carburetor is a (mostly*) mechanical device that uses air pressure to pull fuel from a small reservoir called the bowl. That is, for the sake of this article, what defines them: Physics does all the work.

* I say "mostly" because some carburetors from the late 1970s and early '80s, called feedback carburetors, had some form of electronics -- they took data from primitive sensors and used it to tweak the fuel mixture. However, the basic method of moving fuel out of the bowl and into the intake manifold was all done with air pressure. As far as I know, the engine will still run even if you unplug the electronic bits -- assuming, of course, the carburetor was working before you unplugged it ... and having owned two cars with feedback carburetors, I can put the odds of that at roughly 60/40.

Fuel injection (usually*) uses an electric pump to squirt fuel into the engine. The engine control unit (ECU) gets data regarding the airflow into the engine and tells the electric injector(s) exactly how much fuel to squirt in. Physics, as applied in the carburetor, is out of a job; if you unplug the fuel-injection system, the engine won't run.

* I say "usually" because there are mechanical fuel-injection systems, though I don't believe they've been used in U.S.-market cars since the 1960s, maybe even the 1950s. For the sake of this topic, we can safely pretend that all fuel-injection systems are electronic.

In the early days of fuel injection, automakers developed throttle-body fuel injection (TBI) as a cheap way to switch over to fuel injection. TBI mounts the injector(s) right in the throttle body. It's a single compact unit that mounts atop the intake manifold in pretty much the same way as a carburetor -- hence the appeal to automakers. With TBI, few parts of the engine need to be redesigned, and they deliver most of the consumer benefits of fuel injection (easier starting, smoother running and no need for routine adjustment).

The problem with TBI is that it isn't very precise: Like the carburetor, it can't adjust for the needs of individual cylinders. Automakers quickly developed the multiport fuel-injection systems we see today -- systems with individual injectors for each cylinder (mounted on the intake manifold) and a much smaller throttle body, which is often located further away from the manifold. (Today, cars are going to direct fuel injection, which sprays directly into the cylinder, but they still have plumbing for each cylinder.)

Mixing up carbs and TBI is an easy thing to do. Unfortunately, there are some young'uns who think that anything that isn't multiport fuel injection must be a carburetor. Wrong. Wrongwrongwrong.

You also can't assume that just because a car is old, it's carbureted. Several companies sell aftermarket TBI units that are bolt-on replacements for carburetors.

So how do you tell the difference? Visually, it isn't always easy. Look at the two photos I've posted here: At first glance, they look pretty darn close. In some TBI systems, you can see the injector hardware suspended at the top of the throttle body, and it's easy to mistake the bits inside the carb barrels for fuel injectors. One helpful thing: Most carburetors have a choke -- a squared-off flap that covers at least two of the barrels -- but, again, not all do. (The carb in the photo I picked has no choke.)

The easiest way to tell is in the way you cold-start the engine. With a fuel-injected engine, you don't touch the gas; you just turn the key and it starts, because the injector automatically spritzes fuel into the engine.

With a carbureted engine -- generally speaking -- you usually have to pump the accelerator once or twice* before starting. This closes the choke, moves the throttle plates to the fast-idle position and squirts a little raw fuel into the manifold. If you don't pump the accelerator, chances are the car won't start.**

* Once or twice. Usually just once. There's no need to stomp up and down on the throttle like you're trying to kick the stuffing out of it; in fact, too much pumping will flood the engine and make it even more difficult to start.

** If the engine is even remotely warm, you don't have to pump the gas (though sometimes half-throttle will help). My old carbureted Dodge will start with no help if it's been run at all in the past 18 hours or so. Once I rented a Volkswagen Thing in Culebra, Puerto Rico, that had no choke; because the weather was so temperate, I never needed to pump the gas to start it. See, this is why I have my doubts about people making it to the end of the article. You might die of boredom.

Bottom line: Carburetors and TBI units often look the same, but they aren't.

So now you know the difference between a carburetor and throttle-body injection. If I hear about you getting it wrong, you will be sentenced to a week of trying to start a carbureted Dodge Omni in zero-degree weather.

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For the Last Time, Throttle-Body Fuel Injection Isn't a Carburetor! - Autotrader