Barrel valves and slide valves are two other configurations sometimes found on throttle bodies, usually in racing applications. A barrel valve is a cylinder with a large channel drilled through its diameter. This type of valve sits inside of a cylindrical hole; rotating the barrel in its hole allows more air to pass through the channel. Slide valves (which are comprised of a single throttle plate sliding outward from the throttle body case) are even less common in automotive applications, but they're quite common on motorcycles.
Comparison
Barrel valves and slide valves allow for more airflow than china butterfly valves, but they don't control air as well. Even when completely open, a butterfly valve's throttle shaft and plate block a certain amount of the air passing through. Additionally, the butterfly valve's throttle plate causes turbulence in the air stream, which can be a bad thing when they're mounted very close to the cylinder head or fuel injector. The turbulence coming off of the throttle plate whips around behind it like a mini tornado, leaving a dead spot of stagnant air just behind the throttle plate. If this dead spot happens to coincide with the fuel injector location, then its fuel mist won't mix with the airstream, leading to an uneven fuel burn in the combustion chamber and poor performance.
However, this turbulence also helps to make butterfly valves more precise than the other types of valves. It's for this reason that barrel and slide valves are used almost exclusively on powerful, high-RPM engines; racecar drivers are usually either completely on or completely off the throttle, so the on/off nature of these types of valves doesn't negatively impact performance.
Comparison
Barrel valves and slide valves allow for more airflow than china butterfly valves, but they don't control air as well. Even when completely open, a butterfly valve's throttle shaft and plate block a certain amount of the air passing through. Additionally, the butterfly valve's throttle plate causes turbulence in the air stream, which can be a bad thing when they're mounted very close to the cylinder head or fuel injector. The turbulence coming off of the throttle plate whips around behind it like a mini tornado, leaving a dead spot of stagnant air just behind the throttle plate. If this dead spot happens to coincide with the fuel injector location, then its fuel mist won't mix with the airstream, leading to an uneven fuel burn in the combustion chamber and poor performance.
However, this turbulence also helps to make butterfly valves more precise than the other types of valves. It's for this reason that barrel and slide valves are used almost exclusively on powerful, high-RPM engines; racecar drivers are usually either completely on or completely off the throttle, so the on/off nature of these types of valves doesn't negatively impact performance.