When the throttle body on an engine is opened, the engine revs higher.

  • Why does this happen?
  • What is the connection between the crankshaft and the throttle body?
  • What vehicle are you repairing?
    – cory
    Mar 8, 2017 at 13:54
  • 1
    @cory This is a question about engine theory (and is tagged as such) which were overwhelmingly decided as on-topic on this forum in the Meta discussion Mar 8, 2017 at 13:58
  • It's too broad anyways. There are tons of different types of engines and thus too many possible answers to fit here. I assumed, for example, that you were talking about steam engines, so your question made no sense.
    – cory
    Mar 8, 2017 at 14:35

2 Answers 2


tl dr: Airflow

Think about an engine. It's an air pump. Air comes into the engine, mixes with gas (petrol), gets compressed, explodes, then flows out the exhaust. The more air you can get into an engine, the faster it will run. The throttle body poses a huge restriction which keeps air from coming into the engine. When you open the TB, more air can come in, which allows the engine to run faster.

There are other controls which help with all of this, to include the sensors (tells the computer whats going on with the engine) and fuel pump (to provide the fuel to the air so it can ignite and go boom). The ignition system which fires at the right time to provide the spark to make it go boom. All of this (and more) assists in making the crankshaft go faster, but it all starts with the air going into the engine which has to get past the gatekeeper, otherwise known as the throttle body.


In a carbureted engine, the fuel is pulled in by vacuum. More air flowing through the carburetor draws in more fuel. Since the air is restricted as Paulster2 stated, the engine at a given RPM can already pull in more air than it is. So opening the throttle body allows this extra air to come in, which pulls in more fuel (via a venturi effect, I believe) through jets sized mathematically to keep the air:fuel ratio in the proper range.

In a modern engine, it's all controlled by computer, but often to the same effect. The throttle controls air entry, sensors detect the amount of incoming air and fire more fuel out of the injectors to keep the ratio the same and give you more power. In some vehicles, even opening the throttle is controlled by computer; the pedal in my truck isn't wired to the throttle, it's wired to a sensor that tells the computer to open the throttle body. So the computer not only controls fuel to keep the mixture right, but can constrain air to prevent stalling under load, or for emissions control, or for whatever other reasons the designers/programmers want.

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