So last night on my way home I managed a serious mechanical over rev for the second time in my life. First time was 17 years ago when I shifted my Improved Touring Fiesta from 3rd to 2nd instead of fourth as I was exiting Big Bend at Waterford Hills at max revs. The result then was a wrecked flywheel, sheared flywheel to crank bolts, damaged crank and main bearings. Kind of what you expect. Last night on my way home I hit upon the holy grail, my favorite long double apex on ramp with a short straight between the two apexes (is that the plural?). Well coming in I’d changed down to third, accelerated through the first apex and was accelerating down the short straight and just before turning in I went for 4th. I fluffed it and it didn’t go in so I grabbed the gear leaver and in my hast pulled towards myself and into second. Weeeeeeeee went the revs. Now, I wasn’t’ on the track so I’m not talking hitting 2nd from Max rpm in 3rd, but it was definitely a serious over rev. The engine felt fine though. I tucked my tail between my legs and hypermiled it home. I then changed the oil in 18F on the driveway. I didn’t notice anything in the filter, but it was borking cold so I didn’t go looking for a magnet. Today the engine feels happy as can be. Thoughts? Am I sitting on a time bomb?
Like my undies...
... Depends.
But chances are, if it feels fine now, it's probably fine.
If it didn't blow up on the spot or start making any odd noises immediately, it's fine.
You're probably fine. Very recently my friend did a money shift. Went from 5th to 2nd(meant to go into 4th) and saw the tach hit 8k rpm in his VW Tiguan. Was able to limp the rest of the way home. I still haven't heard what internals he destroyed, but things. Lots of things. He's a bit hamfisted.
Gearheadotaku wrote:
which car?
The C30. The Lincoln is an auto and the Saab is waiting for a new FPR and some vacuum hoses.
GameboyRMH wrote:
If it didn't blow up on the spot or start making any odd noises immediately, it's fine.
Yeah I agree mostly. Other potentially weakened areas would be:
- Torn Engine mounts
- accessory drive belt can be stretched by extreme acceleration. Same with timing belt. Basically lose a few thousand miles of life out of them.
If you don't hear a knock you should be fine with bearings.
Advan046 wrote:
GameboyRMH wrote:
If it didn't blow up on the spot or start making any odd noises immediately, it's fine.
Yeah I agree mostly. Other potentially weakened areas would be:
- Torn Engine mounts
- accessory drive belt can be stretched by extreme acceleration. Same with timing belt. Basically lose a few thousand miles of life out of them.
If you don't hear a knock you should be fine with bearings.
Good point on the timing belt. The Factory interval is something like 105K miles or 10 years. I'm at 92K miles and 6 1/2 years. Probably something to add to the list come spring time.
There have been a few Miatas expire on track following an overrev. Not immediately, but the same day. The load on the rods goes up exponentially with rpm, so I suspect they get overstressed and fail shortly after. Of course, they're still operating in a high-stress environment, not just driving to the mall.
That said, there's not much you can do and maybe you did get lucky. So carry on.
When it grenades, it will be while gently downshifting for a stop light. Its fatigue life has been shortened, but it will still work until the fatigue life is gone.
Pops has a theory about personal health, that runs in much the same way: Each one of us is given so many heart beats; no point using them all up doing "fitness" or some such nonsense.
My first auto x ever in my E21 I over revved the engine. It wasn't a downshift but I was unfamiliar with the car and at the end of an open slalom section I glanced down and saw the needle at about 7800 and climbing. Redline was 6500 IIRC. It had 275k on it and I never had any issues with it after that.
The major side effect of a "slight" mechanical over-rev that doesn't appear to damage anything is that you get low compression from a wee tweak in a valve when they start to float and snuggle with pistons. This often happens before rods start stretching and bolts start shearing because factory valve springs are soft. If your compression is still good I'd say you might have gotten away with it.
I'd also say if you didn't zing it more than 100 or two RPM over the factory redline there is probably some room to have zero damage. Some ECUs store the max RPM. If you can pull it and it's not more than what tuning chips allow for additional revs you can put your mind at ease. For instance... BMW sets a 6850 for stock soft limiter on S52s and tuners give you all the way to 7250 so if you had some RPMs left in 3rd when you found 2nd .... maaaayyybe you didn't really zing it?
The first time you over-revved an engine, you had to deal with a lot of drivetrain inertia that essentially "drove" your engine to the grenade point. In the most recent mishap, you missed a shift and the engine freewheeled for a second. You're probably good on all counts except maybe the timing belt should be replaced since it's near the end anyway. I agree with GPS about the valve springs being weaker than the crank and rods etc.
Also, go to confession.
Jerry From LA wrote:
Also, go to confession.
This is my confession at the church of holy spanners.
For your penance, do three oil changes on early Miatas. Don't forget to hit the donation box on your way out.
Did you happen to catch what neighbourhood the tach ended up in?
Whiteblocks are pretty well built, especially newer ones. I4 or 5? It's very possible that although you exceeded the programmed red line, you still stayed in a range thats relatively safe.
twopointwo wrote:
Did you happen to catch what neighbourhood the tach ended up in?
Whiteblocks are pretty well built, especially newer ones. I4 or 5? It's very possible that although you exceeded the programmed red line, you still stayed in a range thats relatively safe.
The stock redlines are really low and the rods look way too beefy/pistons look too light for the bottom end to be an issue. Kissing a valve is a possibility but sometimes they fail right away, sometimes you pull the engine apart 50k later for some other reason and notice some of the valve heads sitting crooked.
Rotaries have a "nice" failure mode when overrevving. It's like the five point palm exploding heart technique. Overrev a 3mm seal engine and everything is okay until the first time you take it up to 6200rpm, then it FODs itself. BTDT three times.
Knurled wrote:
Rotaries have a "nice" failure mode when overrevving. It's like the five point palm exploding heart technique. Overrev a 3mm seal engine and everything is okay until the first time you take it up to 6200rpm, then it FODs itself. BTDT three times.
I'm glad you added that. As I was clicking on this thread I thought to myself - "I wonder if you can over-rev a rotary..."
Instantly, the first time you so much as touch redline. Seriously, your windshield will shatter.
rcutclif wrote:
Knurled wrote:
Rotaries have a "nice" failure mode when overrevving. It's like the five point palm exploding heart technique. Overrev a 3mm seal engine and everything is okay until the first time you take it up to 6200rpm, then it FODs itself. BTDT three times.
I'm glad you added that. As I was clicking on this thread I thought to myself - "I wonder if you can over-rev a rotary..."
(Raises hand) Only engine I ever killed was a 13B. It was at an autocross on my last run, and I really nailed the corner coming on to a longish straight and spun it a little too fast trying not to shift into third. Braked for the next corner, got back on the gas and chug-chug-chug, it was done. Toasted apex seal.
My only experience with serious over rev... was a friends 289 Mustang... His result... broke the cam in the middle... front 4 cylinders ran fine... back four were dead
Mike wrote:
Instantly, the first time you so much as touch redline. Seriously, your windshield will shatter.
Hahaha, this also happens in the Stunts series.
I think it really depends on just how high it went. Did you let off the clutch fully before realizing what you did?
When I was younger and dumber, I had gotten into the habit of downshifting the auto in my old Camry to slow down on snow/ice easier. In inclement weather I'd flip the OD switch off for the whole trip and leave it in 2nd in town, then downshift from there to help with the new driver and lack of ABS issues. I was going down a backroad with several inches of snow on it one evening when a deer jumped in the road a 50 yards or so out. I went for the shifter and overshot into 1st, between the E36 M3ty tires slipping on snow and only going 40mph or so to begin with, I doubted it went much past 7000rpm. After that I practiced my threshold braking a bit more and quit relying on engine breaking so much. That spring I pulled away from a stop sign a bit more vigorously than normal on my way to school(maybe half throttle), and the #2 rod bearing spun so bad it seized the ~180k mile well maintained 5S-FE. Like pile of metal on the balance pack bad. That January night is the best explanation I can come up with for the failure.
