At Motor Authority, we love manual transmissions because they make driving more fun. But modern manual transmissions have a big problem, according to Engineering Explained's Jason Fenske.
That problem is "rev hang," and it's a result of pairing manual transmissions with electronic throttle bodies. An electronic throttle body eliminates the mechanical connection between the accelerator pedal and the throttle valve on an internal-combustion engine. Instead of a driver's foot action tugging on a cable, as in mechanical throttle bodies, a position sensor tells a small electric motor how much to open the throttle.
An electronic throttle body can also communicate with other sensors in a car, which helps enable certain driver aids, and allows throttle behavior to be tuned for specific functions, such as heating up catalytic converters or providing smooth acceleration in off-road-specific driving modes. They also improve pedal feel, Fenske noted, because the response of mechanical throttle bodies is inherently nonlinear.
But while electronic throttle bodies are great at interfacing with electronic systems in modern cars, they don't speak the same language as analog manual transmissions, Fenske said. The result is rev hang, when revs stay elevated even after the driver has let off the throttle and slowly drop off. Putting the clutch in abruptly drops the revs, causing a jerking motion.
Rev hang tends to be an issue with electronic throttle bodies because of emissions, according to Fenske. They are usually programmed to cut fuel and delay throttle closure when the driver lifts off the right pedal, in order to maintain a consistent air-fuel ratio for emissions purposes, he said.
So if you own a relatively new car and have trouble pulling off smooth shifts, the car may be part of the problem. Watch the full video for complete technical details on rev hang.