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Throttle body linkage for positive control
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The single (or dual) throttle body used to control air flow in a plenum type EFI system (not ITB) can be much larger than a carburetor used on an engine a similar state of tune. This reduces pumping losses and increases peak power. Intake pressure loss as low as .100 psi (.204” Hg) are possible, but have serious consequences as to response, control and general driveability.
By comparison, a carburetor almost ceases to meter accurately, even at full throttle, below .5 psi (1.018” Hg).
Very large throttle bodies are “nervous”, and very sensitive to small pedal movements at low speeds. The amount of air admitted by even a few degrees of throttle disc rotation is very large, and tends to produce unwanted acceleration. A throttle shaft rotation of only 10° from idle may produce as much air flow as 20° or more from a smaller throttle body.
The total shaft rotation of either size is almost equal: about 10° open for idle (the throttle never closes completely), and the remaining 80° to fully open, where the disc is parallel to the direction of airflow and the bore axis. |
The sensitivity can be reduced, and more precise control added without loss of power by either adapting, modifying or fabricating a cable control similar to the BMW shown here (click the image for a larger view).
The principle is very simple: the cable end is at 90° to the throttle arm at idle, this yields direct motion but its speed (vs. the rotation of the throttle shaft) is very low because the arm is long from the throttle shaft center to the cable end. |
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As the cable is pulled, the throttle arm rotates back. As it does so, the critical length from the throttle shaft center to the cable end becomes shorter, which increases the speed dramatically. For example, assuming that the radius from the throttle shaft center to the cable end is 3” at idle and gradually decays to 1” in 90° of rotation, the throttle opening is only 1/3 as much per degree of throttle movement at and near idle, speeds up to 1/2, then 2/3 and finally becomes very fast approaching full throttle.
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