gota911

I did not say the engine would continue to increase in speed (RPMs) once the fuel has been cut off. I said the the engine wouldn't stop immediately when the fuel was cut off. There is fuel feeding the engine when the rev limiter cuts off the gas supply. There is also fuel "in the pipeline" on its way to the engine when the fuel supply is stopped. That is why I said the engine would not stop immediately. By the time the rev limiter cuts the fuel, the engines is already past red line.

If you took a car without any over revs and accelerated past the rev limiter once, there will be some revs in range 1 after that one rev limiter occurrence. If there has never been a missed downshift on a car, or a shift that caused a mild over rev, why would it have revs in any of the rev ranges?

I would love to see a DME readout of a PDK car that has bounced off the rev limiter a few times. I am 99.9% certain that there WILL BE over revs. PDK protects the car from a missed shift and eliminates the possibility of a mechanical over rev, but it is my contention that PDK cars are not immune over revs (a.k.a revs above red line RPM). If it is impossible to have a "money shift" (mechanical over revs) in a PDK car and over revs are present, then how did the over revs get there?

alexb76

Jeez, I can't beleive you had to explain all of that to the other guy ... when it's super easy to VISUALLY see how you can get into over-revs.

Just get on the gas, while revving high, engage clutch and pay attention to rev gauge, there's a blip! it's so easy to see... now if you were at the red line, this blip gets you to range 1 (and maybe 2) very easily!

simsgw

Not a chance. The time intervals in a modern computer, that is anything less than forty years old, are so much faster than gross physical events like fuel delivery that none of that happens. To a computer, the time it takes a flame front to propagate across the cylinder after the spark is enabled is like... like a weekend in Vermont to a human. In a briefing at Vandenberg Air Force Base, we were explaining a new launch control computer and I calculated that our computer could track a round emerging from a 45 caliber pistol and fire a round of its own to intercept it at a distance of... I think it was ten feet from the muzzle. This was in 1972. And my cell phone has a faster processor today.

The time between a Carrera engine reaching 7299 rpm and 7300 rpm is determined by a measurement of events that are terribly slow to the computer controlling things. Times written as 0.01 seconds sound fast. To us. To a computer that's enough time for a million instructions, picking one example at hand here.

If it wants to stop a 'blip' at 7299 rpm, it simply will not fire the fuel injectors. They open mechanically, but the decision whether to open them is electronic and electrons are bloody quick. If the injectors don't open, then fuel doesn't move into the cylinder. Without additional fuel, the piston that would have delivered thrust is acting as a vacuum pump instead. The crankshaft does not accelerate to a higher rotational speed. No power delivery, no increased energy in the crankshaft. It has to come from somewhere and when that computer shuts down fuel injection, that's it.

We do have to recognize that originally engines were almost completely mechanical devices and the recognition and prevention of over-revs is more difficult with early engine technology. All of the above considers my own DFI engine. The further back in model history you go, the less precise such things are going to be. Physics hasn't changed, but it is true that older control systems needed longer times to recognize an external event and act appropriately to control it. So I'm sure you can find an early attempt at rev-limiters somewhere on this planet that could not recognize a potential over-rev in time to prevent it absolutely. But that was then. Computers are too fast now to struggle with an event this slow motion unless the designer was a twit. And while I concede the possibility, I resist the notion that Porsche assigns a twit to such an important part of the engine.

Now if you 'bounce' an engine off the rev limiter, you're hearing a design decision made months or years earlier. Some people sat around a conference and argued about whether they should program the engine control computer to let the engine continue to deliver power intermittently to hold the crank at 7299 rpm, let it waver between 7200 and 7299 to provide some slight feedback, or apply an arbitrary shutdown for a fixed time like one second to make the driver pay attention. I have no idea how a Porsche is programmed, but in no case will it continue to accelerate to an rpm higher than whatever limit the coders were told to enforce.

It is a different question whether they stop it short of published redline or do not. I actually don't see a number for the redline in my owner's manual. I do see one on the tach, that looks like 7500 rpm. So range one of the DME report is telling us how long the engine was held near the redline, not over it.

I'm not sure if you can fool the gearholding logic of a PDK, but I agree that if it's implemented correctly, you cannot induce an over-rev by it downshifting. If you 'bounce' it off the rev limiter, we'll find out which decision that design team reached, but my personal bet would be that you'll see nothing but the range one ignitions. Those are not over-revs if I'm reading the tach correctly. They are just the last two hundred rpm of the permitted range.

Actually, we have to ask a PDK owner. What does the manual say? If you put it into manual mode, using the buttons or paddles, will it hold gear even at the redline or will it automatically shift upward if you fail to?

Whichever is the case, the rev limiter on a computer-controlled car can't be outsmarted by a blip. No fuel, no increase in rpm. Period.

Gary

pilfjd

Range 1 is not considered overrev. According to my tachometer, redline is 7500. If range 1 is 7300-7500, then yes, you will have recordings in that range if you hit the rev limiter.

Skibum

LOL@Alex...
Great post Gary.

Epic thread in the making.

jhbrennan

Different ranges for different engines too:

Range 1

997 Turbo >6800 <7000
997 >7300 <7500
997 GT3 >9000 <9200
987 >7200 <7500

gota911

+1 on Gary's post. Good info, good post.

I have not seen the minimum and maximum levels in each rev range for a 997.2, so I have been referencing the 997.1 ranges.

jhbrennan

I think the ranges I posted are for 997.1 engines...at least the Carrera and Turbo - don't know about the GT3.

gota911

I went on the Porsche web site and looked at a couple of close up pics of the 997.2 tach. Those pics show that red line is 7,400. My assumption is that Rev Range 1 would begin at 7,500, but not certain about that.

gota911

I guess you have a different tach than is shown on the Porsche web site. The attached pic shows four marks between 7k and 8k, which makes each mark worth 200 RPM, so redline is 7,400 in the pic.

rodsky

In manual, PDK will not up shift. It just bounces off the rev limiter. It also will not let you downshift into a red line gear. So theoretically I don't think you can induce over revs.

911CSMe

Seems this may be an appropriate area to ask (as a new member and one looking to buy my first 911 997?), with all the discussion about PPI and Overrevs and DME etc. to consider when looking to purchase used...it can get daunting. What are the implications regarding overrevs etc. in a Tiptronic (2005-2007)? Are there similar concerns if one uses the manual shifting of Tip? Are they of the same magnitude of the concerns as manual 6 speed trans? It's a "Brave new World" or should I say you have to "Brave" this "new World" out there when looking to acquire a used Porsche! Look forward to learning much more from y'all! Thanks

alexb76

Probably not as an issue as you can't downshift from 4th to 2nd on Tip... regarldess you MUST do a PPI inspection for any car before purchase.

simsgw

That's not physically possible. Inertia does not work that way. The only way you get revs in range four is downshifting while the car is going too fast. Unless of course you found a way to block the rev-limiter function completely.

Gary

911SLOW

997 GT3:
https://rennlist.com/forums/8311935-post37.html

997.1:
https://rennlist.com/forums/8327284-post11.html

997.2:
https://rennlist.com/forums/8559272-post10.html

996GT3:
https://rennlist.com/forums/996-gt2-...ed#post8347773


997.1Turbo:
https://rennlist.com/forums/8369222-post5.html

996 carrera:
https://rennlist.com/forums/8988434-post21.html