jhbrennan

Maybe it's not the correct word to use - momentum - but I don't see how the rev limiter can stop the engine at exactly the defined rpm limit when you are moving through the gears in a spirited manner. Not that I would try this, but if you reved the engine by blipping the throttle while stationary, would the rev limiter stop the engine at exactly 7,200 (if that were the limit)? I think not so if it goes to 7,205 you would record a range 1 over-rev. Sorry i didn't pay attention in physics class and even if i did it was so long ago that i don't remember anything anyway.

jhbrennan

What if the dealer CPO'd the car with range 4 and 5 readings and disclosed the readings prior to sale?

jrotsaert

Bad example. the clutch disconnects the gearbox from the engine so all of a sudden the engine has nothing to work against and therefore speeds up for a very short time. If you watch an off shore boat race, every time the propellers get out of the water (nothing fighting vs the engine) the throttle man has to cut the throttles off or else the engine would increase revs and break. Same concept here except it's not water

Mussl Kar

Confusion between acceleration and momentum. Throttle man cuts power so drive train does not get massive shock load. Nothing to do with over rev and I highly doubt that a modern marine engine would NOT have a rev limiter to protect the engine.

It takes power to accelerate to a given speed/rpm. Acceleration, weather radial as in RPMs increase or linear as in a launch, requires power. Remove power and acceleration stops. Load on the engine has nothing to do with it. Settings of the rev limiter have EVERYTHING to do with it.

blake

Brilliant point... The problem is the subjectivity of this issue. I would imagine that the selling dealership would be more inclined to honor the CPO that they "certified" than a dealer out of geography who has a different Porsche zone rep approving warranty work.

I asked your very question to the local service manager of the car I noted in my earlier post. His response was that even though the car was CPOd by another dealership, he was confident the Porsche rep would refuse a major warranty claim (engine) as he had seen this happen before. I pressed him on that point - citing that the DME indicated abouse prior to the CPO date, and that the car should not have been CPOd. His response - "these things happen, this is not an exact science"...

Caveat emptor....

-B

Alstoy

Thanks Blake. What do the numbers correspond to? Being a bit thick today.

sandwedge

In other words, "this particular CPO policy may or may not include coverage for catastrophic engine failure".

alexb76

Could very well be the case, I will run it again to check the time of each and see if it's before CPO Date.

gota911

A 997-1 DME records the the number of ignitions in following rev ranges:

Range 1: 7300-7500 RPM
Range 2: 7500-7700 RPM
Range 3: 7700-7900 RPM
Range 4: 7900-8400 RPM
Range 5: 8400-9500 RPM
Range 6: 9500-11000 RPM

At 7200 RPM you have 120 revs per second X 3 ignitions per revolution or 360 ignitions per second.

Let's assume a car has the following time in each rev range, assuming the midpoint of each range as the actual RPM.

Range 1 - 3,603 ignitions / 370 per second = 9.74 seconds
Range 2 - 407 ignitions / 380 per second = 1.07 seconds
Range 3 - 36 ignitions / 390 per second = 0.09 seconds

As has been stated earlier, ranges 1 - 3 should not cause you any problems with engine damage or with the warranty. Range 4 gets into the gray area, but revs in Ranges 5 and 6 are red flags for the potential to have the engine warranty voided by Porsche.

Hope this helps.

Alstoy

Thanks Tim. If the higher revs are so bad, why isn't there some sort of gov to stop that?

jhbrennan

There is a rev limiter but it won't prevent mechanical over-revs caused by a missed shift.

No HTwo O

I'd imagine the engine revs faster, than the actual needle shows on the dial too. In other words, the dial lags the actual revs logged.

Mussl Kar

When I last spoke with someone at PCNA I was told that it was not possible to damage the engine with the throttle. Cannot prevent a blown shift though.
I think it may be possible to make some sort of robo gate thingy that would prevent a downshift if revs are wrong. Prob open a can of worms that would lead to all sorts of bad things.

gota911

As jhbrennan stated, there is a rev limiter which automatically cust the fuel feed to the engine when you hit red line at 7,200 RPM. However, that does not stop the engine immediately (just as hitting the brakes won't stop you immediately) and will exceed 7,200 RPS. That is when the DME records the number of ignitions in that particular rev range and the time (stated in engine hours) in which the last ignition occurred in that rev range.

If you miss a shift (going from 5 to 2 instead of 4) the transmission will cause the revs to go beyond red line. That is called a "money shift" because if the engine revs high enough, it could cause severe damage the the valves, pistons and valve shafts, resulting in a lot of $$$$ to repair.

Skibum

Hi Tim

I do believe your analogy is flawed.
When you brake at no time does your speed INCREASE. You could correctly say that your engine would not STOP immediately when fuel is cut, but increase in RPM??
Can you point me in the direction of the physics behind an engine continuing to increase in RPM after fuel has been cut?

Thanks