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04-28-2011, 09:30 PM | #1 |
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Possible 6MT mechanical overrev mistake!!
Been driving MT for over 10 years now, but might have pulled a potentially serious fail out of clear oversight.
So I'll cut right to the chase here. I was traveling approximately 70 MPH (no more than 75 but can't be 100%) downhill on the freeway as I'm getting ready to exit (was in 3rd engine braking down from high revs). The ramp is goes off the right and up hill. It is nicely banked to the right - so it's one of my favorites to hold tight in. Anyway, I entered the ramp at more or less the speed stated above and as i begin to go up the hill I downshifted into second. It was a good downshift, but I was not cognizant of my speed. I would have slowed down more before shifting if I knew it was that high. It had to have still been around 70 to 75 but i'm hoping it was less. I sort of failed to measure the speed at which the hill was slowing me down and assumed I was going slow enough before i shifted (as is typically always the case - third gear and up hill!). Car revved all the way up to into yellow - not sure exactly where - might have been just in or not even. I was terrified and couldn't tell. It kept me from enjoying what might have been the most amazing sound and behavior I have noticed from this engine to date. the needle really slowed as it approached the red and nothing came on the dash or iDrive. My question is - did I dodge a bullet or could the car still have overreved without the tach indicating the same? Presumably, if I mechanically overreved the car beyond 8400, the needle would have been pretty far into the red right?? Alternatively, does the rev indicated on the tach match what is actually occurring in the engine in the case of a mechanical overrev? I hope I'm just being paranoid but I found this, which doesn't help my concern. I'm hoping the omnicient Bimmerpost members can let me know instead of having to ask the dealer... Thanks!
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04-28-2011, 09:43 PM | #2 | |
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I am about to go turn it on again and see if anything seems unusual. After this event occurred, I still reached upwards of 7000 on my way home (but only during acceleration). I probably should have taken it easy - but i really was in denial at that point. I kept telling myself "there's no way you got that high, it would have showed lights and warning all over." It wasn't until i got home that i started to freak out again and write my OP.
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04-28-2011, 10:06 PM | #3 | |
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04-28-2011, 10:07 PM | #4 |
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That's the money shift. The rev limiter doesn't protect you when you redline it on a down shift. Impossible to tell how high it was for how long without a digital readout. (I believe the dealer can scan you for overrevs). Hopefully not an issue. I think I saw the 2nd gear redline is about 70, but I'm not positive. If you hit 9k, it probably isn't the end of the world. I'm not sure when this motor goes into valve float (chance that valves hit the pistons) - that is the death result of the money shift. This is typically a 5th to 2nd instead of 4th shift. At this point, no reason not to just drive it, not much you can do about it. Ultimately have the dealer scan it next time you bring it in.
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04-28-2011, 10:16 PM | #5 |
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Hmm. I'm still pretty worried now.
I guess the key question is this: If it never entered the Red (which I am sure of), have I avoided harming my engine?
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04-28-2011, 10:30 PM | #6 |
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your fine. with an 8200 rpm redline, you really have to do something stupid to money shift and do damage. This car likes to be in the yellow, you should drive it there more often
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04-28-2011, 10:33 PM | #7 | |
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04-28-2011, 10:35 PM | #8 |
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Even if it was 75mph you were only about 4% over the maximum 2nd gear speed of 72 mph (http://www.m3post.com/forums/showthread.php?t=292787). So that would be about 8700 RPM. It is probably fine. And if you are positive it was not even in the red then you probably were only going 70 mph and are definitely totally fine.
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04-28-2011, 10:42 PM | #9 | ||
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What about the variable tach in this situation. Would it have immediately dropped the red to 6000 or 6500 RPMs? I have seen that happen when I hit the rev limiter on acceleration. Anyone else seen that before? I've had it happen about 3 or 4 times in my 5 months with the car.
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04-28-2011, 10:51 PM | #10 | |||
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04-28-2011, 11:04 PM | #11 | |
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As to the tach, I'm getting worried now about it varying quickly like that. Need 6MT guys to speak up to confirm, but I think that's normal. I think it only happens when you hit the rev limiter so that you can give the engine a second to compose itself before you beat it up some more. It rises back up to normal within seconds (1-4 seconds or so). I would think guys who track their car have seen this, but if not, then I better get my car in for service.
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04-28-2011, 11:09 PM | #12 |
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Did you feel the car accelerate when you downshifted? If yes, you were NOT into the red zone. If engine didn't have any power after the downshift (with gas pedal depressed, of course), that's a sign you overrevved the engine. But at 75, you were barely there. Pay attention next time .
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04-28-2011, 11:14 PM | #13 | ||
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I think your car is safe. I'd like to know more about this business about the sliding limiter dropping to 6k tho. That's really strange if that's how it is supposed to work for 6MT cars. |
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04-28-2011, 11:34 PM | #14 | |
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04-28-2011, 11:39 PM | #15 | |
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04-28-2011, 11:54 PM | #16 |
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I'll comment on the tach needle being slow and I do not intend to get into DCT stuff.
In DCT when I'm on 7th for example driving 70mph and I want to go to third quickly. Instead of 4 clicks I put the pedal to the metal and down shift. That takes the car to the third around 6K. I keep my hand ready to quickly upshift cause in 2 seconds the shiftlights become red yet the needle is still in the 7K range. That happens so often and gets worse if you go 5th to 2nd for example while on 55mph. I think the needle is unreliable when it gets to high rpm downshifts. Saying that I think your car is fine. If being in yellow is bad my car would have been gone long ago.
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04-29-2011, 12:43 AM | #17 |
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At 70mph you would have still been in the power band in 3rd gear and had plenty of engine brake,no reason to try to blip and go down to second.Use the brakes to slow down the car,unnecessary stress on the drivetrain is never a good thing.Car should be fine tho,I don't think you over revved the engine.Drive on
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04-29-2011, 12:44 AM | #18 |
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04-29-2011, 12:49 AM | #19 | |
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Anyhow, I'm sure your car's alright.
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04-29-2011, 01:02 AM | #20 |
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After working with many different types of cars for many years, this what i have learned. Now this is not a fact for all engines but something i have learned from old school techs.
When a car maker makes an engine and sets the red line at lets say at 6,000rpm, the engine can actually go safely to 6,500rpm with out being damaged. In other words, the car maker tests the engine to its full limit on the dyno, then they subtract 10% or so and set the red line. The % or rpm they subtract varies on the engine design. Last edited by KGB7; 04-29-2011 at 01:08 AM.. |
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04-29-2011, 01:07 AM | #21 | |
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I would just observe the car over the next few days and see if there are any noises, leaks, etc. Once the data is gathered, you can go and take appropriate action.
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04-29-2011, 01:10 AM | #22 |
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sudden lowering of redline with variable tach
That's interesting. I can't believe no one so far has seen that. I may have to start a whole nother thread on that to get more peoples eyes on this question and see what I can come up with. It's bizarre and has rarely happened, but the fact that no one else has seen it is concerning. I guess I'm not too worried, as the car has driven just fine after each time it happened. But what does it mean??
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