991.2 Over Range 5 and 6 Bad?
07-16-2018, 01:01 PM
#46
Rennlist Member
I think the real solution is to get education out there so people's immediate reaction to a handful of old over revs isn't "RUN FOR THE HILLS, SHE'S GONNA BLOW!!", which seems to have little basis in observed reality.
07-16-2018, 01:49 PM
#47
Like that (education) is really going to happen (not).
Sometimes we tend to forget that most Porsche owners and buyers aren’t frequent forum visitors.
With range 5/6 overrevs I would pass on this one and search for the next car.
Sometimes we tend to forget that most Porsche owners and buyers aren’t frequent forum visitors.
With range 5/6 overrevs I would pass on this one and search for the next car.
Last edited by Mark993TT; 07-16-2018 at 03:19 PM.
07-16-2018, 02:51 PM
#48
Too many 911s out there and available to purchase to warrant me buying on that had 5/6 overrevs.
I dont care how much education you do, I am simply going to stay away and buy something else. Even if the risk is low, I wouldn’t want to take on that tomebomb no more than the seller will continue to resume the liability on it.
Perhaps if the seller put enough money for a engine rebuild in escrow to cover potential problems during my ownership....but I suspect the sellers would never agree to that. And I wouldn’t purchase it for those very same reasons.
I dont care how much education you do, I am simply going to stay away and buy something else. Even if the risk is low, I wouldn’t want to take on that tomebomb no more than the seller will continue to resume the liability on it.
Perhaps if the seller put enough money for a engine rebuild in escrow to cover potential problems during my ownership....but I suspect the sellers would never agree to that. And I wouldn’t purchase it for those very same reasons.
07-16-2018, 04:34 PM
#50
07-16-2018, 07:24 PM
#51
The more serious question is why you're asking THAT question seriously.
it's a ****in Porsche 911. Drive it! If you're not hitting the rev limiter you're not driving you're car. You're simply *****-footing it thinking you're hard.
I hit rev limiter many times in my PDK.
protip: if you keep your throttle pinned to redline in 2nd and 3rd gear for 3 seconds or longer it will stop bouncing off the limiter and smooth out the throttle right at redline for a smoother and natural balance in a corner.
it's a ****in Porsche 911. Drive it! If you're not hitting the rev limiter you're not driving you're car. You're simply *****-footing it thinking you're hard.
I hit rev limiter many times in my PDK.
protip: if you keep your throttle pinned to redline in 2nd and 3rd gear for 3 seconds or longer it will stop bouncing off the limiter and smooth out the throttle right at redline for a smoother and natural balance in a corner.
Not the same as an over rev in a manual. The PDK engine management is more protective of the mechanics of the powertrain. The manual will allow you to do physical damage to the car that the PDK will prevent. If you don't believe me, downshift to 2nd on an empty freeway while you are traveling 100, and let me know how that goes. Then try it in a PDK- it will just shift through the gears as fast as it can WITHOUT DOING DAMAGE to your car.
07-16-2018, 07:35 PM
#52
As an undergraduate, more than fifty years ago, I drove and competed in a rally-prepped Volvo PV444. With stiffer valve springs, milled head for higher compression, re-worked carbueration, etc. it was frequently revved over the factory redline of 6500 rpm for a period of approximately three years. While demonstrating the mods to me, the mechanic, who prepped Volvo factory team cars, actually revved through valve float to show that it was possible. Eventually, after years of abuse, the crankshaft snapped in two. A relative who was an aerospace engineer working for an aircraft engine company told me that it was a "classic crankshaft fatigue failure" due to repeated dynamic overstress.
As a Ph.d. structural dynamics analyst, I suspect that a very limited number cycles of high dynamic overstress due to a money shift are most likely to lead to immediate failure, or failure on a relatively short term basis afterward. It also, however, could possibly result in reduced fatigue life under more normal conditions. It could possibly lead to fatigue failure after a number of normally non-damaging cycles of loading, particularly those approaching normal redline. Without a detailed technical analysis, using specific material fatigue properties, detailed stress calculations, etc. the situation is impossible to evaluate with certainty.
As a Ph.d. structural dynamics analyst, I suspect that a very limited number cycles of high dynamic overstress due to a money shift are most likely to lead to immediate failure, or failure on a relatively short term basis afterward. It also, however, could possibly result in reduced fatigue life under more normal conditions. It could possibly lead to fatigue failure after a number of normally non-damaging cycles of loading, particularly those approaching normal redline. Without a detailed technical analysis, using specific material fatigue properties, detailed stress calculations, etc. the situation is impossible to evaluate with certainty.
Last edited by verstraete; 07-16-2018 at 10:27 PM.
07-16-2018, 07:39 PM
#53
I wouldn't touch it. Trying to explain that when you go to sell it would not be easy.
07-16-2018, 08:07 PM
#54
Addict
Rennlist Member
Rennlist Member
Correct me if I'm wrong but I doubt these cars make max hp at around redline - optimal shift should be a bit earlier so what's the point of redlining the car all the time? I try to shift the 991.1 slightly before 7k (~6.8-6.9), makes the most sense looking at the dyno chart, don't think i'm losing anything in straight line acceleration given how the car runs on straights vs similar cars
The reason that this is better than shifting at peak hp is because to maximize performance, you want to maximize your average hp. When you shift at redline, your revs in the next higher gear are in a much more advantageous point than if you had shifted at peak hp.
You need to shift beyond peak hp, so that in the following gear you are not too far below peak hp. Redline is carefully calculated to give best performance when shifted there. If it was not, they would lower redline to improve longevity.
With a CVT gearbox, you would want to stay at peak hp constantly (below redline), but with a gearbox with 6 or 7 performance ratios, you are constantly changing rpm. You are only at peak hp for a small fraction of a second - you need to consider average revs and power...
Last edited by GrantG; 07-16-2018 at 08:23 PM.
07-16-2018, 11:28 PM
#55
Rennlist Member
Hours since it happened don't make it better. Years might. I wouldn't want to take that bet. Only time I've had a Porsche engine fail on me in 28 years was while I was cooking along at 150~ mph at Daytona....a tired race engine someone "zinged the hell out of" at a previous event, or so I was told. I do know one thing: I didn't over-rev it in three sessions before it put a hole in the case. Didn't make the tow back into the pits or the inevitable conversations any more fun. Buying a modern Porsche street car and getting left with the bill for a complete engine? Sheesh. Someone said it best above: There are plenty of these cars for sale. Even if there aren't, there will be. YMMV, as may your opinion. And that's all good—everyone gets to place their bets. I'd skip this car.
07-17-2018, 12:14 AM
#56
Rennlist Member
Your track story on a "tired race engine" at 150mph (not all that relevant to a street car, and its race history surely confounds things further...) would seem to conform to the notion that if a money shift breaks the car, it lets you know in pretty short order.
