djcxxx

There may not be ECU mods now, but there will be. Porsche must anticipate this sort of potential problem in advance. I have to say that there is a fair amount of neurosis on this forum.

Petevb

There are two options.

1. The RPM limits are not actually any lower. Instead Porsche simply chose to reduce the over-rev margin because the electronics should now prevent over-revs, so an over rev should only the result of something un-natural and possible grounds for warranty denial, such as tuning or (?- downshifting while airborne?).

2. The over-rev margin is actually lower.

If it's the latter then robustness appears to have decreased by a significant amount: the 11,000 rpm point where "damage has generally occurred" on the 997 is at 28 meters per second piston speed, vs 24.5 meters per second on the 991. While this might not sound like much it is a huge difference in stress level. Thus if the 991 really does give up that much lower it would seem to be much, much weaker.

Even if this is the case, however, there are some reasons it might be intentional and not a bad thing.

For example: cams are one component that get very compromised at high revs. A cam produces more power the quicker it opens and closes the valve- it's called "ramp rate", and for the best power at any given speed you want the ramp to be steep. Unfortunately a steep ramp rate will "float" the valves at high rpm, which is a recipe for a blown motor. To counter this you need either a) a shallow ramp rate, or b) very stiff valve springs, which wear out components and rob power through friction. Neither of these is good. But if you know your motor is never going to see over 9.5K rpm, you can use a steep ramp and relatively soft valve springs, giving you more overall power below 9k.

This is only an example and I don't know what tradeoffs were actually made, if any, but I do know that as an engineer I'd design a different motor for a PDK only car to take advantage of the increased margin of safety. I'd raise the redline, change the power band, or otherwise optimize the motor for the gearbox. Perhaps that's some of what we're seeing here.

Manifold

^ Interesting. And maybe that partly explains why manual wasn't offered?

<3mph

Really great explanation Petevb. Thanks! And it would have been fascinating to listen in on the engineers' deliberations as they worked on this engine.

MileHigh911

Can one Over-rev this engine when they "double pull" the flappy paddles and rev it up to impress the by-standers? When the 2 engine failures first occurred, that was the first thing that came to my mind. With 9,000 rpm sounding incredible, I thought maybe some new owners were finding paddle neutral and over-revving engine. Does anyone know?? Maybe when in paddle neutral, the throttle is cut out early...to prevent momentum from the over-rev?????

Mike in CA

I'm not silly enough to try impressing anyone in that way, but I'm certain the engine is rev limited whether in or out of gear.

Macca

Im my opinion the OP document does not smell at all like an official PAG type recommendation. This must be something locally produce for the NA market (I stand to be corrected)?

The language used is alarmist to say the least the the rev ranges remarkably low. There is no way they built the engine to within 4% over rev failure tolerances. My old 993 has no top end work and has over revved beyond spec on a few occasions (my fault) without damage (at least to springs and valves when inspected).

Apart from the fact you can not over rev this engine it was tested to 10,000 rpm by the factory and im 99% sure that's recorded in an interview with AP or documentation direct from the factory - I will go away and have a look.

If the OP wasn't of such resolute standing on this board Id say this was "trolling" ;-P

TRAKCAR

Slow don't troll.

Sorry, maybe a stupid question;
If you are accelerating at max force, specially in 1st, 2nd or 3rd wouldn't the increasing speed of the car push the car over the rev limiter of you hold gear? I mean the increasing momentum does not instantly stop the millisecond the car hits cutoff?

neanicu

+1

neanicu

That's what I thought Range 1...
That's how I explain some overrevs in Carreras with PDK...

Mike in CA

Not stupid; interesting. I'm not an expert on this so maybe someone can give a more complete and technical explanation. But just intuitively, it's not like the engine is freewheeling. With a 12.9:1 compression ratio there is a lot of resistance to engine rotation, and once fuel and spark are cut things slow down fast. In the case of PDK-S, part of the equation could also be an almost instantaneous decoupling of the clutches (< 100ms is pretty quick) once redline is reached, something that can't be done nearly as quickly with a MT.

Zulu Alpha

Interesting analysis Petevb, very impressive. Just to mention something I'm sure you already know, that this 991 GT3 engine uses rocker arms with a damper as this engine was designed with high RPM in mind. The reason they have this rocker arm set up with a damper is to prevent the "float" from the cam you speak of at high RPM.

TRAKCAR

Mike, I had not thought about the car throwing itself in neutral based on overrevs and does not sound right, if you max out revs in say second gear in a slow corner and forget to up shift the car could throw itself off the track if the load abruptly goes away like that?

I find this all very interesting maybe just cut off will start early enough to not be able to damage the engine?

Manifold

Take the scenario a step further: at 9K already bouncing off rev limiter, then entering a steep downhill corner. Going into neutral there would be bad, but if it stays in gear, how are revs not going to go up? I guess regular PDK would upshift even if in manual mode, but PDK-S will apparently stay in gear?

qbix

That downhill corner would have to be some kind of cliff. But then I believe you would not think about your engine overreving.

Sorry I couldn't resist. We are talking here about elementry school physics or basic Newton laws.