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Old 09-02-2009, 05:38 PM   #1
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Default Mechanical Over-Rev - How does it cause damage?

Hey guys. Having a discussion with a friend of mine about mechanical overrev of engines, and what causes damage to the engine. So, say you're cruising down the freeway at 75 in your C5Z06. You go to downshift from 5th to 4th and actually get 2nd, dump the clutch. For the sake of argument, let's assume that you're running sticky tires and they don't lock up, but keep traction. In this example, lets say the RPM's hit 8400 as you brainlock and don't react in time to undo what's been done.

So, is it the 8400 RPM that you reached that damages the engine, or is it all the FT-LB of torque flowing the wrong way (from the torque tube into the motor) that damages it, or both? Or is it just the super-rapid acceleration of the motor?

I thought y'all might have some good input as I don't know enough about engines or have enough experience to figure it out myself. Thanks!
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Old 09-02-2009, 06:10 PM   #2
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Assuming that you do not float any valves which in turn would contact pistons, i would say it would be the rapid acceleration of the engine. Since when you go into a lower gear i think the amount of torque flowing through the drivetrain would be directly proportional to the rate at which everything speeds up. But im sure there are smarter people in here that will put my thoughts to shame.
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Old 09-02-2009, 07:01 PM   #3
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Well I'm no Engine Expert, but the theroies on it just being rapid accel of the engine are wrong. If they were right, that means any engine could rev to 20k and be fine.

It's usually the fact that your revving so high that damages the motor, spinning at 8k plus is just to much for the rotating assembly.

I have no idea how torque could flow the wrong way so idk on that part, but i know that revving that high is just to much.
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Old 09-02-2009, 07:39 PM   #4
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Yeah it's basicly the RPM you hit that causes the damage. Mainly the valvetrain and occasionally a bearing if oiling was suspect to begin with..
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Old 09-02-2009, 10:07 PM   #5
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I have no idea how torque could flow the wrong way so idk on that part, but i know that revving that high is just to much.
When the engine accelerates not because of increased combustion but because it's being driven by the drivetrain which is spinning faster than it, that is the torque going the wrong direction I was talking about.
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Old 09-03-2009, 10:32 AM   #6
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Well, I don't know about street cars, but when you turn a sprint car around and you don't hold the tire with the brakes, you have a tendancy to pull the pins out of the bottom of the pistons. This is due largely to the fact not of the over rev, but the sudden accelleration from the engine going 8000 RPM forward to 7000 RPM backwards!!!!

Generally on an over rev like what you are talking about, the valvetrain takes the brunt of the damage. You will float the valves which will cause your seats to get hammered which will cause a lack of compression. We seen that a lot at Katech with the CTSV-R engines. The guys would shift down to quickly and bamn, 9000 rpm. The corvettes didn't have that problem, they had sequential gear boxes.
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Old 09-03-2009, 01:00 PM   #7
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Quote:
Originally Posted by MikahB View Post
When the engine accelerates not because of increased combustion but because it's being driven by the drivetrain which is spinning faster than it, that is the torque going the wrong direction I was talking about.
No the damage doesn't come form "motoring" the engine. It comes from the rpm way above design limits. Motoring is done all the time in OEM testing to measure friction and perhaps pumping losses. I suspect high dollar NASCAR teams do the same thing.

Most overrev damage is valvetrain related. In a recent article about valvesprings in race engine TECHNOLOGY a race team reported that valve springs in race engines that were not overreved (wrong gear selected) lasted 10 times as long as those occasionally overreved. One driver went thru valve springs regularly, one did not. Obviously piston/valve during overrev can cause bad problems.

The other problem is piston g loads. They increase exponentially with rpm. One excursion to 8400 in a stock engine probably won't fail parts, but numerous ones might.

GM included a rubber-like ring in the center of the driveshaft in early M6 C5 Vettes because the drive shaft whipped enough during this kind of overrev to scrape the tunnel. They discovered that when GM employees had that accidental problem. I don't know if it was test drivers, engineers or Corporate brass that had the problem. No ring on the A4 because you can't overrev an unmodified one.

I don't know if that ring is still there. Perhaps newer Vette shafts are stiffer and don't whip as much.

Jon
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Old 09-03-2009, 04:37 PM   #8
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Every car I have ever worked on that was over rev'ed the valves kissed the tops of the pistons. I replaced all the valves and good as new.

I personally have over rev'ed more than a few cars. The cars I have done this too with no frills engines did not drag the tires and the engine rpm shot up very fast. I was able to catch it and stab the clutch before any thing was damaged.

The more high performance cars that I have done this drug the tires because of lack of traction and the rpm still shot up but much slower. My guess is because more compression??? Maybe a high compression engine has more resistance to being over rev'ed. Maybe it just had to do with traction.

As a guy I used to work with said if you have never missed a shift or over rev'ed your car than you are not shifting fast enough. I don't know that I agree with that but some people are more willing to gamble with good parts than others.
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Old 09-03-2009, 06:05 PM   #9
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I had an Oldsmobile 350 that was regularly revved to 8k+rpm. It eventually stressed the rod bolts enough to let both halves of a bearing stack on top of each other, resulting in that piston kissing the head. It also scored the crank, of course.
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Old 09-03-2009, 08:09 PM   #10
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A over rev has nothing to do with motor TQ one way or the other really.
Has do with the stress of going over the Redline floating the valves lose. LACK OF COMPRRESION in cylinder. Rods breaking bolts get over worked bearing spin. piston hit valves and break ****.

As for the Sprint car engine that spin around its the 8k to the stop of the piston to change directions in a milsec that rips the wrist pin out.
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Old 09-03-2009, 09:34 PM   #11
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Thanks for all the excellent input fellas. Looks like the general consensus is RPM does the damage, not the hard hit of torque. Appreciate all the insight.
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Old 09-06-2009, 09:21 PM   #12
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y bother with the state of the conn rods when the pistons have been back reamed by an exhaust valve.....
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Old 10-02-2009, 02:56 PM   #13
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so if its only rpm related, then damge due to over-revving whether its due to missing a shift or just normally revving the motor past its limiits (if possible) are the same, right?
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Old 10-02-2009, 04:07 PM   #14
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Quote:
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so if its only rpm related, then damge due to over-revving whether its due to missing a shift or just normally revving the motor past its limiits (if possible) are the same, right?
Most likely you could overrev much higher by a bad downshift say 5>2 (instead of 5>4) at high revs in 5th than you could by just buzzing it with no load. If your valvetrain (or better yet engine computer) gave up @ 7100 and limited the revs, the 8500 or so spurt from the 5>2 shift would be tons worse. That's "mechanical overrev".

The engine parts really don't know or care how they got to those revs. The g-loads do the damage.

Homework problem:

You want to test your flywheel/clutch to see if it will blow up if your engine overrevs to 10000. Do you need a 10000 rpm-capable engine to test it? If not, what's a fairly cheap way to do it? No, not 5>2 downshifting.

Building the test cell to absorb the flying bits when the flywheel blows is the expensive part. Railroad ties lining the entire inside of a poured concrete room works fairly well.

Jon
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Old 10-03-2009, 12:58 AM   #15
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Quote:
Originally Posted by Old SStroker View Post
Homework problem:

You want to test your flywheel/clutch to see if it will blow up if your engine overrevs to 10000. Do you need a 10000 rpm-capable engine to test it? If not, what's a fairly cheap way to do it? No, not 5>2 downshifting.

Building the test cell to absorb the flying bits when the flywheel blows is the expensive part. Railroad ties lining the entire inside of a poured concrete room works fairly well.

Jon
I tested mine shifting from 2nd to 3rd. The clutch or flywheel (does it reall matter when it happens to you) exploded. It took out the tranny, bell housing, clutch, flywheel, the block, heater box, firewall, floorboard, framerail, Ac condenser, driveshaft, gas pedal, hood, fenderwell on the right side, right front tire, there were holes in the floor, carpet and dashboard. It missed my leg and foot by millimeters. I learned a valuable lesson though.
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Old 10-03-2009, 09:18 AM   #16
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Quote:
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I tested mine shifting from 2nd to 3rd. The clutch or flywheel (does it reall matter when it happens to you) exploded. It took out the tranny, bell housing, clutch, flywheel, the block, heater box, firewall, floorboard, framerail, Ac condenser, driveshaft, gas pedal, hood, fenderwell on the right side, right front tire, there were holes in the floor, carpet and dashboard. It missed my leg and foot by millimeters. I learned a valuable lesson though.
Skip shift from 2nd to 4th?

Sorry to hear you popped the flywheel. Glad to hear it missed you.

OEM flywheels are tested WELL above any rpm the engine can ever reach, but they probably are not abused with max launches with sticky tires a lot of times before they are spin tested. The metal remembers every load and every temperature change it ever encounters. If you toasted the clutch a few times, the 2-3 shift was't the root cause, it was just the straw that finally did in the dromedary.

FWIW, you battle damage sounds like it was a traverse engine FWD.


Jon
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Old 10-03-2009, 11:39 AM   #17
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Originally Posted by Old SStroker View Post
Skip shift from 2nd to 4th?

Sorry to hear you popped the flywheel. Glad to hear it missed you.

OEM flywheels are tested WELL above any rpm the engine can ever reach, but they probably are not abused with max launches with sticky tires a lot of times before they are spin tested. The metal remembers every load and every temperature change it ever encounters. If you toasted the clutch a few times, the 2-3 shift was't the root cause, it was just the straw that finally did in the dromedary.

FWIW, you battle damage sounds like it was a traverse engine FWD.


Jon
No it was a 77 Camaro but parts and pieces went everywhere. It let go at 7K. The lesson was that I shouldn't of cut corners on safety items. The shift didn't cause it, like you said. It was hard launches and the other abuses the clutch and flywheel endure. The car looked like a bomb went off under the hood. I'm pretty glad it missed me too.

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Old 10-03-2009, 12:00 PM   #18
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holy hell mang^ you have pics of that damage? good to hear it missed you!
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Old 10-04-2009, 09:53 AM   #19
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When my LS6 was stock heads and valvetrain, I had about 8 or so over-rev conditions. 7 of them were partials (aka saves) and the last one was the doozy with a 3@ 7000rpm @3.73 gearing into a 2nd gear. (I hate the shifting mechanism of T56's for this very reason)

Pistons were perfectly fine. The springs were gone though; lost about 10rwhp valves were fine though. If you over-excite the spring, you'll wear it out to where its too fatigued to maintain the same pressure.

Also, you'll need new tires, I had flatspotted all 4.
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Old 10-04-2009, 03:23 PM   #20
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Quote:
Originally Posted by Schwanke Engines View Post
Well, I don't know about street cars, but when you turn a sprint car around and you don't hold the tire with the brakes, you have a tendancy to pull the pins out of the bottom of the pistons. This is due largely to the fact not of the over rev, but the sudden accelleration from the engine going 8000 RPM forward to 7000 RPM backwards!!!!
Does your engine spin backwards when going in reverse as well?
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Old 10-04-2009, 03:23 PM
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