What Is 'Rev Hang' And Why Do Petrolheads Hate It?
When you dip the clutch in a car, the revs will drop. This is due to the inertia of the flywheel, and the lack of air going in the intake as the throttle is release. After the gear change is over and the clutch is released, the engine speed will go back up again, meeting the transmission speed.
However, rev hang is when the engine speed doesn’t immediately drop once the clutch is actuated, and instead descends very slowly, as if there were some form of restriction. In extreme cases the engine speed will even rise slightly when the engine and transmission are disengaged.
This obviously isn’t ideal for changing gear. If the engine speed doesn’t drop enough during the process of dipping the clutch, changing gear and then releasing the clutch, the engine speed and the rate at which the flywheel is rotating will be much higher than the speed that the friction plate is rotating at.
This creates a horrible jerk as a braking force occurs when the powertrain and transmission are reengaged and forced to spin at the same speed. This can be frustrating if you’re wanting to perform a quick, slick gear change, as you have to wait for the revs to drop to a reasonable level before upshifting to the next gear to avoid stunted progress.
The cause of rev hang can be put down to the war on emissions. Recently, manufacturers have been intentionally implementing rev hang within ECU programming to save themselves from a potential emissions scandal. When the throttle is released and the clutch is dipped, an ECU with rev hang is programmed to leave the throttle slightly more open than usual.
This strategy is used because it was found that suddenly closing the throttle produced a sudden spike in gas pressure within the crankcase which emphasised the rate of oil vaporisation, thus adding to emissions from the engine. Also, when the throttle is suddenly shut, the fuel mixture suddenly becomes lean which creates NOx (oxides of Nitrogen) which also contributes to emissions.
By keeping the throttle open for a small period of time after the pedal had been released, this pressure spike was avoided and cleaner emissions were achieved. Found most prominently in US-spec vehicles to meet the emission rules over the pond, this pre-programming has frustrated many owners since its development. Rev hang can even be found in Subaru’s WRX STI, along with prominent cases in VW and Hyundai products.
To help eradicate this high pressure within the crankcase, a PCV (positive crankcase ventilation ) valve is used which can reduce the need for a rev hang system when fully functioning. This PCV valve is used to regulate the pressure within the crankcase that can fluctuate to high levels when gear changes occur. In internal combustion engines, it is inevitable that a small amount of ‘blow-by’ occurs, which is when gases from the combustion chamber seep past the piston rings and travel down into the crankcase. The job of a PCV valve therefore is to remove a proportion of the blow-by fumes away from the crankcase, thus reducing the amount of high pressure gas contained within.
Although an intentional rev hang system would go a long way in saving your oil levels and help your car sail through an emissions test, most petrolheads only see it as a hindrance to their driving pleasure. With a quick reprogramming of the throttle body control codes and a bit more competence when it comes to checking oil levels, rev hang can be eradicated and your gear changes can be as smooth and as quick as you desire.
Do you have a car that features rev hang as a setting within the ECU from factory? Let us know your experiences!
Comments
My 2016 hyundai accent has this and i can tell you its not pretty when you try to make a nice smooth shift but all you get is a very unpleasant jerk that just ends up in broken dreams…
Same thing. Quite annoying. BMW e46 330ci
It’s very prominent in my 2010 Subaru Outback 6M. It’s such a pain in my rear.
I drive a 2.0L diesel Kia Cee’d (so Euro spec car, obviously) and also gets rev hang, but it’s so much worse because of the typical diesel torque.
My 97 ram does this. It makes it hard to look cool when I rev my friends quiet littl vw
I have an extreme - and extremely annoying - rev hang problem with my 2012 Jetta GLI, 6-speed manual. (stock, VW E888 2.0L Turbocharged engine). Not only don’t the RPMs fail to decline rapidly ensuring upshifting can be done quickly, and without extreme wear on the synchro-rings, just after hard throttle acceleration, the RPMs actually climb 200-500 RPM, when the throttle is chopped and the clutch depressed simultaneously. The Revs then drop as slowly as drying paint.
It’s really the only thing I don’t like about the car. BTW, this phenomenon is not noticable in the same car configured with the DSG manumatic gearbox, as the next higher gear is engaged so rapidly there is no time to experience the rev hang, but I like driving a stick, better - - at least until this emission control mechanism was engineered into the electronically actuated throttle valve.
Of course it’s been to the dealer where “new” factory software updated was installed under warrantee on several occasions, but none have done the slightest to alter the rev hang issue.
Thus my question is, is there a way to defeat, or at least minimize this rev hang via re-programming? Please don’t tell me that this is merely an issue of having a very heavy flywheel. While high rotational inertia would also create a slow rev-down, I have friends who tried to solve this problem with a low-mass flywheel to no avail. Their engines do rev a bit more quickly (and are more prone to wheelspin when nailing it in 1st & 2nd gears coming our of corners), their engines still hover a high RPMs when they chop the throttle.
The fix is clearly in the programming, yet, while good with a wrench, I know zilch about reprogramming an engine control module (ECM). Any help in resolving this would be deeply appreciated as it’s the only significant bug in an otherwise excellent car, which I’ve driven for 30K miles ever mindful of it and annoyed by it.
Additionally, if anyone knows of a commercial “Stage 1” ECM reprogram that eliminates, or even markedly reduces the dreaded VW/Audi rev hang problem, I’m ready to buy it. Of course I wouldn’t object to the additional 50-65 HP and even more torque boost the typical “Stage 1” chip re-tunes offer, but not if it did not markedly reduce the rev hang. Please help
You can still get smooth gear changes even with a car that has rev hang.
You just need to feather the clutch a little more. It’s still just as fast if done with practice.
Megane 3 2011 petrol also behave similar. Earlier vetsuon from 2006 same engine, revs drop in few seconds to idle 780 rpm. This one slows slowly, holfs 1000 rews 3-4 seconds and than drops to idle 700-800 revs. Very frustrating to changing gears. Also i like to brake with engine idle, not working much ob this car. Which solution is?
Our otherwise perfect 2019 GLI has horrible rev hang, maybe DSG next and would DSG stop the problem?
i have a question. my car has the rev hang behavior as your article mention.
is this a feature ? or an issue?
I reset the ECU learning (plug out battery, and turn the key acc on for a while, then start engine), the rev hang all gone.
If I want it back, how can I do it?
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