Quick 981 DME Report Question
12-08-2017, 03:46 PM
#16
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Holy crap. I've never met anyone who tracked and didn't go to the rev limiter. To each their own, though. Some folks drive 6/10 at the track and safely enjoy themselves, and others go 9/10+ and raise the risk and (IMO) the fun.
12-08-2017, 09:02 PM
#17
. Good luck.(Edited the ignitions per second. Initially divided 126 rev/sec by 3 ign/rev (or 42), when it should be multiplied (378), to end up with ign/sec. Brain fart on my part
)
Last edited by JCtx; 12-12-2017 at 04:27 PM.
12-08-2017, 09:32 PM
#18
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I'm going to disagree with some folks thinking. The rev limiter is there to protect the engine. And that's exactly what it does. Ive bounced off my rev limiter a couple of times. It's no big deal. Porsche will still allow a dealer to offer the car as a CPO, because it does not hurt the engine. If it was harmful to the engine, Porsche would lower the revlimiter redline and the rpm at which those revs are counted.
There is a different used Porsche you do not want to buy, that is rev related - a used Porsche with some miles, that has never seen higher rpms. That engine has been "lugged". There can be ridges built up on cylinder walls in the engine, that are to low, if never revved higher. Those ridges will be hit when you buy that used one and rev it higher. This is why no Porsche driver should simply leave it in "Drive" in normal mode. Especially if its a PDK, and you don't like to manually force it in a lower gear, drive it in Sport+ only, and hit the redline once in a while, but only after your oil temp is 180°F, minimum. Don't play in the higher rev range, until your oil temp is up. I try to limit my revs to less than 4500, until the oil is hot.
There is a different used Porsche you do not want to buy, that is rev related - a used Porsche with some miles, that has never seen higher rpms. That engine has been "lugged". There can be ridges built up on cylinder walls in the engine, that are to low, if never revved higher. Those ridges will be hit when you buy that used one and rev it higher. This is why no Porsche driver should simply leave it in "Drive" in normal mode. Especially if its a PDK, and you don't like to manually force it in a lower gear, drive it in Sport+ only, and hit the redline once in a while, but only after your oil temp is 180°F, minimum. Don't play in the higher rev range, until your oil temp is up. I try to limit my revs to less than 4500, until the oil is hot.
Last edited by PCA1983; 12-08-2017 at 09:53 PM.
12-10-2017, 05:05 PM
#19
Rennlist Member
There is a different used Porsche you do not want to buy, that is rev related - a used Porsche with some miles, that has never seen higher rpms. That engine has been "lugged". There can be ridges built up on cylinder walls in the engine, that are to low, if never revved higher. Those ridges will be hit when you buy that used one and rev it higher.
Any upper bore ridge (the ridge being the original bore diameter, unworn because the top piston ring is below the top of the piston and does not travel the full length of the upper portion of the bore) is the byproduct of wear, not RPM. We see this on any old engine ready for a rebuild. I can agree RPM is a factor in how fast an upper bore ridge can develop do to wear, but because it wears out the bore faster and that, would be the opposite of your statement.
While lugging, truly lugging an engine (under 1000 rpms with a wide throttle opening) will make it work harder, increase temperature, lose efficiency, and in some extreme cases might mess with timing -- I have never heard of it being the reason for an upper bore ridge.
We know high RPMs are not bad for the engine, however, it does increase stress, thereby wear on the valve train and rotating mass ultimately decreasing service life. Tooling around at 1500 to any mid-speed RPM will increase service life of an engine with all else being equal.
(apologies for going off topic gang)
12-10-2017, 05:29 PM
#20
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I'm going to disagree with some folks thinking. The rev limiter is there to protect the engine. And that's exactly what it does. Ive bounced off my rev limiter a couple of times. It's no big deal. Porsche will still allow a dealer to offer the car as a CPO, because it does not hurt the engine. If it was harmful to the engine, Porsche would lower the revlimiter redline and the rpm at which those revs are counted.
There is a different used Porsche you do not want to buy, that is rev related - a used Porsche with some miles, that has never seen higher rpms. That engine has been "lugged". There can be ridges built up on cylinder walls in the engine, that are to low, if never revved higher. Those ridges will be hit when you buy that used one and rev it higher. This is why no Porsche driver should simply leave it in "Drive" in normal mode. Especially if its a PDK, and you don't like to manually force it in a lower gear, drive it in Sport+ only, and hit the redline once in a while, but only after your oil temp is 180°F, minimum. Don't play in the higher rev range, until your oil temp is up. I try to limit my revs to less than 4500, until the oil is hot.
There is a different used Porsche you do not want to buy, that is rev related - a used Porsche with some miles, that has never seen higher rpms. That engine has been "lugged". There can be ridges built up on cylinder walls in the engine, that are to low, if never revved higher. Those ridges will be hit when you buy that used one and rev it higher. This is why no Porsche driver should simply leave it in "Drive" in normal mode. Especially if its a PDK, and you don't like to manually force it in a lower gear, drive it in Sport+ only, and hit the redline once in a while, but only after your oil temp is 180°F, minimum. Don't play in the higher rev range, until your oil temp is up. I try to limit my revs to less than 4500, until the oil is hot.
12-10-2017, 10:02 PM
#22
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That was always my understanding as well. But a prior post stated generally that the rev limiter protects the engine from over rev and I assumed he was referring to a manual transmission vehicle where that feature, while providing some protection in the event you simply accelerate to or through red-line, does not protect against a mechanical over-rev.
12-10-2017, 11:36 PM
#23
...but the point 09redGTS is making is that it is OK to run the engine up to rev limiter value. This IS the engine speed that Porsche has determined is safe.
He did NOT say the rev limiter would prevent over-revving from an errant downshift.
Experienced autocrossers (any marque) often chooses to bounce of the limiter for a second or 2 to avoid an upshift followed by a quick down shift.
Porsche will still allow a dealer to offer the car as a CPO, because it does not hurt the engine. If it was harmful to the engine, Porsche would lower the revlimiter redline and the rpm at which those revs are counted.
Experienced autocrossers (any marque) often chooses to bounce of the limiter for a second or 2 to avoid an upshift followed by a quick down shift.
12-11-2017, 12:18 AM
#24
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...but the point 09redGTS is making is that it is OK to run the engine up to rev limiter value. This IS the engine speed that Porsche has determined is safe.
He did NOT say the rev limiter would prevent over-revving from an errant downshift.
Experienced autocrossers (any marque) often chooses to bounce of the limiter for a second or 2 to avoid an upshift followed by a quick down shift.
He did NOT say the rev limiter would prevent over-revving from an errant downshift.
Experienced autocrossers (any marque) often chooses to bounce of the limiter for a second or 2 to avoid an upshift followed by a quick down shift.
12-11-2017, 02:16 AM
#25
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Well, the ignitions equate to barely over 1 second (42 ignitions per second) in that range, so just a couple of runs to or near redline. The hours don't make any sense there. They're clearly the TOTAL hours car has been driven, which is a crapload of hours for just 4K miles. That equates to just 12 mph average, meaning most of the car's life was spent commuting, apparently. Doubt you'd find something better than that, but hopefully the owner was equally careful taking care of the car both cosmetically and mechanically. Good luck.
. Good luck."7200 rpm divided by 60 seconds = 120 rps. With 3 ignitions every revolution 120 rps = 360 ignitions every second."
If the calculations in the line above are correct, it would appear that the DME report I posted reflects about 1/7 of a second in Range 1 (49 ignitions) and not "barely over 1 second" as calculated by elp_jc. Does anyone agree or disagree with this conclusion?
The car has 10K miles on it so the 12mph average is inaccurate.
And what is the significance of the "Operating Hours" heading? Total hours the engine has run? Or is it related to the number of hours on the engine when the momentary over rev was detected?
12-11-2017, 11:37 PM
#26
Is the math in the post I quoted correct? I found what I think is conflicting information and I'm a tad confused. Here's what someone on a 911 forum posted:
"7200 rpm divided by 60 seconds = 120 rps. With 3 ignitions every revolution 120 rps = 360 ignitions every second."
"7200 rpm divided by 60 seconds = 120 rps. With 3 ignitions every revolution 120 rps = 360 ignitions every second."
. Only every other revolution is a combustion (ignition) cycle.
Last edited by billwot; 12-12-2017 at 09:54 PM.
12-12-2017, 01:39 AM
#27
The operating hours listed is when the range 1 ignitions occurred. Notice I said ignitions and not over-revs. Range 1 - 3 ignitions are generally fine. The operating hours is the total time the engine has been running. If the car idles a lot, the average speed will therefore register as lower.
Elsewhere on the report should be the total operating hours. That figure is most useful with range 4 and above ignitions. For example, if the car has some ignitions in range 4, but they were over 100 hours ago, it's probably not a big deal. I forget how many hours need to have passed for CPO to be possible.
Elsewhere on the report should be the total operating hours. That figure is most useful with range 4 and above ignitions. For example, if the car has some ignitions in range 4, but they were over 100 hours ago, it's probably not a big deal. I forget how many hours need to have passed for CPO to be possible.
12-12-2017, 04:18 PM
#29
In sport+ auto, you'd hit the redline every time. Even at part throttle. It's just for the track. But yes, it's good to reach redline every once in a while. Or at least drive aggressively. I don't redline my car every time I use it, but always drive in manual mode, and always drive it semi-aggressively, always downshifting thru the gears (except 1st, unless I need the acceleration. Just like a manual, trannies don't like it too much) to hear that glorious PSE revmatch. He he.
That's absolutely correct. I thought I multiplied by 3, but divided it instead. Ha ha. Brain fart. In our case, it's 7,600/60=about 126 revs/sec. I divided that by 3, when it should be multiplied by 3 ign/rev indeed, therefore 378 ignitions per second at redline. In your car, it definitely was just once. The reason for the 3 is in a 4-cycle engine, it takes 2 full revolutions to complete a cycle, and the engine being 6 cylinders, it's 3 ignitions per revolution (6 ignitions per 2 revolutions). Will edit my original post. Sorry for the error.
. Even at part throttle. It's just for the track. But yes, it's good to reach redline every once in a while. Or at least drive aggressively. I don't redline my car every time I use it, but always drive in manual mode, and always drive it semi-aggressively, always downshifting thru the gears (except 1st, unless I need the acceleration. Just like a manual, trannies don't like it too much) to hear that glorious PSE revmatch. He he.That's absolutely correct. I thought I multiplied by 3, but divided it instead. Ha ha. Brain fart. In our case, it's 7,600/60=about 126 revs/sec. I divided that by 3, when it should be multiplied by 3 ign/rev indeed, therefore 378 ignitions per second at redline. In your car, it definitely was just once. The reason for the 3 is in a 4-cycle engine, it takes 2 full revolutions to complete a cycle, and the engine being 6 cylinders, it's 3 ignitions per revolution (6 ignitions per 2 revolutions). Will edit my original post. Sorry for the error.
12-12-2017, 06:19 PM
#30
Rennlist Member
That's absolutely correct. I thought I multiplied by 3, but divided it instead. Ha ha. Brain fart. In our case, it's 7,600/60=about 126 revs/sec. I divided that by 3, when it should be multiplied by 3 ign/rev indeed, therefore 378 ignitions per second at redline. In your car, it definitely was just once. The reason for the 3 is in a 4-cycle engine, it takes 2 full revolutions to complete a cycle, and the engine being 6 cylinders, it's 3 ignitions per revolution (6 ignitions per 2 revolutions). Will edit my original post. Sorry for the error.
Thanks for the clarification. It's all coming into focus now.