Over Rev???
05-20-2015, 01:46 PM
#1
Instructor
Thread Starter
Over Rev???
So I was thinking, which typically gets me into a lot of trouble, what exactly is an over rev? Or better what causes one?
Is it simply if you miss a gear a the engine suddenly "over revs"?
Can you cause one if you accelerate in one gear to say 5000 RPM before shifting to the next gear?
If Porsche considers a "bad" over rev at 5000 and 6000 the why does the tach redline at over 6000 RPM?
May be stupid questions but can't find the answers anywhere and I'd like to know for my own education to help avoid causing one. I'm kinda paranoid about driving the car, making a mistake, and having a black mark on the record.
JC
Is it simply if you miss a gear a the engine suddenly "over revs"?
Can you cause one if you accelerate in one gear to say 5000 RPM before shifting to the next gear?
If Porsche considers a "bad" over rev at 5000 and 6000 the why does the tach redline at over 6000 RPM?
May be stupid questions but can't find the answers anywhere and I'd like to know for my own education to help avoid causing one. I'm kinda paranoid about driving the car, making a mistake, and having a black mark on the record.
JC
05-20-2015, 01:51 PM
#2
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Quick summary:
There are different ranges, and different people have different opinions on them..
Most (including Porsche) agree that range 1 and 2 (as defined in the DME) are fine. They are normal approaches and bounces off of red line.
Anything higher is mechanical, or a "mis-shift" that the computer can't protect. For example, shifting from 3rd to 2nd accidentally, instead of 4th.
Depending on which range you visit (goes up to 6), this can obviously cause unnecessary stress on the engine.
There are different ranges, and different people have different opinions on them..
Most (including Porsche) agree that range 1 and 2 (as defined in the DME) are fine. They are normal approaches and bounces off of red line.
Anything higher is mechanical, or a "mis-shift" that the computer can't protect. For example, shifting from 3rd to 2nd accidentally, instead of 4th.
Depending on which range you visit (goes up to 6), this can obviously cause unnecessary stress on the engine.
05-20-2015, 01:52 PM
#3
Rennlist Member
Can't believe you couldn't find the answers anywhere - there has been as much written about this topic as there has been on oil choices. Try your search again...or even try Google "porsche dme over rev".
05-20-2015, 02:09 PM
#4
Instructor
Thread Starter
JC
05-20-2015, 02:13 PM
#5
Actually if you read the threads, that is covered, as well as what each range means and a host of other information on the topic to exhaustion.
Search on here and 6speed and you will find all that info and more.
I know, because I have posted a great deal of it myself....
DC
Search on here and 6speed and you will find all that info and more.
I know, because I have posted a great deal of it myself....
DC
05-20-2015, 02:23 PM
#6
Instructor
Thread Starter
Very sorry to have caused an issue. I've searched and read but did not find the answer to my specific question, must have missed it, I will search again....
JC
JC
05-20-2015, 04:11 PM
#7
Rennlist Member
You create an over rev when you exceed (go "over", hence the term) the rpm limit for a designated rpm range - the rpm ranges differ by engine but let's say range 1 is 7000 to 7400 rpm. If you exceed 7000 rpm you will have registered an over rev ignition. In ranges 1 and 2 this can happen if the engine rpm limiter kicks in. The higher ranges can be exceeded if you miss a shift, say going from 4th to 3rd instead of from 4th to 5th. So in answer to you basic question, yes you can create an over rev by delaying an upshift and bouncing off the rev limiter.
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05-22-2015, 10:20 AM
#8
Instructor
Thread Starter
Thank you very much for the answer. I tried but couldn't find the answer to holding on to a gear too Long.
The car moves so fast I actually did just that the other day and hit just over 4000 rpm before shifting.
Anyway thanks again and sorry if I was annoying seriously couldn't find that answer.
JC
The car moves so fast I actually did just that the other day and hit just over 4000 rpm before shifting.
Anyway thanks again and sorry if I was annoying seriously couldn't find that answer.
JC
05-22-2015, 10:48 AM
#9
Instructor
Here you can find a lott: https://rennlist.com/forums/997-turb...s-and-cpo.html
Also here some thing out the topic:
I hardly ever fully accelerate to rev limits in first gear. Its over so soon that when the car shoots forward, I directly shift into second gear. I think the revs are around 5000-5500.
In second gear its common for me to hit the rev limiter. Reason is that I just can't notice when the engine reaches top revs. In my Alfa's its was very easy to predict the revs by listening to the engine. The engine sound of the TT is like a loud washing machine with no sound changes in higher revs.
From third gear on I'm able to watch my rev counter and shift before the limiter.
The revs overshoot might happen sooner in first gear than higher gears because of the mentioned inertia of the flywheel. Which accelerates very fast toward rev limits in first gear.
Although the laws of physics tell us that when the rev limiter cuts in there is no more energy added to the flywheel and acceleration of the flywheel should stop, it might be that the whole drivetrain is winded up so much that it reacts like a flexible torque tube and gives the flywheel an extra swing in the higher revs over the limiter.
In addition to the inertia of the flywheel and energy release of the drive train there is an other factor I completely overlooked.. turbo pressure.
There is a boost of 1 bar (0,1 MPa) over ambient air pressure. When the rev limiter cuts in, ignition is cut of and the pressure release valve should open. I can imagine there is a time gap between ignition cut of and pressure valve opening. The compressed air in the system after the (turbo) compressor works like an air engine and drives the revs up for a little while till the release valves open.
Don't know how the intake air in the 997 TT is regulated. If this is a throttle body, then what does this do when the rev limiter cuts in. Does it close?
Also here some thing out the topic:
I hardly ever fully accelerate to rev limits in first gear. Its over so soon that when the car shoots forward, I directly shift into second gear. I think the revs are around 5000-5500.
In second gear its common for me to hit the rev limiter. Reason is that I just can't notice when the engine reaches top revs. In my Alfa's its was very easy to predict the revs by listening to the engine. The engine sound of the TT is like a loud washing machine with no sound changes in higher revs.
From third gear on I'm able to watch my rev counter and shift before the limiter.
The revs overshoot might happen sooner in first gear than higher gears because of the mentioned inertia of the flywheel. Which accelerates very fast toward rev limits in first gear.
Although the laws of physics tell us that when the rev limiter cuts in there is no more energy added to the flywheel and acceleration of the flywheel should stop, it might be that the whole drivetrain is winded up so much that it reacts like a flexible torque tube and gives the flywheel an extra swing in the higher revs over the limiter.
In addition to the inertia of the flywheel and energy release of the drive train there is an other factor I completely overlooked.. turbo pressure.
There is a boost of 1 bar (0,1 MPa) over ambient air pressure. When the rev limiter cuts in, ignition is cut of and the pressure release valve should open. I can imagine there is a time gap between ignition cut of and pressure valve opening. The compressed air in the system after the (turbo) compressor works like an air engine and drives the revs up for a little while till the release valves open.
Don't know how the intake air in the 997 TT is regulated. If this is a throttle body, then what does this do when the rev limiter cuts in. Does it close?
05-22-2015, 11:05 AM
#10
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Here you can find a lott: https://rennlist.com/forums/997-turb...s-and-cpo.html
Also here some thing out the topic:
I hardly ever fully accelerate to rev limits in first gear. Its over so soon that when the car shoots forward, I directly shift into second gear. I think the revs are around 5000-5500.
In second gear its common for me to hit the rev limiter. Reason is that I just can't notice when the engine reaches top revs. In my Alfa's its was very easy to predict the revs by listening to the engine. The engine sound of the TT is like a loud washing machine with no sound changes in higher revs.
From third gear on I'm able to watch my rev counter and shift before the limiter.
The revs overshoot might happen sooner in first gear than higher gears because of the mentioned inertia of the flywheel. Which accelerates very fast toward rev limits in first gear.
Although the laws of physics tell us that when the rev limiter cuts in there is no more energy added to the flywheel and acceleration of the flywheel should stop, it might be that the whole drivetrain is winded up so much that it reacts like a flexible torque tube and gives the flywheel an extra swing in the higher revs over the limiter.
In addition to the inertia of the flywheel and energy release of the drive train there is an other factor I completely overlooked.. turbo pressure.
There is a boost of 1 bar (0,1 MPa) over ambient air pressure. When the rev limiter cuts in, ignition is cut of and the pressure release valve should open. I can imagine there is a time gap between ignition cut of and pressure valve opening. The compressed air in the system after the (turbo) compressor works like an air engine and drives the revs up for a little while till the release valves open.
Don't know how the intake air in the 997 TT is regulated. If this is a throttle body, then what does this do when the rev limiter cuts in. Does it close?
Also here some thing out the topic:
I hardly ever fully accelerate to rev limits in first gear. Its over so soon that when the car shoots forward, I directly shift into second gear. I think the revs are around 5000-5500.
In second gear its common for me to hit the rev limiter. Reason is that I just can't notice when the engine reaches top revs. In my Alfa's its was very easy to predict the revs by listening to the engine. The engine sound of the TT is like a loud washing machine with no sound changes in higher revs.
From third gear on I'm able to watch my rev counter and shift before the limiter.
The revs overshoot might happen sooner in first gear than higher gears because of the mentioned inertia of the flywheel. Which accelerates very fast toward rev limits in first gear.
Although the laws of physics tell us that when the rev limiter cuts in there is no more energy added to the flywheel and acceleration of the flywheel should stop, it might be that the whole drivetrain is winded up so much that it reacts like a flexible torque tube and gives the flywheel an extra swing in the higher revs over the limiter.
In addition to the inertia of the flywheel and energy release of the drive train there is an other factor I completely overlooked.. turbo pressure.
There is a boost of 1 bar (0,1 MPa) over ambient air pressure. When the rev limiter cuts in, ignition is cut of and the pressure release valve should open. I can imagine there is a time gap between ignition cut of and pressure valve opening. The compressed air in the system after the (turbo) compressor works like an air engine and drives the revs up for a little while till the release valves open.
Don't know how the intake air in the 997 TT is regulated. If this is a throttle body, then what does this do when the rev limiter cuts in. Does it close?
05-23-2015, 05:29 AM
#12
Instructor
But, it stays unclear why some of you guys come up with overrevs counts while they claim they didn't do a wrong gearchange.
Theoretically it shouldn't be possible to overrev in 1-st, 2-nd or whatever gear when you put the pedal to the metal.
The ignition system cut off and stops the energy input in the engine. With this further acceleration (and increase in rpm's) is stopped.
- Energy input = acceleration
- No energy input = no acceleration (and in a world without losses a constant speed)
Because of the drivetrain losses and wind drag the revs should drop immediately.
But it doesn't.
Like mentioned before the answer COULD be:
- the energy stored in the drive train which reacts like a flexible torque tube and swings the engine up
- the overboost in the (turbo) compressor reaction like a air engine which drives the engine in overrevs. This overboost is released by the turbo overboost pressure valve. And could (also) theoretically shut towards the engine of by the valve in the throttle body.
But my knowledge for Porsche cars is limited due to the short time I owe one and the limited work I have done with it. I don't know how the throttle body of a Turbo works , if there is a throttle body and if its designed tho withstand compressor overboost.
05-24-2015, 01:46 AM
#13
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I would expect it just to be inertia. If you hold it steady at (for example) 7k rpm then stop the fuel/ignition then it can't exceed 7k rpm; revs must fall.
But, if you're accelerating then I would expect the revs to exceed 7k even with no fuel/ignition due to momentum. By what amount they exceed 7k depends on the rate of acceleration and the amount of 'braking' due to friction/pumping in the engine.
But, if you're accelerating then I would expect the revs to exceed 7k even with no fuel/ignition due to momentum. By what amount they exceed 7k depends on the rate of acceleration and the amount of 'braking' due to friction/pumping in the engine.
05-26-2015, 12:51 AM
#14
Race Director
But, it stays unclear why some of you guys come up with overrevs counts while they claim they didn't do a wrong gearchange.
Theoretically it shouldn't be possible to overrev in 1-st, 2-nd or whatever gear when you put the pedal to the metal.
The ignition system cut off and stops the energy input in the engine. With this further acceleration (and increase in rpm's) is stopped.
- Energy input = acceleration
- No energy input = no acceleration (and in a world without losses a constant speed)
Because of the drivetrain losses and wind drag the revs should drop immediately.
But it doesn't.
Like mentioned before the answer COULD be:
- the energy stored in the drive train which reacts like a flexible torque tube and swings the engine up
- the overboost in the (turbo) compressor reaction like a air engine which drives the engine in overrevs. This overboost is released by the turbo overboost pressure valve. And could (also) theoretically shut towards the engine of by the valve in the throttle body.
But my knowledge for Porsche cars is limited due to the short time I owe one and the limited work I have done with it. I don't know how the throttle body of a Turbo works , if there is a throttle body and if its designed tho withstand compressor overboost.
Theoretically it shouldn't be possible to overrev in 1-st, 2-nd or whatever gear when you put the pedal to the metal.
The ignition system cut off and stops the energy input in the engine. With this further acceleration (and increase in rpm's) is stopped.
- Energy input = acceleration
- No energy input = no acceleration (and in a world without losses a constant speed)
Because of the drivetrain losses and wind drag the revs should drop immediately.
But it doesn't.
Like mentioned before the answer COULD be:
- the energy stored in the drive train which reacts like a flexible torque tube and swings the engine up
- the overboost in the (turbo) compressor reaction like a air engine which drives the engine in overrevs. This overboost is released by the turbo overboost pressure valve. And could (also) theoretically shut towards the engine of by the valve in the throttle body.
But my knowledge for Porsche cars is limited due to the short time I owe one and the limited work I have done with it. I don't know how the throttle body of a Turbo works , if there is a throttle body and if its designed tho withstand compressor overboost.
Called being in a state of denial. Any over rev beyond what is controllable by the rev limiter is a wrong gear change, with possibly an exception.
The exception is the car is redlined down a downgrade and the engine experiences overspeed condition due to the car being affected by gravity.
There is no inertia. All the moving parts of the engine represent drag/friction/resistance to motion. Just try to turn an engine over by hand. This drag doesn't go away at some high RPM but is present all the time. And with the transmission in gear there is the drivetrain's friction too. It exacts between 10% to 15% loss from the engine's output and that is always present.
As soon as the throttle is reduced RPMs drop.
If the engine behaved any other way it would be I think underivable. It would be hard to control the throttle to get a smooth take off as this "inertia" would cause the desired RPMs to be exceeded and by varying amounts due to engine/oil temperature and accessory loading.
Shifting would be a real experience if one managed to get the car moving.
05-26-2015, 01:56 PM
#15
Instructor
But, it stays unclear why some of you guys come up with overrevs counts while they claim they didn't do a wrong gearchange.
Called being in a state of denial. Any over rev beyond what is controllable by the rev limiter is a wrong gear change, with possibly an exception.
The exception is the car is redlined down a downgrade and the engine experiences overspeed condition due to the car being affected by gravity.
There is no inertia. All the moving parts of the engine represent drag/friction/resistance to motion. Just try to turn an engine over by hand. This drag doesn't go away at some high RPM but is present all the time. And with the transmission in gear there is the drivetrain's friction too. It exacts between 10% to 15% loss from the engine's output and that is always present.
As soon as the throttle is reduced RPMs drop.
If the engine behaved any other way it would be I think underivable. It would be hard to control the throttle to get a smooth take off as this "inertia" would cause the desired RPMs to be exceeded and by varying amounts due to engine/oil temperature and accessory loading.
Shifting would be a real experience if one managed to get the car moving.
Called being in a state of denial. Any over rev beyond what is controllable by the rev limiter is a wrong gear change, with possibly an exception.
The exception is the car is redlined down a downgrade and the engine experiences overspeed condition due to the car being affected by gravity.
There is no inertia. All the moving parts of the engine represent drag/friction/resistance to motion. Just try to turn an engine over by hand. This drag doesn't go away at some high RPM but is present all the time. And with the transmission in gear there is the drivetrain's friction too. It exacts between 10% to 15% loss from the engine's output and that is always present.
As soon as the throttle is reduced RPMs drop.
If the engine behaved any other way it would be I think underivable. It would be hard to control the throttle to get a smooth take off as this "inertia" would cause the desired RPMs to be exceeded and by varying amounts due to engine/oil temperature and accessory loading.
Shifting would be a real experience if one managed to get the car moving.
What interest me is this phrase : "The exception is the car is redlined down a downgrade and the engine experiences overspeed condition due to the car being affected by gravity".
This sounds logic, the same logic I use to point at (turbo) compressed intake air that gives the engine an (delayed) extra swing (like a inverted air compressor = > air engine).
Here the extra energy is not coming from gravity but from the car itself.
