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Discussion Starter · #41 ·
I'm as confused as ever. I can guarantee these readings did NOT come from downshifting. I certainly, from time to time (mostly autoxing, but not completely) have bounced off the rev limiter. I hope Ara is correct and the rev limiter on mine is simply set higher, a definite possibility. Stan, if you're out there-you have bounced off the limiter, too, at autoxs, right? I'd love to see your next data dump. Any other autoxers???

I really don't think I've done anything wrong, and in any case I'm having no mechanical problems for warranty consideration currently. Let's hope it stays that way.
 

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Jer...take you car back to service a couple more times so that you can get a 9500 RPM limit!!!

Rev limiters can get tricky...and the logger itself might get weird at times.

I can have my own free standing data logger independently measure the engine RPMs achieved. We'll see what I can come up with. Hopefully I can try this out before the weather turns bad!

Stan
 

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Discussion Starter · #43 ·
I look forward to it. In any case bring on spring! I can't wait to get on track again. I might do a winter SOLO 2 event next Sunday at Maple Grove Raceway here in PA, if I have enough brownie points saved up at home...I promised her the season was over before finding the SCCA chapter (Blue Mountain) yesterday.
 

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I'm not sure if anyone specifically pointed this out, but...

If when accelerating, you hit the rev limiter before shifting, and push in the clutch before letting off the gas. It would seem possible that the significantly lower load might make it really hard for the computer to cut the gas quick enough. It might only take one cycle at full gas and no load to push it up 300 rpm.

Or when accelerating down a really steep hill. :)
 

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To help me understand & to possibly shed some light on the situation...

If I'm in a very fast car (a very fast, heavy car, for the sake of argument), if I'm *really* accelerating & hit the clutch at 45 mph, my speed will not stop dead at 45, will it? Won't my forward momentum carry up the velocity a bit more?

to help me out more: If I were in a rocket, going straight up & accelerating at 3 times that of gravity, once I ran out of fuel, wouldn't I start decelerating at 9.8 m/s^2 (or whatever gravity is - it's been a while since I was in school)? If that's the case, wouldn't my acceleration start decreasing from 3 * 9.8, or, for the first few seconds, wouldn't my speed still be increasing? (because "net acceleration" is still on the positive side?)

So, if all that is at least somewhat accurate, then I would assume you could overun by a few hundred rpm (or a fwe miles per hour), especially in 1st, if you were accelerating pretty quickly when you hit the limiter & then did not quickly hit the clutch.
 

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Grudkin, what kind of failure did you have? If it was a bottom end failure your warranty should not be voided because it is nearly impossibel to cause that by an over rev. Bent valves, scarred pistons, etc are tell tale signs of an over rev. Also there is no doubt in my mind that your engine was capable of an extra 292 rpms. Those same engines in stock form consistantly run to 8000 rpms in many people's cars. I hope you didn't have to come out of pocket from this. As far as another car that bounce off the fuel cut off before the AS service my top 3 high rpms were all in the 8700s and at least two of those occured when hitting the limiter. I would just consider yourself lucky yours kicks in higher. I wish mine went to 9k.
 

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BrianK said:
To help me understand & to possibly shed some light on the situation...

If I'm in a very fast car (a very fast, heavy car, for the sake of argument), if I'm *really* accelerating & hit the clutch at 45 mph, my speed will not stop dead at 45, will it? Won't my forward momentum carry up the velocity a bit more?

to help me out more: If I were in a rocket, going straight up & accelerating at 3 times that of gravity, once I ran out of fuel, wouldn't I start decelerating at 9.8 m/s^2 (or whatever gravity is - it's been a while since I was in school)? If that's the case, wouldn't my acceleration start decreasing from 3 * 9.8, or, for the first few seconds, wouldn't my speed still be increasing? (because "net acceleration" is still on the positive side?)

So, if all that is at least somewhat accurate, then I would assume you could overun by a few hundred rpm (or a fwe miles per hour), especially in 1st, if you were accelerating pretty quickly when you hit the limiter & then did not quickly hit the clutch.
Yes, you speed stops dead at 45mph, and your 3 * 9.8 (m/s^2) would turn into negative 9.8 (m/s^2) (assuming you're still at sea level, otherwise, gravity will be less). The basic check to keep in mind w/ physics (well the stuff we are talking about here anyway) is that energy doesn't come from nowhere. Kinetic energy = 1/2 m v^2. So to increase your speed you need to dump in more energy. Aside from the engine, there is no engergy to be had in your car (unless you have a strong tailwind/downhill, etc).

momentum = m * v, and yes that won't dissapear when you lift the gas, it will just be conserved. You wont get any more of it though until you put more engergy into the system.

PS:
m = mass (kg)
v = velocity (m/s)
(except in the units, then m = meters)
 

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It's really pretty simple. When the force disappears, so does the acceleration. There is no "momentum" that will continue acceleration past the point when the outside force is no longer being applied. There is no such thing as "excess kinetic energy".

What may be giving the appearance of this "momentum" has been identified by a few people here and that is: exactly when is the force interrupted? How long after the RPM limit has been reached does it take the ECU to recognize that and cut the fuel off? How quickly does the fuel cutoff take to completely stop the fuel flow? What effect does the fuel/air mixture in the intake manifold have on when the engine stops producing enough power to cause the car to accelerate? What if you push the clutch in to shift before pulling off the gas, right before the rev-limiter would have kicked in? What RPM is really programmed into the ECU?

I would expect these things to happen almost instantaneously, but there clearly would be some time lag. Enough to account for 200-300 RPM? I don't know.
 

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Grudkin, what kind of failure did you have? If it was a bottom end failure your warranty should not be voided because it is nearly impossibel to cause that by an over rev. Bent valves, scarred pistons, etc are tell tale signs of an over rev. Also there is no doubt in my mind that your engine was capable of an extra 292 rpms. Those same engines in stock form consistantly run to 8000 rpms in many people's cars. I hope you didn't have to come out of pocket from this.
It cost me about 9K out of pocket for a rebuilt motor, installed. Then that motor went within 3K miles, replaced under warranty, but I had to pay labor. Another 3.5K. I've since fire-saled it. I will NEVER buy another BMW. A letter writing campaign was fruitless. I am confident that I could have won a legal challenge, but I don't have the time or inclination to go that route. Americans sue too much, and I won't be a part of it unless maligned.

Back on topic, there was a bent valve -- more details I don't have.

FYI HRM has this Carrera RS replica. I saw it when I picked up my car and it looked very nice.
I know this car. It was built by TRE in North Hollywood (where I take my cars), and later brought to NY and further modified. Sweet ride, and I actually made an offer to the prior owner this summer, but he wanted too much for it. Sweet car at maybe 38K, but not worth more IMO, though I believe the claim that there's over 100K in it. Better to buy 'em built than to build 'em
 

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BrianK said:
If I'm in a very fast car (a very fast, heavy car, for the sake of argument), if I'm *really* accelerating & hit the clutch at 45 mph, my speed will not stop dead at 45, will it? Won't my forward momentum carry up the velocity a bit more?
Nope, not one bit...momentum is the tendency to stay at the speed you are going. There isn't an acceleration equivalent of momentum, which would cause your acceleration to taper off if the force was removed instantly. Momentum is what resists acceleration, and deceleration.

to help me out more: If I were in a rocket, going straight up & accelerating at 3 times that of gravity, once I ran out of fuel, wouldn't I start decelerating at 9.8 m/s^2 (or whatever gravity is - it's been a while since I was in school)? If that's the case, wouldn't my acceleration start decreasing from 3 * 9.8, or, for the first few seconds, wouldn't my speed still be increasing? (because "net acceleration" is still on the positive side?)
As soon as you lost thrust, your acceleration would change from +3g to -1g, instantly. Your speed would peak there, although you would continue to move upward until the negative acceleration ate up all the speed you had built up.

So, if all that is at least somewhat accurate, then I would assume you could overun by a few hundred rpm (or a fwe miles per hour), especially in 1st, if you were accelerating pretty quickly when you hit the limiter & then did not quickly hit the clutch.
The reasons you would continue accelerating past the rev limit were given earlier in the thread, I believe by Evl: When the rev limit is crossed, first the ECU has to notice. That may take a small fraction of a second, because RPM is generally calculated from the time between two pulses on a sensor connected to the crankshaft, or similar means. Then the ECU cuts off either spark or fuel or both, and only then does the engine stop producing power.

Similarly, when you pull your foot off the accelerator, the throttle slams shut but there is still some air in the intake manifold which can still be used to burn fuel. The engine will continue to produce power until that air is expended.

But if you push in the clutch like you describe in your first scenario, the above two issues don't apply. No more power from the engine means no more acceleration.

Don't be confused by the talk about "residual acceleration" or "excess kinetic energy", because as they are used above they are not valid physical concepts.
 

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Where did that "excess kinetic energy" thing come from. I'm not usually that stupid. Make that never. :( I hang my head in shame.
 

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And the ECU is *fast*

Back of the envelope time:

Spark timing wants to be accurate to one degree or less. Power gains and losses are on the table at 2 degrees error, possibly lots of gains. In my bike tuning days, 2 degrees of error could cause your two-stroke to melt.

So the ECU needs to be able to know crank position something like 360 times the redline. Converting to time, we get:

8,000 RPM * 360 = 2,880,000 degrees per second

The time per degree is .3472 micro seconds at that speed.

I don't know how many degrees the ECU averages to get claimed RPM. Crank speed varies between power pulses as energy goes into the system from the pistons (power stroke) and out of it (compression stroke). So the instantaneous RPM jitters way too much.

Averaging over one full rev (I just pulled this out of a hat) is 125 microseconds. THe ECU is CLEARLY on top of things at this rate, because spark and injection timing have to happen far faster than this.

So the cutoff time can be very, very fast. Assume that the rev limiter averages in the RPM of 8 revs, that is still 1 millisecond between onset of overrev and cut out.

If the revs are climbing at 8,000 rev a second (full tach sweep in one second, wich is pretty quick) then using the asumed rev limiter averaging above, the engine gains 8 RPM between first rev above limit and rev limiter smack-down. That is to say, it goes from 8,000 RPM to 8,008 RPM before the rev limiter declares over rev.

Fully digital ECU are *fast*.

If the ECU decides "no fuel, no spark" the engine becomes an air compressor in one millisecond. The ECU might say, "less fuel, retard timing" to be gentle, but the guillitine [sp] option means you feel like you are going through thick mud. I've done it on my motorbike once, and it feels like the machine just died underneath you.
 

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Yeah, I think you make some good points, but I don't think its as fast as you claim.

Typically crank/cam position is determined with a wheel with some number of teeth (~30ish) and a missing tooth. The computer measures the rate of the pulses and interpolates to determine the current position. It doesn't actually sample 360 times per revolution (which is the sampling frequency you'd need to measure 2 degree increments directly).

Keep in mind also that the code that does the rev limit isn't necessarily the same code that does the spark/fuel, so it could be running slower, and based on the measurements we have, we I think have to conclude that the computer isn't checking the limit this often.
 

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And if a cylinder has fuel/air mixture and spark is present when ECU decides it is time to cut? Too late... overrev. In a 4-cylinder 4-stroke engine, one cylinder is always in the power stroke, so cutting fuel/spark will not affect one power stroke. If you're at 8000 RPM or so and the engine is under load, there's going to be power to reach a higher rev.

It will overshoot the rev limit every time when accelerating hard, I say, unless the ECU cuts fuel/spark preemptively (at a lower threshold).

BUT not by a lot. The real culprit has to be something else, like how often the ECU checks for rev limit. Overshoot is common in 1ZZ and 2ZZ with stock Toyota ECUs and replacement ECUs like Power FC.
 

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Since the discussion about acceleration and force seems to be continuing I will add my 2 cents in the hope that it helps solidify the things already said about your acceleration stopping the instant the force goes away.

I find that when I'm teaching physics pictures are far more effective at explaining a subject than math and words are...so here's an attempt at ASCII graphics pictures to explain what is happening. The pictures are going to have arrows representing forces on a car. The arrows will show the direction of the force and the length of the arrow indicates the amount of force in that direction.

============================

Situation I: Foot on the floor accelerating.

Force from engine -------> CAR <-- Force from air drag etc

The two arrows are in opposite directions so they partially cancel...when you add them together you get:

Net force on car: -----> CAR

As has already been said, force causes acceleration in inverse proportion to the mass of the thing being accelerated. Or, in math: F = M*a or a = F/M. In this case the force points to the right so the car is accelerating to the right.

===========================

Situation II: No force from the engine...this could be because clutch peddle is on the floor, you have gone to neutral throttle, spark has been cut, etc.

Now the force diagram looks like

CAR <-- Force from air drag, etc

Now the net force is to the left, so the car is accelerating to the left (most drivers will call this deceleration).

Note that there is no mention of what was happening BEFORE you had no force from the engine. It doesn't matter...the current forces have no memory of the previous forces.

==========================

Of course, the real issue when it comes to rev limiters, as has already been discussed, is how quickly the force generated by the motor can be turned off. If it is turned off slowly, the acceleration of the car will decrease slowly...if it is turned off instantaneously, the acceleration of the car stops instantaneously as well.

Anyway, sorry for being so teachy...I just got done sending out my latest round of applications to open professor positions in university physics departments so I'm kind of in that mode. :)
 

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Just because I'm in the mood, I think it's important that we all remember that when accelerating in the car, all of the force causing the acceleration of the car is located at the tires, "where the rubber meets the road" so to speak, and is due to friction. Hence stickier tires and tires with more contact to the ground are better for cars with enough power to get them moving, and why spinning your tires is bad (at least from a maximum acceleration point of view, since the coefficient of static friction is always greater than the coefficient of kinetic friction). I think I will draw a free body diagram...

Now, in this FBD (Which is not to scale), we have some lovely sources of friction, mainly air friction and rolling friction, represented in yellow. The lovely green color represents the normal forces of the road on the tire in the y direction (which, assuming the car is balanced 50:50, are equal). The nice magnetic blueish color is the weight of the car (mass time acceleration of gravity). And finally, we have the force of road on the tires in the x-direction, which is due to the engine/drivetrain/etc. applying a force on the wheel, which puts a force on the road in the negative x-direction (I have defined, if you haven't noticed, postive x to be in the direction of motion of the car, and negative x to be the opposite) (Also, on that note, engineers are at the center of the universe, because we pick the coordinate system). Now, a FBD of a body in motion is not complete without a kinetic diagram as well, but those are rather boring as they only show the accelerations, which in this case would be an acceleration of the body at the center of mass and a moment due to the fact that the force of the road on the tire is not completely offset by the forces of friction.

I'm sure if I made a mistake in here one of you will catch me, but I would appreciate it if you did it quickly, as my Dynamics II exam is on Friday morning. And if you wanted to change this from acceleration to deceleration, all you have to do is change the magnitude of that lovely force of the road on the tires in the positive x direction, since the rest of the forces are either based on velocity (friction) or weight (normal forces, etc.). You can change it by braking or taking your darn foot off the floor of the car and letting it come down from 8500 rpms and whatnot. Also, this FBD was for a car going in a straight line... a car that is turning is obviously completely different.
 

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Back to the topic at hand...

I just got my 1000 mile service done. My highest rev was 8640. The next one down was in the 8500s.

So, Surferjer, as far as your max revs being higher than mine... 100 RPM variance is not a terribly significant difference (barely over 1%). I suspect you have found the actual cutoff RPM.

Ara
 

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I can confirm that in the Miata, the crankshaft angle sensor is a disc on the front of the crankshaft with about 4 teeth sticking out, which move past a magnetic pickup. So the crankshaft angle has to be determined by interpolation, if 2 degree accuracy is required for the spark timing. That implies that if there are significant variations in the speed of the crankshaft through a single revolution, they must be regular and predictable.

Evl does have a point that the routine in the ECU that knows the crankshaft angle accurately enough to time the spark, may not be talking to the routine that handles the fuel cut on overrev. Speed can be calculated from position, but a calculation takes processing time. The programmers may have chosen to calculate engine speed less frequently (or compare the speed to the rev limit less frequently) in order to conserve processing time for higher-priority calculations. The spark timing might need to be updated as fast as possible to maintain a 2 degree accuracy, but the rev limiter can kick in 100 RPM before the nominal value and not damage anything.
 

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dfa2100 said:
Just because I'm in the mood...
Sounds good in paper ;)

1st the balance is not 50:50, there is more weight in the back of the car.

2nd, spining tires does help you go faster, what you want to acheive is maximum friction without getting the tires loose. This is more aparent on bikes and high power dragsters, if you look at the telemetry you'll see that they are constantly spinning tires and the closer you are to the edge of getting loose the better it's for you. Of course at your own risk. of course you can sping them too much but that's getting the tires loose and you loose all the friction.

3rd, acceleration happens at the rear wheels, but deceleration happens mostly in the front wheels. Just pointing it out.

4th there is a downforce coeficient that increases with speed and increases your contact patch making your tires more efficient.

5th Tires flex, hard to see on our cars, but it happens, not that it matters to your write up but just pointing it out, also tires will get taller and skinier and the contact patch changes depending on the speed and temperature, more contact towards the sides of the tires and less on the center of the tire. On our tires and at the temperatures they operate you can't tell visually but it does happen.

6th the forces of the road on the tire are null, (action reaction) pressing harder with your hand on a table causes the table to press harder against your hand until a breaking point if the table or the hand cant take it. In quantum physics they say that mater doesn't like to compress.

7th, the faster you go, time goes slower and space compresses ... nevermind ;)
 
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