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?".

My logic behind this is that fuel injector will be supplying more fuel at higher loads and hence letting go off accelerator at higher load will result in more amount of fuel injected before it responds and cuts off fuel injection. To burn of this extra fuel completely then you would require more air and hence increased duration over which the throttle valve has to be kept open by the ecu (i.e rev hang). Also in port fuel injected vehicles there will be some fuel that wets a portion of intake runner and this fuel that wets the runners cause more rev-hang in port fuel injected vehicle.

Here is the video explaining rev hang: https://www.youtube.com/watch?v=mm6SSeLivqE

Also a counter argument to this could be that the throttle valve will be opened more at higher loads so the ecu doesn't need to maintain/keep the throttle valve open for an even larger duration to burn the excess fuel injected.

Here is video that seems to show that you have more rev hang at higher load: https://www.youtube.com/watch?v=INtfe3xJXF8

My question is does rev hang depend on the engine load? Also what are all the factors that can affect rev hang?

  • $\begingroup$ So see mechanics.stackexchange.com/q/83391/10976 $\endgroup$
    – Solar Mike
    Jul 5, 2021 at 13:36
  • $\begingroup$ I was the one who asked that question in motor enthusiast site. I have asked here since it couldn't be answered till now. I could delete the question in motor enthusiast now that i have posted it here. $\endgroup$ Jul 5, 2021 at 14:01
  • $\begingroup$ Do you think that because the rpm is higher then the engine takes longer to slow down? Why do you think F1 engines have a low rotational inertia? And slow rpm engines more mass, ie heavier flywheels etc. $\endgroup$
    – Solar Mike
    Jul 5, 2021 at 14:05
  • $\begingroup$ I guess F1 engines were made high revving engines to reduce engine size. I think that's also why they have low torque since they are generally made over square engines and also they idle quite high. But i don't think whether inertia has to be considered as a factor when we talk about rev hang because if we are talking about the amount of fuel that ecu tries to burn off completely then rev hang is just a matter of amount of fuel you have to burn off till it starts dropping down at normal pace again. Unless there are other factors affecting rev hang, which is my question. $\endgroup$ Jul 5, 2021 at 14:17
  • $\begingroup$ Interesting question, but it sounds like this is ECU controlled so different manufacturers might use different algorithms, I'm not a car expert though... $\endgroup$
    – Drew
    Jul 5, 2021 at 17:58

1 Answer 1


"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?"

This is solely up to how the ECU programming is performed. There is nothing stopping the designer from completely cutting off the injectors immediately when the throttle input (accelerator pedal position) is at zero (or when the position changes rapidly towards shut), but in drive by wire systems like we're discussing here we can't just turn off fuel when we want only for emissions. There are a multitude of competing things the software designer is trying to balance, with driveability likely at the top of the list. Different manufacturers will approach the problem differently (for example, General Motors cars were known for coasting easily at accelerator-off, which tells me they weren't closing the throttle aggressively at all). At the highest levels of vehicle performance (F1), I would expect throttle-off engine characteristics to be customized to the driver and the driver's style.

  • $\begingroup$ If you could shut off the injectors very quickly just as you release the accelerator instantly then the manufacturers shouldn't be introducing the rev hang in the name of emission control. Also a car that has a high rev hang is not actually good looking from the stand point of drivability since you need the revs to drop sufficiently to shift smoothly. At least most people do complain about drivability with rev hang. And especially for those people who upshift at very high rpms. $\endgroup$ Jul 6, 2021 at 4:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.