The Big Problem With Modern Manual Transmissions - Rev Hang
Why Motor Vehicle Maintenance & Repair Stack Exchange recommends it
Mechanics Stack Exchange users link it when explaining why RPM drops slowly after a shift, since it lays out the emissions and electronic-throttle reasons in plain terms.
Recommended by 2 people across 2 different months, 2020 to 2021.
What it is
An Engineering Explained video on rev hang, where engine RPM lingers after you press the clutch and let off the throttle in modern manual cars. It argues the cause is electronic throttle bodies programmed for emissions and smoothness, not heavy flywheels. Jason Fenske covers how electronic throttles differ from mechanical ones and what they enable.
What people said
Recommends Jesse Reza Khorasanee · 2020-11 · Motor Vehicle Maintenance & Repair Stack Exchange · on “Hesitation in RPM drop when shifting manual transmission, 2007 Mazda6”
This is Rev hang [LINK] . It releases less emissions than a standard manual gearshift. Modern car manufacturers have strict emissions guidelines to meet and in a manual transmission the ECU has less control of the engine and thus less control of emissions released. Keeping the throttle open after a gear change allows airflow to the engine to combust all remaining gases in the chamber. This is what the catalytic converter is designed to process, so emissions will be better processed than if we closed the throttle early. If you're interested I highly recommend Engineering Explained's video where he explains this very well: [this video]
Recommends Harper - Reinstate Monica · 2021-08 · Motor Vehicle Maintenance & Repair Stack Exchange · on “Why do mechanics give the wheels a spin after raising a car for inspection?”
@davidbak those can also be caused by applying high amounts of power/throttle at those moments (and on modern cars with ludicrously high horsepower engines, "high" actually means "low/medium"). Cross power off the list by not touching the accelerator at those moments and seeing if the squeal ceases. Note that you can't come rapidly off the throttle in a modern car, because of "rev hang" [this video] ... if you tend to overstomp the gas to compensate for it feeling unresponsive, see that video.
Cites Heatgunner · 2021-07 · Motor Vehicle Maintenance & Repair Stack Exchange · on “What are the factors that rev hang depend on? Does rev hand depend on the engine load?”
I know that rev hangs are intentionally introduced to modern vehicles that operate on electronic throttle bodies, by programming the ecu to close the throttle valve slowly (in gasoline engines) even when you let go off the accelerator pedal quickly ,so that you can reduce emissions. I have read that when you close the throttle valve quickly it takes some time for the fuel injectors to cut off the fuel supply making the air-fuel mixture available in the engine to be richer thus causing higher hydrocarbon emissions. So my question is "Will letting go off accelerator at high engine load conditions (say you are flooring it and letting it go completely) will increase the duration of rev hang?". M