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z06fun said:
Seems like those numbers are all much too close together to all have happened from downshifting. Maybe the source of the discprepency with the fuel cuttoff limit will be more clear after more people start getting this type of "later in life" ECU printouts
THat was my thought when I saw the numbers. Having spent a lot of time looking at race car data, it looks like the normal variation from a cut-off style rev limiter. It appears that this IS the rev limit of this car.

I am getting my 1000K service done on Monday. I'll see what the numbers are in comparison.

I can tell you for a fact that Mazda Miatas from the 90s don't have the same limit from ECU to ECU. We hunt out the higher revving ones for the race cars...

Ara
 

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I have a Power FC and FC Commander in my Spyder (1ZZ, not 2ZZ) and have the rev limit set to 7300. I've seen peak RPM (it is saved) of over 7500 and it wasn't from downshifting... it was when I was intentionally "bouncing off the rev limit" to avoid upshifting (at an autocross). The rev limit works by cutting the fuel... how else would it possibly work? So there can be some overshoot for a brief period. It takes time for the computer to receive and process RPM input and more time to cut fuel. Sorry if this is a repeat of what others have said... I didn't read them all carefully.
 

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ConeFusion said:
Acceleration stops immedatately once you stop applying power. A body in motion maintains its speed (not acceleration) if no external forces are applied to it.
This is not true. A body at constant velocity will maintain its speed without external force (and no friction). An accelerating body will lose its rate of acceleration without external force, but that does not mean it will not accelerate at all. Thus it is possible to have the engine cut fuel or spark yet climb above the RPM at which the cutoff is done.

All this would likely be considered in the design of the rev limiter. There are also tolerances on all systems.
 

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Vantage said:
Shay,
If you mechanically force the engine revs higher than what is prudent (e.g. downshift), a limiter cannot react quick enough to stop it from happening in most cases.
Limiters do not affect overrevs by downshifting. E36 M3's are also known for blown engines due to mis shifts while downshifting.

The rev limiter cuts fuel, spark or both. Under a downshift the rear wheels drive the system and the revs will match the tire road speed (or lock up).
 

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ex-M3 said:
An accelerating body will lose its rate of acceleration without external force, but that does not mean it will not accelerate at all.
The main thing to remember here is

F = m a

(force = mass x acceleration)

or alternatively,

a = F / m.

when F = 0 (as in when the rev limiter kicks in), a = 0. Assuming no drag, otherwise we just go to negative accerleration, aka deceleration. So not only will it loose its "rate of acceleration" (by this do you mean the third derivative of position?) but it will not accelerate or decelerate in the absense of forces.
 

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ex-M3 said:
An accelerating body will lose its rate of acceleration without external force, but that does not mean it will not accelerate at all.
As Evl already explained, that's wrong. You might be confusing acceleration with velocity. No force, no acceleration. Just think of it from an energy point of view. A body that gains speed (accelerates) gains kinetic energy. The energy of a system can only increase if external energy is added. As soon as you stop adding energy (applying a force), the energy remains constant, which (neglecting energy loss through friction, etc.) means the velocity remains constant, therefore the acceleration (which is the derivative of the velocity) is zero.
 

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Evl,

The term you are looking for is "jerk": the derivative of acceleration. This neat term defines the very thing that makes engines like the one in our cars fun. Increasing rates of acceleration feel "good".

The rest of you: do we have ANY data that suggests that this is NOT the actual rev limit of the cars? We may be scratching our collective heads for nothing. Does anyone have a rev limiter printout from known bounces against the limiter that are lower?

Ara
 

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I hope you're joking, Dan!
Surferjer,
Or course I was kidding!
:)


----


I thought an object in motion tends to stay in motion? With this in mind, I see the revs going a little higher, resulting in your numbers.
 

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Ara said:
Evl,

The term you are looking for is "jerk": the derivative of acceleration.
And I am one when I reread my earlier post :rolleyes: Landed from Tokyo at 5 pm today so I am working a good case of jet lag.

You are correct about jerk and acceleration. All I was trying to say is when you cut power the engine will overshoot the revs at the time power was cut because of inertia. The rate of acceleration will quickly decrease to zero. My post did say it will continue to accelerate which is not right, but it will continue to gain a few revs.

Maybe I should go to sleep, but that is impossible too because my body says it is 2 pm.
 

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Right about the rate of acceleration decreasing quicky. Let's say the fuel and spark are cut instantly at X RPM. Some rotating parts such as flywheel, pistons, still have kinetic energy that exceeds the amount they would need to maintain a constant forward speed, assuming you hit X RPM on a rapid upswing. The amount of overshoot would depend on the excess kinetic energy then, which the ECU could compute or look up in a table and factor in so as to have a more precise limit, but it is probably doing something very simple like cutting fuel and/or spark at a fixed RPM (and allowing it again at a somewhat lower RPM... that's what gives it the "bounce"... ba ba ba ba). The max RPM with worst case overshoot isn't going to be all that more dangerous than that with no overshoot. Lazy programmer says, "just cut it off at X" and not worry about it because it will never exceed X+Y when accelerating.
 

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Ok, back on topic. ;) I did some searching through old topics, and came across 8500rpm for the rev limit several times. This would be for short periods of time. The sustained maximum seems to be lower, numbers around 8300rpm came up a few times.

So the numbers that Jer is seeing are definitely higher than anything I would have expected from previous information. :confused:
 

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FWIW, BMW denied a warranty claim for a blown engine in my M-coupe with a single over-rev of 296 rpm (7296 vs 7000). Interesting, since a Dinan chip in the same engine (s52) brings the rev-limiter to 7500 -- so mechanically, that over-rev should have been a non-issue. We'll see what Lotus does with these, but given the overall climate, I would expect the warranty to be void on the drivetrain in this case. Something to think about when tracking cars.

In my case, the blown engine may well have been due to an over-rev from downshifting, though the 'black box' (ECU) does not tell you WHEN the over-rev occurred. Theoretically, it could have been before I bought the car, since I picked it up used. I would advise anyone buying a used car these days to have the dealer run the ECU, to make sure you are starting fresh on any remaining warranty.

Another question for you mechanical guys/gals -- I can see how a downshift can force an over-rev, but can you defeat the rev-limiter in an unloaded state? Meaning, if you rev the car in neutral, is it possible that the engine spins up so fast that the measured revs surpass the electronic limit?
 

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Oh, and the ECU also had my 'top-registered speed.'

I track my cars, so it was high. But it's interesting to consider that this type of information can and IS being used in accident investigation now. It would be easy for a lawyer to paint a driver as reckless and irresponsible to a jury using these bits of info. I think it is only a matter of time before you'll be downloading this info to re-register your car.

So, another reason to have a track-only car, and get your ya yas out legally. That's where I am heading.

Big brother is always watching now...
 

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My car spent hours at WOT yesterday on Road Atlanta. This info should be interesting. By the way, what was the top speed?
 

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135 for the M.

I've been to 145/150 in my Porsches on same tracks (Willow Springs in CA). The M just didn't inspire the same confidence for me, but then again, I was running a stock set-up with pretty average street tires on the BMW

Recently replaced the M with a '74 911 with some nice suspension work and a 3.0 engine -- really looking forward to getting that one out. About 200 hp and 2400 lbs. Hoping for 320 hp and 2200 lbs when I'm done modifying. Going for an RSR-replica thing:cool:
 

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It would be interesting to get some data from other people's ECUs, but if we assume there is a constant time lag between the engine hitting the rev limit's rpm and the ECU stopping acceleration, then the number of revs that the engine gains in that time should be linear in the gear ratio. This is because the torque at the wheel is multipled by the ratio, so the amount of acceleration that the car does will be determined by that.

Doing a least squares fit of the data, I get that in "inifinte gear" (ie ratio 0), where you have no power to the wheels, you get to 8680rpm, which is our measured fuel cut-off rpm. According to the "gear chart" thread, the transient rev limit is supposed to be 8600, so we aren't too far off of that.

BTW, congrats on getting up to 155mph! :)
 

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grudkin said:
Recently replaced the M with a '74 911 with some nice suspension work and a 3.0 engine -- really looking forward to getting that one out. About 200 hp and 2400 lbs. Hoping for 320 hp and 2200 lbs when I'm done modifying. Going for an RSR-replica thing:cool:
FYI HRM has this Carrera RS replica. I saw it when I picked up my car and it looked very nice.
 

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When all else fails, RTFM :)

Quote from pg 51:
"The engine management system limits the maximum continouous engine speed to 8000 rpm once normal running termperature has been reached. Very short bursts up to 8500 rpm are allowed during maximum acceleration through the lower gears.

A 6000 rmp limit is imposed on a cold engine in order to reduce possible damage from a harsh driving style."
 

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eliseowner2b said:
ExM3

Sleep deprivation can do wonderful things! Take advantage!!!
Yea and I plan on catching up on Monday afternoon at work. Around 2 pm is the witching hour when the jet lag really takes over.
 
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