simsgw

Subject to the discussion of "soft limit" strategies, yes. When the engine reaches whatever speed the computer is programmed consider an absolute limit, then no overshoot is possible. Not unless you're pointed down a steep hill and/or you let out the clutch in a gear too low, so you use the kinetic energy of the car to increase the engine speed.

Gary

simsgw

The comforting thing about science, as opposed to the soft end of campus, is that we don't have to care whether people agree with us. I give you joy of it, Tim.

That's quite... brave of you Tim.

Gary

911SLOW

Didn't read the whole thread but in case you are wondering.
The rev limiter cannot always stop the engine from revving exactly at the maximum revs entered at the DME's set point.


For example, here is a logged run with overrevs (2nd and 3rd gear) for a 2004 GT3 with a rev limiter set at 8200rpm, for the first 4 gears.



Time RPM
02:28:06.3437500 777
02:28:06.5937500 745
02:28:06.7812500 757
02:28:06.9687500 764
02:28:07.1562500 782
02:28:07.3593750 771
02:28:07.5468750 780
02:28:07.7343750 774
02:28:07.9218750 787
02:28:08.1093750 741
02:28:08.3125000 755
02:28:08.5000000 759
02:28:08.6875000 756
02:28:08.8750000 772
02:28:09.0781250 741
02:28:09.2656250 757
02:28:09.4687500 764
02:28:09.6562500 772
02:28:09.8437500 815
02:28:10.0468750 793
02:28:10.2343750 743
02:28:10.4375000 734
02:28:10.6250000 759
02:28:10.8125000 803
02:31:14.1718750 3362
02:31:14.3750000 3412
02:31:14.5781250 3470
02:31:14.7656250 3536
02:31:14.9531250 3605
02:31:15.1406250 3665
02:31:15.3281250 3729
02:31:15.5312500 3810
02:31:15.7187500 3864
02:31:15.9062500 3922
02:31:16.0937500 4001
02:31:16.2812500 4041
02:31:16.4843750 4090
02:31:16.6718750 4148
02:31:16.8593750 4213
02:31:17.0468750 4252
02:31:17.2343750 4297
02:31:17.4375000 4324
02:31:17.6250000 4376
02:31:17.8125000 4419
02:31:18 4458
02:31:18.1875000 4502
02:31:18.3750000 4539
02:31:18.5781250 4570
02:31:18.7968750 4614
02:31:18.9843750 4651
02:31:19.1718750 4693
02:31:19.3593750 4709
02:31:19.5781250 4757
02:31:19.7656250 4795
02:31:19.9531250 4838
02:31:20.1406250 4865
02:31:20.3281250 4896
02:31:20.5312500 4936
02:31:20.7187500 4961
02:31:20.9218750 4994
02:31:21.1093750 5020
02:31:21.2968750 5041
02:31:21.5000000 5065
02:31:21.6875000 5150
02:31:21.8750000 5196
02:31:22.0625000 5433
02:31:22.2656250 5553
02:31:22.4531250 5763
02:31:22.6562500 5917
02:31:22.8437500 6107
02:31:23.0468750 6303
02:31:23.2343750 6520
02:31:23.4218750 6719
02:31:23.6250000 6935
02:31:23.8125000 7156
02:31:24 7321
02:31:24.1875000 7647
02:31:24.3750000 7583
02:31:24.5781250 7843
02:31:24.7656250 7985
02:31:24.9531250 8203
02:31:25.1562500 7774
02:31:25.3437500 7798
02:31:25.5468750 7039
02:31:25.7343750 6149
02:31:25.9218750 5968
02:31:26.1093750 6057
02:31:26.2968750 6174
02:31:26.4843750 6305
02:31:26.6875000 6413
02:31:26.8750000 6503
02:31:27.0625000 6653
02:31:27.2500000 6749
02:31:27.4375000 6871
02:31:27.6406250 6949
02:31:27.8437500 7193
02:31:28.0312500 7203
02:31:28.2187500 7312
02:31:28.4062500 7448
02:31:28.6093750 7536
02:31:28.7968750 7667
02:31:28.9843750 7730
02:31:29.1718750 7833
02:31:29.3593750 7939
02:31:29.5468750 8017
02:31:29.7500000 8102
02:31:29.9375000 8188
02:31:30.1250000 8264
02:31:30.3125000 8260
02:31:30.5156250 8354
02:31:30.7031250 7870
02:31:30.8906250 6895
02:31:31.0781250 6224
02:31:31.2656250 6542
02:31:31.4531250 6380
02:31:31.6562500 6513
02:31:31.8437500 6569
02:31:32.0312500 6617
02:31:32.2187500 6655
02:31:32.4062500 6710
02:31:32.6093750 6621
02:31:32.7968750 6671
02:31:32.9843750 6532
02:31:33.1718750 6563
02:31:33.3593750 6527
02:31:33.5468750 6386
02:31:33.7500000 6303
02:31:33.9375000 6224
02:31:34.1406250 6120
02:31:34.3281250 6005
02:31:34.5156250 5948
02:31:34.7187500 5857
02:31:34.9062500 5777
02:31:35.0937500 5733
02:31:35.2812500 5617
02:31:35.6406250 5523
02:31:35.8281250 5460
02:31:36.0156250 5416
02:31:36.2031250 5350
02:31:36.3906250 5306
02:31:36.5781250 5256
02:31:36.7812500 5201
02:31:36.9687500 5146
02:31:37.1562500 5144
02:31:37.3437500 5109
02:31:37.5312500 5071
02:31:37.7343750 5044
02:31:37.9218750 5000
02:31:38.1093750 4969
02:31:38.2968750 4917
02:31:38.4843750 4892
02:31:38.6875000 4825
02:31:38.8750000 4780
02:31:39.0625000 4757
02:31:39.2656250 4708
02:31:39.4531250 4646
02:31:39.6562500 4584
02:31:39.8437500 4579
02:31:40.0312500 4529
02:31:40.2187500 4497
02:31:40.4062500 4455
02:31:40.6093750 4410
02:31:40.8125000 4369
02:31:41 4344
02:31:41.1875000 4292
02:31:41.3750000 4264
02:31:41.5625000 4222
02:31:41.7656250 4186
02:31:41.9531250 4144
02:31:42.1406250 4101
02:31:42.3281250 4081
02:31:42.5156250 4030
02:31:42.7187500 3981
02:31:42.9062500 3959
02:31:43.0937500 3914
02:31:43.2812500 3888
02:31:43.4687500 3863
02:31:43.6562500 3849
02:31:43.8593750 3830
02:31:44.0468750 3818
02:31:44.2343750 3786
02:31:44.4218750 3758
02:31:44.6093750 3746
02:31:44.8125000 3723
02:31:45 3705
02:31:45.1875000 3689
02:31:45.3750000 3663
02:31:45.5781250 3636
02:31:45.7812500 3617
02:31:45.9687500 3601
02:31:46.1562500 3579
02:31:46.3437500 3558
02:31:46.5312500 3533
02:31:46.7343750 3514
02:31:46.9375000 3493
02:31:47.1250000 3471
02:31:47.3125000 3449
02:31:47.5000000 3424
02:31:47.6875000 3406
02:31:48 3374
02:31:48.1875000 3351
02:31:48.3750000 3327
02:31:48.5625000 3311
02:31:48.7500000 3290
02:31:48.9531250 3276
02:31:49.1406250 3253
02:31:49.3281250 3253
02:31:49.5156250 3240
02:31:49.7187500 3225
02:31:49.9218750 3205
02:31:50.1093750 3189
02:31:50.2968750 3190
02:31:50.4843750 3186
02:31:50.6718750 3151
02:31:50.8750000 3157

simsgw

You probably should read the thread, John. The behavior of a 2004 GT3 engine isn't relevant to what we're discussing.

[Sorry for that crisp answer, but I was a little rushed right then. The 2004 GT3 engine is different for being an M96-79 obviously, but the important point is the age of the electronics. Computer technology develops extremely quickly and the number of sensors and the number of instructions executable in each revolution have both changed tremendously since the M96-79 was developed. Moreover, I honestly don't know which specific version they used in the GT3's. Was it the Motronic 7.8 or something like that? Car magazines rarely mention engine electronic versions and I'm retired so I don't bother to get the journals these days. Anyway, the point is I'm darned if I know how they handled limiting at that point in the technology. I'm fairly sure it wasn't an actual rev-limiter even that long ago, but I can't even swear to that so my comments about a completely computer-controlled engine probably do not apply.

Nice run by the way. Duratronic log record?]

Gary

gota911

Gary, that ^ is a great line! I love it!

Thanks for taking the high road by writing "brave" instead of "foolish!"

jhbrennan

Maybe it's the inaccuracy of the tach that is the culprit - I've seen a video over on Teamspeed comparing various exhaust systems on 997 turbos and one video shows the tach needle revving in the high 6,000's and even blipping over 7,000. From that video I have to assume the rev limiter cannot cutoff spinning at 6850 (or whatever the threshold is)...but then I don't know if the ECU was modified with a higher rev limiter either. As I said before, always an interesting discussion. I'd still like to know why my Durametric readings show 1 over rev increments across multiple ranges - not only does the 1 ignition seem doubtful but how could I have 1 ignition increases in ranges 2,3 and 4 all with the same hour reading? I've asked Durametric tech support but never a response.

911SLOW

The 996GT3 used the ME 7.8 Motronic (Bosch) engine management system that allowed the continuous variable timing of the inlet camshafts (VarioCam), and had a gear-dependent control of the maximum engine speed.
Gear recognition took place through a map in the Motronic control unit, which calculated the selected gear from the vehicle speed (from the ABS control unit) and the engine speed.

The 996/997 cars (as of model 2002 and 2003 for the Boxster) used the same DME and electronics. So the diagnostic functions of their Motronic control unit correspond largely with those found on the 996GT3 mk2.


Porsche changed the engine management with the introduction of the 997 with DFI engines.
A completely new DME control unit EMS SDI 3.1 was developed by Continental (Siemens) for the DFI flat-six engines and designed specifically to meet the requirements for the use of direct fuel injection. Compared to the previously used Bosch ME 7.8 DME control unit, the EMS SDI 3.1 DFI control unit features greater processing power with triple the clock speed and approx. 65% more memory capacity. It is particularly so because the injector control during multiple injection and the comprehensive data maps require greater computing power.
A voltage of approx. 75 volts is required to open the DFI high-pressure fuel injectors. This requires 12-to-75 volt DC/DC converters, which are installed in the DME control unit. (they get very hot hence the use of aluminum coolers on top)

The newer DME control unit features the following key functions:
• Direct fuel injection (DFI) control
• Control of the DFI fuel injectors
• Control of the flow control valve for high-pressure control
• Control of the demand-controlled oil pump
• Communication with the optional Porsche Doppelkupplung transmission (PDK)
• Communication with the PSM
• Communication with the optional PTM
• Control of the shift indicator
• Control of the maximum RPMs through a speed sender (engine speed and crankshaft reference mark)


So to summarize, what you see in my test run (old run specifically made to test the rev limiter before sending the DME to RS tuning for increasing it to 8450rpm and making it compatible with 100cpi cats), is similar to what you would see if you test any non DFI 996 / 997 car with a manual transmission.

Sadly I haven't logged a DFI car to confirm that the newer DME shows similar characteristics in governing the maximum engine RPMs.

john8642

Does anyone know what years did the computers start recording overrevs, and which models do so?

No HTwo O

Well, for sure with the 996 and 986 (1999-2004- only 2 over rev ranges) and the 997 and 987 (2005 and newer- with 6 over rev ranges).

I don't know what Porsche is doing with the new 991 but I'm sure they have something.

And I know my 993 (1996-1998) does not log over-revs.

San Rensho

Why in the world are you over-revving the motor? Max HP comes in at 500-1000 rpm before the rev limiter kicks in and if you are downshifting at 7k and over-revving you are an idiot. Please.

fast1

When you are rapidly shifting through gears, as you would in a drag race, you should stay in the lower gear to red line so that you will be in the sweet spot for the next gear, and of course the mechanical advanage gained of being in a lower gear easily offsets the lower engine power that you have in those last 1K rpms.

Notwithstanding the views of the experts listed earlier, I would not buy a car with overrevs in range 4 or higher. Just because the engine did not blow, does not mean that it hasn't sustained micrscopic damage which could make it more susceptable to failure in the future. Although I don't have a Mechanical Engineering degree, I have learned a lot from the school of hard knocks: two blown engines when there wasn't a rev limiter to save me when I was power shifting at the drag strip.

USMC_DS1

Similar to you... my car has 1 ignition in rev level's 3-6 per my DME printout from the p-dealer and my Durametric tool. From what I understand... it is supposedly impossible to have 1 ignition since each crank of the engine requires numerous ignitions. Several Porsche techs have indicated that this is an anomaly and should just be ignored.

San Rensho

The sweet spot is right between max torque, around 4.5k rpms and max HP, around 6.5, so you if you shift at max hp, you will be at or above max torque and never have to redline the car. And why in the world would you ever drive like this on the street?

In the motorcycle world theres a term called squids. If you are going to the redline on the street, then you are a Porsche calamari.

X51

So does Porsche only CPO cars with over revs in ranges 1-3, or something different, or do we know?

MICHAELWWW

You can ask this guy if over revs matter. Definitely does when it is repair time.
http://www.6speedonline.com/forums/9...ew-engine.html