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    Thread: School me on Rev hang

    1. Member worth_fixing's Avatar
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      08-01-2020 09:11 AM #1
      So Rev hang is the new norm in order to reduce tailpipe emissions by ensuring combustion is complete in a controlled reduction in engine speed. It dulls the whole driving experience when paired with a manual trans (which is the only way to go) and often results in grinding 2nd gear due the the large ratio difference between 1st and an engine speed too fast for smooth synchromesh engagement.

      It's a pain in the ass, but it's better for the environment so...yeah. We either live with it or we tune it out at the detriment to greater tailpipe emissions.

      ...but I had a couple of questions. Why is rev hang much worse on certain engines, and how on earth do quick-shifting automatics reduce engine speed so quickly and pass emissions? I'm not even talking about the farting autos out there (which I assume fart to burn off NOx); just the regular torque converters fitted to normal or higher performing engines. Many of these automatics shift gears in a small fraction of the time it takes from physically shifting from 1st to 2nd in a manual and yet we're grinding gears because of emissions. Is there a way to have an engine behave like it was bolted to an auto with its engine speed agility + emissions friendliness? There's something I'm not understanding here.

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    3. Member
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      08-01-2020 10:13 AM #2
      Grinding 2nd usually has more to do with a large ratio spread (also for emissions) than rev hang; everything transmission-wise that happens while the clutch pedal is in is past the input shaft. Engine speed only comes into play when the clutch pedal is lifted--so rev hang will usually result in a lurch if you're trying to shift fast, but shouldn't cause a grind. If you're literally grinding second, you might be moving the lever before the pedal is down sufficiently or moving the lever with too much force. If you just mean the lurch from lifting the clutch up, then sorry for getting into semantics.

      At any rate, you're right that rev hang definitely is annoying, and trying to shift fast through low gears on late model cars does often cause a lurch unless you baby the clutch. On most automatics it's not an issue (though on older SMG-style ones it is!!), but for different reasons.
      • In the case of dual-clutch boxes, the transmission ECU (aka TCU) and engine ECU are actually talking to each other. Because the engine knows exactly what the transmission is going to do, and since the next gear is already meshed, it knows exactly how long it is going to take to do it. So the ECU can intentionally drop revs to avoid the lurch. Various ways to effect this--some of which generate the 'fart' noise--but I believe they will typically do so through engine timing.
      • For torque-converter automatics, it's much less an issue as the TCU can unlock the transmission and the differences are absorbed by the fluid coupling.
      • For ZF8-style hybrids, the electric motor is presumably absorbing some of the jolt. The nice thing about electric motors is that they can build or drop torque very quickly. Still, these cars tend to be slightly jerkier IME than torque converter ZF8s. Incidentally, that's also a bad thing about electric motors, and why steering feel is much harder to find in cars with EPS.


      So, the answer is a probably yes--they could implement technology to make rev hang not an issue. But humans tend to be less consistent in shifting for a few reasons--I mean, I'm certainly not going to bang through gears in traffic, and I certainly am at the track. ECU doesn't know that though, and one of the beautiful things about MTs is that you can change every shift to suit what you're doing. That just happens to be at odds with rev hang.
      Last edited by ghost03; 08-01-2020 at 10:16 AM.

    4. Member VR6ix's Avatar
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      08-01-2020 10:18 AM #3
      · ·we're only gonna die for our own arrogance that's why we might as well take our time...
      · · /
      · · · · · · · · · · · · · · · · · On a long enough timeline, the survival rate for everyone drops to Ø

    5. Member Cradle2theDave's Avatar
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      08-01-2020 10:23 AM #4
      In a manuel, a lighter flywheel will greatly reduce rev hang issues.

      When I had the stage 1 clutch in my car with the aluminum flywheel, rev hang was essentially non existent. Unfortunately I wasn't a fan of doing a clutch job every 20,000 miles, so I had to go with a South Bend stage 2, and now the rev hang is atrocious, the flywheel must be made of solid granite.

    6. Member
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      08-01-2020 10:27 AM #5
      Oh, one more thing. In some cars, especially with smaller engines, the extra load from the AC will help bring revs down faster. I know in my Mk5 GTI I would run the compressor even with the windows down on occasional for that reason.

    7. Member GLI Dan's Avatar
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      08-01-2020 10:27 AM #6
      I'm just going to say it, unless you live near some wonderful driving roads (NC/CA and surrounding areas I'm looking at you), a manual is not the way to go, and definitely not "the only way to go." #unpopularopinion

      That said, if you're grinding second you might want to work on your foot hand coordination. As described above rev hang will effect the engine engagement with the drive train, not the selection of the gear.
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    8. Member
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      08-01-2020 10:30 AM #7
      Quote Originally Posted by GLI Dan View Post
      I'm just going to say it, unless you live near some wonderful driving roads (NC/CA and surrounding areas I'm looking at you), a manual is not the way to go, and definitely not "the only way to go." #unpopularopinion
      I love the PDK in my Cayman, but ironically the place I miss having a MT most is the commute. On back roads and the track there's enough interesting stuff going on I don't even think about it, but if I'm driving around town pressing paddles is a lot more boring than shifting.

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      08-01-2020 02:46 PM #8
      These were the first car i drove with noticeable rev hang with the manual V6

    10. Member Bicycle019's Avatar
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      08-01-2020 08:10 PM #9
      Quote Originally Posted by ImpeccableNEW View Post
      These were the first car i drove with noticeable rev hang with the manual V6
      Yup, these and the mechanically identical Contour V6 5mt had terrible rev hang. Easy to fix though with a restrictor made from a copper pipe end cap and drill bit added to the line between intake boot and EGR. Transformed how the car drove by eliminating the rev hang.

      My Mk7 GTI 6mt has some hang. Hard to really rush the 1/2 shift due to it, but not nearly the show stopper it was on earlier cars. Echo other posters comment on running the A/C helping neutralize it.
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    11. Member elite.mafia's Avatar
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      08-01-2020 08:48 PM #10
      My 2011 Civic Si had rev hang. It was annoying because it was extremely inconsistent. Sometimes you get rev hang, sometimes you don't. For the most part you end up ignoring it and just slipping the clutch to smooth it out.
      So far I don't think my 05 celica GTS has rev hang, though it is drive-by-wire. The flywheel feels very light and the revs drop extremely fast when you go to shift, making it kind of hard to be smooth because the revs just drop too fast. Kind of the opposite issue to revhang.
      Last edited by elite.mafia; 08-01-2020 at 08:51 PM.
      2zz-ge 6mt trying to rev to mars

    12. Member S1ack's Avatar
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      08-01-2020 08:52 PM #11
      I have not read the walls of text posts here.
      I think I may at a later date.

      I am just going to say that the 'rev hang' in a 19 Jetta GLI, is abhorrent, when I compare and contrast it to a 16 Golf R. Both of which I 'daily drive'.

      Perhaps 'abhorrent' is too string a term. But I just don't get it.
      It is so very different. Night and day.

      Both are 6MT.

      Go figure.
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    13. Member GoVdubSPEEDGo's Avatar
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      08-01-2020 11:22 PM #12
      new WRX has crazy rev hang, but with a Cobb tune it’s gone, so that’s what I did!


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      SHIFT

    14. Member Karl_1340's Avatar
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      08-06-2020 09:24 AM #13
      There is no noticeable rev hang in my Cadillac ATS.

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