Rev ranges...

[Durzel]

Moog72 said:

Naive question perhaps, but how exactly do these over-revs occur? I always thought that the rev limiter (both soft and hard) are there to prevent these sorts of things from happening. Or is it simply not quick enough to cut in if you accidentally go from say 4th to 1st?

I'm not sure there is a soft and hard rev limit, though willing to be told different. There is an ECU-controlled limit beyond which it will cut power if reached.

Overrevs occur simply because the mechanics of a manual gearbox being connected to the engine make it possible. On a manual car you can force the car into a gear that is too high ratio for the current speed, and when you release the clutch the gearbox drives the engine at far greater RPM than the valvetrain is rated for. The ECU can't prevent this from happening, or the wrong gear selection to happen in the first place.

As has been said by Henry it can cause damage that is not immediately perceptible (like blowing a hole in the engine) or is otherwise untimely going to result in a total failure of the part(s) affected.

[mrdemon]

What happened to the no warranty even with range 3 over revs rumor a few weeks ago, is this true or not ?

I would not touch it with range 4 on any way , as you have seen a dealer like Henry would not buy it and Porsche would have to do a compression test.

I guess this is why the car is with a Ferrari dealer and not a Porsche one.

[Slippydiff]

Henry-F said:

With the greatest of respect you are wrong.

This week we have seen a car which would pass your test with flying colours, has done significantly in excess the one hour you suggest but which has issues which will turn out to be terminal.

You are confusing something like valves clipping piston crowns with other possible damage.

Thankfully there are technicians other than your "gold" ones who take a slightly more scientific approach.

Henry

Agreed. I know of a 996 Turbo engined car that was spun on a trackday, just over nine running hours later it snapped its timing chain terminally.
I think your oft stated 100 hours safety window after the over rev event, is the minimum applicable to ensure nothing goes terminally wrong as aresult of the over rev.

[Rockster]

TB993tt said:

I understand what you mean and certainly on a low power high japanese revver this certainly won't happen but on a big boost twin turbo Porsche it appears to happen a lot.

If you see the Durametric trace below, this is in second gear with the rev limit at 7400rpm, you can see the revs reached 7518rpm (note the low MAF indicates the rev limiter is breached - probably) so some how the engine managed another 118rpm with no fuel ?




Apart from this point of "physics" the Porsche ECUs do read some very spurious over revs, I have loads of data and it has recorded up to 7800rpm when I know for a fact I have never missed a shift, I don't know whether these are just erroneous "spikes" but I know that the Porsche reading tools also see them.... Another reason why I think the lower 1-5 rev ranges are bks

You are assuming the values obtained via the OBD2 port by querying the DME are the actual rpms. These may not be the actual rpms that are written to the the over rev counters.

[Rockster]

Slippydiff said:

Agreed. I know of a 996 Turbo engined car that was spun on a trackday, just over nine running hours later it snapped its timing chain terminally.
I think your oft stated 100 hours safety window after the over rev event, is the minimum applicable to ensure nothing goes terminally wrong as aresult of the over rev.

The cam/valve gear represent very little loading. This has to be for all that hardware to spin at high rpms and for the valves to avoid floating. These engines experience relatively large and quick changes in rpms on almost constant basis so if there was any possibility the inertia of the cam/valve hardware stressing the chains to the breaking point cam chains would be snapping all over the place.

What happened is the Turbo engine had an inherently bad chain that as happens with bad chains the chain snapped. The saying a chain is only as strong as its weakest link applies here both literally and figuratively.

To try to attribute this to a spin that occurred hours previously is ridiculous.

[fioran0]

Slippydiff said:

Agreed. I know of a 996 Turbo engined car that was spun on a trackday, just over nine running hours later it snapped its timing chain terminally.
I think your oft stated 100 hours safety window after the over rev event, is the minimum applicable to ensure nothing goes terminally wrong as aresult of the over rev.

but the chain in this instance is a result of the engine being forced to turn backwards by the drivetrain rather than an over rev correct? while its possible for both to occur in one "off", one cannot link the ramifications of one cause with another hence my earlier post in the other thread.

one needs to also be wary of asserting that a spin will turn a chain into a ticking bomb too just because it happened here. when sample size is small one cannot draw any conclusions.

on the list of problems (on the GT3) caused by over revs, there is a clear leader and its not even part of the engine.

Edited by fioran0 on Sunday 9th December 18:19