Over-rev update

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Originally Posted by scmx5driver

It's just a matter of a slower revving car not spinning up to over-rev fast enough. The car is going faster than the engine is spinning so the rear wheels break traction and the car slows down before the engine gets past red line.

Now if you're already bouncing off the rev-limiter and downshift your still going to over-rev. But you’d think the synchros would make that hard to do.

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Warning! ::Teacher: moment ahead!!!

An overrev is an overrev, regardless of whether you do it by missing an upshift or downshift. If the rear wheels (and the input shaft of the transmission to which the rear wheels are mechanically connected when the tranny is in gear) are spinning faster than the output shaft of the engine and you disengage the clutch, those two shafts will try to match speeds. They have three ways to do so: overrev the engine, slip the clutch, or break traction on the rear wheels. The energy in the system will follow the path of least resistance, which is usually to overrev the engine. There is a lot less resistance in this path (assuming you’ve got a good clutch in the car).

Don’t believe me? You can test this theory without overrevving the engine. On your way home tonight, run the car up to redline in 1st, engage the clutch, shift to 2nd, and let the revs drop to idle. This will simulate an overrev because we'll have the input shaft of the transmission spinning at a fater rate than the output shaft of the engine. The transmission will backdrive the engine ust as it would in a missed shift. Then, lift your foot off the clutch abruptly. What happened? Did the tires break loose, or did the revs on the tach suddenly jump? I’ll bet you a case of Shiner it’s the latter. :wink:

Not that I need convincing. I’ve overrevved three cars (RX7 2nd to 1st upshift, 6-speed NB 2nd to 1st upshift, 5-speed NB 3rd to 2nd upshift on two occasions), and the rear tires have never lost traction. In all three cases the engine overrevved.

As for synchros, they actually make overrevving easier, since their sole purpose in life is to allow you to shift the transmission without having to match input and output shafts speeds.

On a totally unrelated note, Miata engines are surprisingly resilient to overrevs. :wink:

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Originally Posted by TheChadVIII

Ok, now that makes sense to me...

Oo man get'em to write you a check for the ECU Adam, then you, Nexus, and me can start up a group buy on some Hondata stuff... and some VFACs while we're at it!

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Why get a VFAC when you get all that capability and more in a K-Pro?

Unless you need more buttons to push and another display to look at.;)

This vtec stuff is tricky indeed.:rolleyes:

It is nice having 1 thing control everything though :D

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Originally Posted by altiain

Warning! ::Teacher: moment ahead!!!

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+1 on everything Iain said.

Of course, if my car over-revved because of the rear wheels, I've got a pretty rare RSX ::Clown::

BTW, your story scared me the entire time my car was at my dealer's shop for like 5 days to change the AC compressor and they said afterwards it had a check engine light so they had to send it back in.

But turns out its all fine, but I was nervous and mad until I got it back.

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Originally Posted by altiain

Warning! ::Teacher: moment ahead!!!

An overrev is an overrev, regardless of whether you do it by missing an upshift or downshift. If the rear wheels (and the input shaft of the transmission to which the rear wheels are mechanically connected when the tranny is in gear) are spinning faster than the output shaft of the engine and you disengage the clutch, those two shafts will try to match speeds. They have three ways to do so: overrev the engine, slip the clutch, or break traction on the rear wheels. The energy in the system will follow the path of least resistance, which is usually to overrev the engine. There is a lot less resistance in this path (assuming you’ve got a good clutch in the car).

Don’t believe me? You can test this theory without overrevving the engine. On your way home tonight, run the car up to redline in 1st, engage the clutch, shift to 2nd, and let the revs drop to idle. This will simulate an overrev because we'll have the input shaft of the transmission spinning at a fater rate than the output shaft of the engine. The transmission will backdrive the engine ust as it would in a missed shift. Then, lift your foot off the clutch abruptly. What happened? Did the tires break loose, or did the revs on the tach suddenly jump? I’ll bet you a case of Shiner it’s the latter. :wink:

Not that I need convincing. I’ve overrevved three cars (RX7 2nd to 1st upshift, 6-speed NB 2nd to 1st upshift, 5-speed NB 3rd to 2nd upshift on two occasions), and the rear tires have never lost traction. In all three cases the engine overrevved.

As for synchros, they actually make overrevving easier, since their sole purpose in life is to allow you to shift the transmission without having to match input and output shafts speeds.

On a totally unrelated note, Miata engines are surprisingly resilient to overrevs. :wink:

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Yo Teach!
First of all, if you shift from 2nd to 1st it's a down-shift not up-shift, and if you engage the clutch you can't shift gears, at least not easily, so I think you meant disengage the clutch or engage the clutch pedal.

Second, if you do like you described above... reline in 1st is what, 40 MPH? So when you dump the clutch in 2nd gear from idle the engine revs to like 4K RPM, which is nothing a very easy for the engine to do.

Try this, run up to 4 or 5K in Second gear and DOWN SHIFT to 1st (no rev-matching please). Most likely your rear wheels will break traction as you engine whines up to around 7K RPM. Not sure about the actual RPM numbers it will take to make this happen. So you might have to experiment.

But I think that is more what was described in the earlier post.

Some cars like the S2000 are much quicker to rev than others so they're more likely to over-rev than lock-up the drive tires.

Also, If I tried to shift my Miata from red-line in 4th gear into say 2nd the synchros wouldn't be set for it and I'd have to really grind the heck out of it put it in 2nd.

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Originally Posted by TheChadVIII

This vtec stuff is tricky indeed.:rolleyes:

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Yea, when you get bored of VTEC, you can go play with the VTC.

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Originally Posted by scmx5driver

Yo Teach!
First of all, if you shift from 2nd to 1st it's a down-shift not up-shift, and if you engage the clutch you can't shift gears, at least not easily, so I think you meant disengage the clutch or engage the clutch pedal.

Second, if you do like you described above... reline in 1st is what, 40 MPH? So when you dump the clutch in 2nd gear from idle the engine revs to like 4K RPM, which is nothing a very easy for the engine to do.

Try this, run up to 4 or 5K in Second gear and DOWN SHIFT to 1st (no rev-matching please). Most likely your rear wheels will break traction as you engine whines up to around 7K RPM. Not sure about the actual RPM numbers it will take to make this happen. So you might have to experiment.

But I think that is more what was described in the earlier post.

Some cars like the S2000 are much quicker to rev than others so they're more likely to over-rev than lock-up the drive tires.

Also, If I tried to shift my Miata from red-line in 4th gear into say 2nd the synchros wouldn't be set for it and I'd have to really grind the heck out of it put it in 2nd.

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Sorry, that came out a bit more obnoxious than I meant it to be.

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Originally Posted by scmx5driver

Sorry, that came out a bit more obnoxious than I meant it to be.

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We like obnoxious around here. It makes the popcorn taste better. :-)

There's also another thing to consider that Iain left out of his description... on a botched upshift, the driver generally has the go-pedal floored... So the tranny is only spinning the engine PAST fuel cut-off, reducing the drag effect that might otherwise lock up the rear wheels/slip the clutch.

As regard syncros... they are never "set" for something. They don't do anything at all until you attempt to engage a gear. If the input shaft is slower/faster than the output shaft, the synrco will then and only then work it's little brass self off to get both shafts spinning the same speed.

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Originally Posted by tailchaser

As regard syncros... they are never "set" for something. They don't do anything at all until you attempt to engage a gear. If the input shaft is slower/faster than the output shaft, the synrco will then and only then work it's little brass self off to get both shafts spinning the same speed.

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True, I didn't describe myself very well there.
I guess what I'm trying to say is that the down-shift is such a severe difference, that the syncros can't get everything rev-matched. So it's very difficult to put it into gear.

The RSX-S 6 speed transmissions utilize 3 double and 2 triple cone synchros, so while providing smooth shifting and quick rev matching of the mainshaft and the countershaft, they are not very forgiving of miss shifts.

Gear/Synchro
1/Triple
2/Triple
3/Double
4/Double
5/Double (carbon on 05-06)

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Originally Posted by scmx5driver

Yo Teach!
First of all, if you shift from 2nd to 1st it's a down-shift not up-shift, and if you engage the clutch you can't shift gears, at least not easily, so I think you meant disengage the clutch or engage the clutch pedal.

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If you’re in 2nd and you find 1st while you’re looking for 3rd, I’d call that a botched upshift. Your terminology may differ. Regarding clutch engagement/disengagement, I’ll give you that. I should’ve proofread my work a little more closely. Hopefully you can still get the gist of what I was saying. :wink:

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Originally Posted by scmx5driver

Yo Second, if you do like you described above... reline in 1st is what, 40 MPH? So when you dump the clutch in 2nd gear from idle the engine revs to like 4K RPM, which is nothing a very easy for the engine to do.

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You’re missing the point. What I suggested doing simulates an overrev by backdriving the engine with the rear wheels. Same thing happens in an overrev caused by a botched shift. In both cases the rear wheels – and the transmission input shaft that’s attached to them – will be spinning faster than the output shaft of the engine. The only difference is the absolute engine speeds. The physics don’t change just because you’re going from 1000-4000 rpm instead of 7000-10,000 rpm.

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Originally Posted by scmx5driver

Yo Try this, run up to 4 or 5K in Second gear and DOWN SHIFT to 1st (no rev-matching please). Most likely your rear wheels will break traction as you engine whines up to around 7K RPM. Not sure about the actual RPM numbers it will take to make this happen. So you might have to experiment.

But I think that is more what was described in the earlier post.

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Dude, it’s the same thing, regardless of whether you were trying to upshift or downshift at the time of the overrev. What do you think I was describing earlier? Run up to redline in 2nd gear, go for 3rd and find 1st instead. Explain to me how accidentally shifting to 1st during a botched upshift is any different from intentionally downshifting to 1st like you’re describing? It’s not.

Go back and reread my post. I HAVE done this before. Not once, but multiple times in multiple cars. The wheels don’t break traction. The engine DOES overrev. There is nothing in the engine that prevents it from being mechanically spun beyond redline or fuel cut. Fuel cut is just that – it prevents the engine from overrevving when the engine is the driving function of the system by cutting off the flow of fuel to the cylinders. Unfortunately, in the case we’re talking about the combustion cycle isn’t driving the engine output shaft – the output shaft is being backdriven by the rear wheels.

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Originally Posted by scmx5driver

Yo Some cars like the S2000 are much quicker to rev than others so they're more likely to over-rev than lock-up the drive tires.

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This really has next to nothing to do with the situation we're discussing, since the engine is being backdriven by the rear wheels.

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Originally Posted by scmx5driver

Yo Also, If I tried to shift my Miata from red-line in 4th gear into say 2nd the synchros wouldn't be set for it and I'd have to really grind the heck out of it put it in 2nd.

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Robert, push your clutch pedal in the next time you’re cruising down the freeway and see if you can put your shifter in every forward gear (you won’t get into reverse because there’s a mechanical lockout to prevent you from doing just that while moving forward). If you can shift your transmission into 1st while the rear wheels are turning at 65mph (and I’ll bet you can, unless your synchros are worn), then you can certainly shift your Miata into 2nd when you’re at redline in 4th… although in a botched 4th gear redline upshift you’d be more likely to find 3rd while looking for 5th, I imagine. Either way, 2nd or 3rd, you’ll overrev the engine.

Actually I had a hell of a time in my Miata shifting into 1st at anything over 10-15mph... you'd push it halfway into first and it'd stop then you'd hear the synchro winding the HELL outta it self and it'd sound like an LSD, and pretty much wouldn't go in until I slowed down...but the synchros might'a been worn or something else that caused it, I don't really know how all those work in detail.

It'd go into 2nd at any speed though...

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Originally Posted by altiain

The only difference is the absolute engine speeds. The physics don’t change just because you’re going from 1000-4000 rpm instead of 7000-10,000 rpm.

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The physics doesn’t change at all but that's not to say it doesn’t take much more energy to pack twice as many rev into a space of time. Pistons moving twice as fast valves moving twice as fast, and in your world you can do this with the same energy. If there was no difference in work and friction there would be no over-reving. Your engine could just spin faster and faster for ever.

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Originally Posted by altiain

Explain to me how accidentally shifting to 1st during a botched upshift is any different from intentionally downshifting to 1st like you’re describing? It’s not.

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I never said it was. I was trying to more accurately describe how to simulate the situation that TheChadVIII described in his post that started the whole locking up of the tires. As compared to your low rev over-reving experiment.

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Originally Posted by altiain

Go back and reread my post.

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I think you should go back and reread my original post. Your, basically restating what I already said. "Now if you're already bouncing off the rev-limiter and downshift your going to over-rev."

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Originally Posted by altiain

There is nothing in the engine that prevents it from being mechanically spun beyond redline or fuel cut[/b]. Fuel cut is just that

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Duhh. See above quote. "Now if you're already bouncing off the rev-limiter and downshift your going to over-rev."

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Originally Posted by altiain

you can certainly shift your Miata into 2nd when you’re at redline in 4th… although in a botched 4th gear redline upshift you’d be more likely to find 3rd while looking for 5th, I imagine. Either way, 2nd or 3rd, you’ll overrev the engine.

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If you'll read my original post you'll notice it was in the form of a question.
I've never accedentally downshifted when I was trying to up shift. I have on occasion tried to shift into first when approaching a stop, and couldn't and have missed a 3rd gear upshift and put it into 5th or should I say I would have put it into 5th if the sound of grinding gears hadn't told me to stop.

So the only lesson I needed was... No, the synchros won't stop it. If yours do, they might be worn.

P.S. I guess I set you off when I quoted your post regarding tire lock-up. I guess I should have gone all the was back and quote TheChadVIII's original message, but I was lazy.

And if downshifting couldn't lock-up the back wheels than there wouldn't be much point in heal-toe downshifting, now would there.

:snack:

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Originally Posted by scmx5driver

...And if downshifting couldn't lock-up the back wheels than there wouldn't be much point in heal-toe downshifting, now would there.

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Well there you are heading in Ian's direction. By using heel-toe downshifting to brake you are using the engine to do some of the braking. You do this by doing some rev matching but still using the energy of the car moving forward to drive the engine meanwhile the gas pedal is mostly out so the engine wants to idle. The energy is shifted from the rear (in a RWD car) through the drive line (assuming no slipping/properly working clutch, at this time clutch is fully engaged, and the sychros are just chillin') to drive the engine. If done properly and at a correct/safe RPM, this will overdrive the engine but will not result in dangerous over reving (sky high RPM level).


Wheew... That's my ::2Cents::

As for the other ::Blah:: ::BLee:: ::BS:: ::Argue::
I'm going to follow Magma's example and enjoy the show from the sidelines :snack:

How about we all go and :study: :drive: ::Teacher: and then go :study: some more followed by :drink: :chug: Then we can have a nice big ::Grouphug.

I think you have been :beer:already today