involutions

Hi all,
I'm looking at a 997.1 turbo with a pretty reasonable pricetag, but the owner says the lightweight flywheel has caused false readings of level 5 and 6 overrevs. I know he tracked the car, which he is stating up front, and thus the price. Has anyone else heard of a lightweight flywheel causing false DME readings???

85Gold

I don't know about the LW Flywheel causing the over revs but if it is tuned the rev limiter is usually raised. My 6GT2 had a GIAC tune which raises the rev limiter by 500 rpm IIRC. Personally I would stay away from any car with level 5 and 6 over revs.

Peter

TXE36

Sounds like BS to me - however I'm not really familiar with the 997 or Porsches in general. That said, assuming a manual transmission, there is no way with proper driving for a lightweight flywheel to cause an over-rev. It certainly could be more likely to do this doing some boy-racer ****, i.e. revving the motor in neutral. If motor RPM is not sensed at the flywheel, I have a hard time believing "false DME" RPM readings - RPM is RPM.

How big is a "level 5 and 6 overrev"? I'd be very wary.

-Mike

Frank 993 C4S

Coochas

Sounds fishy. Are we talking about 1 or 2 level 5 and 6 or a whole bunch?
Even throttle blips on a downshift shouldn't be getting that high!
I'm guessing a money shift but I'm not an expert.

mark kibort

how does an engine over rev by 400rpm? it has fuel cut off at anthing above the rev limit. you cant over rev unless you misshift. (money shift)

involutions

Thanks, everyone, for all of your replies. The seller has not yet provided me with the actual data, other than saying it looks "like a mess" with several level 5 and 6 overrevs, and that his mechanic says that's normal with a LW flywheel. I figured that was BS as well. My guess is it was downshifted into the wrong gear multiple times. I'm curious about what would happen first, valve float or a damaged crank/rod bearing.
Assuming no cylinder damage, how much would it cost to have the engine rebuilt?
I guess I don't feel like my pockets are deep enough to replace an entire engine. From a resale value, I also guess I don't want to try making up some BS story about a LW flywheel for the next guy to ponder.

I'll keep looking. I'm sure I'll have more questions for you guys soon.
Thanks,
-Tom

TXE36

In the situation you were responding to a shift is made at redline where the driver does not completely get off the throttle before pushing in the clutch. At the point where the engine unloads, it's RPM will go past redline before the DME can cut the fuel off - think of it as a mini moneyshift in that the engine is setup to overrev and there is nothing the DME can do about it. A lighter flywheel will make this case worse and I would also classify this case as "improper driving".

Rev limiters are not a catch all. In my E36 race car, it's valves were damaged when the shift mechanism failed and it was over-reved because the driver planted the throttle on the floor in neutral. Minor damage that was repaired before I got the car. Bottom line is that it is quite possible to over-rev a "rev limited" motor with the right circumstances.

-Mike

Coochas

Keep in mind a level 6 over rev in a 997 turbo is >9000 rpm. That seems way past redline for simple momentum in a non-lift shift.

Range 1">Range 2Range 3Range 4Range 5Range 6
> 6800 and < 7000> 7000 and < 7200 > 7200 and < 7400 > 7400 and < 7900 > 7900 and < 9000 > 9000

Frank 993 C4S

Here is the upshift data (Speed, RPM, Throttle). Rev limit is 7200 RPMs but the engine accelerates past it in an upshift to 7573 RPMs even though the throttle (bottom) is closed.
.

involutions

Thanks, Coochas and Frank. Great info!

Quadcammer

what you are being told is bs. That said, there is some glitch on the 997 computer that will show a handful of overrevs in certain categories. The trick here is that they will be single digits and similar in number or the higher category being higher than the lower (physically impossible). For instance, my 7tt has 3 overrev ignitions in both ranges 4 and 5, which is impossible. Post the info once you get it and we'll see.

GTgears

mark kibort

Frank, i see the graph, but how is this even remotely possible? if the throttle closes, there is no more increase in power, how does the engine increase rpm with no added energy? It reminds me of the graphs of dyno runs. the graph shows jumps in RPM at the end of a high power pull, but it is just electrical noise.

mark kibort

mike, why do you think RPM can go beyond redline, fuel cut off? it is set up to cut fuel instantly at a certain pulse rate, based on RPM. you can floor your car in idle and it will never go above the rev limt. when fuel cut off is at 6600rpm for example, there is NO more fuel at 6600rpm. the "momentum " thing others have mentioned has NOTHING to do with acceleration forces. no fuel, no acceleration, no increase of RPM. you can do a speed shift... no lift, like the old days of racing, and the rev limiter will kick in at redline RPM regardless of throttle position. the engine cannot "unload" it is not a spring. it is an engine fed by fuel ONLY able to accelerate based on fuel it receives. go have your buddy Dave,who does what you talk about, drive your car and shift at redline with his mis-timing of the throttle between shifts.it wont make that sound anymore. the rev limiter will catch it. ive tried it before , it works.. shift at redline and dont lift.. there will be no overrev. if not, you are saying the DME is not fast enough to respond .. which is nearly impossible.

to your second point. no, sorry, you cant over rev your car in neutral. the rev limiter cuts out at exactly its set point. it doesnt matter how fast it revs to the that set point, the DME can easily cut fuel when it hits it and there is no "momentum" force that supersedes the physics. you need F, to make m , a. (a=F/m)