All the discussions of rev hang I've seen go this way: half the people complain about rev hang, and the other half swear they have no rev hang and they can shift perfectly smoothly. I don't pretend to be an expert at this phenomenon, but I have read quite a bit about it. I think there are some misconceptions out there that lead to the variety of comments on rev hang.
The purpose of this thread is to focus on rev hang, how much it sucks, and how to live with it (including ways to mitigate and perhaps even fix it). No promises on a true fix, as that would likely require custom ECU programming that apparently none of the tuners are interested in at this time.
1. What is rev hang?
After removing your foot from gas pedal and depressing clutch pedal, the engine RPM hangs or floats or drops very slowly at first (in some cases a second or more), before dropping more quickly. It is not just a slower (linear) drop in RPMs than desired, which is what would happen with a heavy flywheel, for example. It hangs before it drops.
2. Why is rev hang annoying?
Because it means you can't shift quickly without putting extra wear on the synchros in the transmission and the clutch. It's a question of passive rev matching. You either have to wait for the RPM to drop to where it wants to be for the next gear (car decelerates, driver behind you gets on your tail), or you have to fight the synchros into the next gear and slip the clutch to get a fast, smooth shift (excess wear). Both are suboptimal and both result in a frustrating shifting experience for the sport-minded driver.
3. What causes rev hang?
Common misconceptions are that rev hang does not exist, that it is due to driver error only, that it is caused by a heavy flywheel or a dual-mass flywheel, that it is caused by the transmission. Rev hang has nothing to do with the transmission or clutch. It has little to do with the flywheel either (although a lightweight flywheel will drop RPMs faster than a normal flywheel).
Removing the CDV (clutch delay valve), removing the clutch pedal spring, and installing a clutch stop are all good modifications that can improve shifting, but they will not fix rev hang.
The main cause is a delayed fuel cut programmed into the ECU. Today's cars including all Mk7s have drive-by wire, so the gas pedal just provides input to the ECU, which then outputs to the throttle. So when you take your foot off the gas, the ECU can delay the fuel cut per its algorithm (what the engineers have programmed in). This is done to reduce emissions--which is a good thing. But frustrating in how it affects our shifting.
Here is a good description of rev hang as implemented by VW specifically, but it's the same principle for all MT cars today:
https://www.autoweek.com/news/techn...revs-slowly-on-purpose-to-curb-nox-emissions/
And here is a detailed video description for the geeks:
4. How can we deal with rev hang to shift more smoothly and minimize transmission/clutch wear?
First, there are two situations where rev hang is much less pronounced: when the engine is cold (first couple of minutes after starting the engine), and when the AC is on. I'm not sure exactly why the AC affects it so much, but the AC compressor is driven by the serpentine belt on the engine so it applies load directly to the engine.
When engine is warm and you are not using AC, here are tips to minimize rev hang:
EDIT: added a poll to the thread, select the two answers that apply (model year and model name). Not including TDI because diesel engines and emissions controls are so different, but if you have a manual TDI and experience rev hang, feel free to comment.
The purpose of this thread is to focus on rev hang, how much it sucks, and how to live with it (including ways to mitigate and perhaps even fix it). No promises on a true fix, as that would likely require custom ECU programming that apparently none of the tuners are interested in at this time.
1. What is rev hang?
After removing your foot from gas pedal and depressing clutch pedal, the engine RPM hangs or floats or drops very slowly at first (in some cases a second or more), before dropping more quickly. It is not just a slower (linear) drop in RPMs than desired, which is what would happen with a heavy flywheel, for example. It hangs before it drops.
2. Why is rev hang annoying?
Because it means you can't shift quickly without putting extra wear on the synchros in the transmission and the clutch. It's a question of passive rev matching. You either have to wait for the RPM to drop to where it wants to be for the next gear (car decelerates, driver behind you gets on your tail), or you have to fight the synchros into the next gear and slip the clutch to get a fast, smooth shift (excess wear). Both are suboptimal and both result in a frustrating shifting experience for the sport-minded driver.
3. What causes rev hang?
Common misconceptions are that rev hang does not exist, that it is due to driver error only, that it is caused by a heavy flywheel or a dual-mass flywheel, that it is caused by the transmission. Rev hang has nothing to do with the transmission or clutch. It has little to do with the flywheel either (although a lightweight flywheel will drop RPMs faster than a normal flywheel).
Removing the CDV (clutch delay valve), removing the clutch pedal spring, and installing a clutch stop are all good modifications that can improve shifting, but they will not fix rev hang.
The main cause is a delayed fuel cut programmed into the ECU. Today's cars including all Mk7s have drive-by wire, so the gas pedal just provides input to the ECU, which then outputs to the throttle. So when you take your foot off the gas, the ECU can delay the fuel cut per its algorithm (what the engineers have programmed in). This is done to reduce emissions--which is a good thing. But frustrating in how it affects our shifting.
Here is a good description of rev hang as implemented by VW specifically, but it's the same principle for all MT cars today:
https://www.autoweek.com/news/techn...revs-slowly-on-purpose-to-curb-nox-emissions/
And here is a detailed video description for the geeks:
4. How can we deal with rev hang to shift more smoothly and minimize transmission/clutch wear?
First, there are two situations where rev hang is much less pronounced: when the engine is cold (first couple of minutes after starting the engine), and when the AC is on. I'm not sure exactly why the AC affects it so much, but the AC compressor is driven by the serpentine belt on the engine so it applies load directly to the engine.
When engine is warm and you are not using AC, here are tips to minimize rev hang:
- Rev above 3000 RPM before shifting. This seems to mostly eliminate rev hang. Although the RPM still does not drop as fast as I would like, it's much better.
- Ensure your foot is completely off the gas before depressing the clutch. Even a little pause between might be worth trying.
- When you don't want to go above 3000 RPM (e.g. slow acceleration to maximize fuel economy), deal with the rev hang and wait for RPMs to drop before shifting (not just before letting clutch out--wait to shift it into the next gear to minimize synchro wear). The goal is to work with the transmission, not against it. It also feels great when you get a perfectly rev-matched shift—there is almost zero resistance to the lever as it slides in. Toward this end, I'd like to make a table of MPH vs RPM in each gear (which will be different for the different flavors of Mk7) to help with rev matching.
EDIT: added a poll to the thread, select the two answers that apply (model year and model name). Not including TDI because diesel engines and emissions controls are so different, but if you have a manual TDI and experience rev hang, feel free to comment.
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