chris walrod

Absolutely! Mine momentarily cuts the CC when I depress the clutch, cancels completely with brake application.

Geoffrey

"I have to disagree with this completely - you're insinuating that the "momentum" from the engine gaining revs will cause the engine to CONTINUE going up in RPM even once the spark is cut by the ECU? I don't think so"

Ok, you do that...I'll rely on my years of experience with these engines and my understanding of how ECUs work...

901aero

Thanks for the tip. Hadn't occured to me that it could be as simple as a cut out switch on the pedal. Especially since I haven't found too many "simple" fixes on the 993!

ZombiePorsche44

I hit the rev limiter all the time without any problems whatsoever.....Miss a down shift (ie; 80mph from 5th to 1st gear) and the rev limiter won't do you a bit of good, but your mechanic will love you when you come for a rebuild.......ZP44

Felix

I would offer that it's not mechanical momentum that makes the engine over-rev in a no-load situation but the inability of the rev limiter circuitry to cut fuel and spark fast enough.

TomF

I had the same thing happen on my 87 cab. The CC switch (actually the little arm attached to it) located on the clutch pedal assy was bent. It is very soft, so it is prone to bending. It is a simple matter of bending it back and testing the actuation. As I recall, there is a pretty good CC testing procedure in the workshop manuals.

Of course it could be something else, but I would start there. Just my .02.

pcar964

No flame intended Geoffrey, but I think you're off base here...

The engine will not continue to increase in rpm once the spark is cut off by the ECU...

As an analogy - if you're accelerating in your 993TT... 50mph, 60mph, 70mph, 80mph... and then lift off the gas exactly as you reach 90mph... the car will not continue to accelerate to 95mph once you lift off the gas.

TomF

A rev limiter only works, so to speak, with the throttle. If you mis-shift, you are "mechanically" forcing the engine to rev higher. You could have the ignition off in this case and still blow or damage the engine. Depressing the clutch is the only possibility to save the engine damage and only if it is done really fast.

The engine can have plenty of momentum to overshoot the rev limiter. I would not suggest that you get in your car, depress the clutch and go for WOT. Your engine will suffer serious damage or failure in spite of the rev limiter.

Of course, I am not talking about the CC in this case.

Relying on a rev limiter more than on occasion is a bad idea....

I will defer to experts like Geoffrey and Chris W. on the more technical side of this.

Red rooster

At one time there was a small piece of test equipment made by Schrick ( Cam people ) that allowed you to hook across the ignition drive and set the start and finish rpm .

Fire the motor up and floor the throttle . The tester would then measure the start to finish rpm time . Armed with a motor inertia number the power developed across that rev range was assesed and could be plotted .

My company used one of these small units to confirm the correct bhp on competitors motors prior and during a single make national series .

The unit rpm cut , by cutting the ignition feed , allowed the maximum rpm to be set just short of the DME rev limiter .
All this was with the pedal flat on the floor during the test . There was enough time for the motor revs to rise with everthing feeling under control !
The motors were more bhp than a 964 !

Geoff
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KS400200,the oldest 964 on Rennlist,unless you know differently !

N51

Hi Geoff,
Interesting turn to the original post. Neat test! If I understand it all, the motor inertia will not exceed the the DME rev limiter. This would seem correct, but I'm not a physics major, just a pharmer. I cannot see how any engine, once the power is cut, can accelerate beyond that point. For it to do so, wouldn't the stored energy have to exceed the frictional/compression resistance? Is that possible?

Noah

Red rooster

Noah,

I found it interesting to have a motor accelerate from , say , 3000rpm up to 7000rpm , foot on the floor and then just sit there until you lifted your foot !

OK, out of sympathy ,the foot lift was reasonably quick but no way did the rpm burst through .You can imagine that that test was all pretty quick !The flywheel
etc was the inertia load .
It was the basic operation of the motor that was providing the inertia drive .
As soon as the spark was cut , no more drive , no more rpm !!

It was just a memory input that made me think that this could be relevant to the discussion on rev limit/throttle etc , etc .

Geoff
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KS400200,the oldest 964 on Rennlist,unless you know differently !

Geoffrey

I think the thing that is missing from this discussion is that there is a heavy flywheel attached to the engine, and the engine does not immediately stop accelerating at the rev limit when the ECU cuts spark or fuel or both. Further, the rev limit is propbably a multiple step revlimiter in the 964 meaning that there is a soft cut at 6800 where spark or fuel is cut, then there is a hard cut where both fuel and spark are cut together. So, if the engine is accelerating at say 1000rpm/sec (and WOT in neutral is probably 3-4krpm/sec) up to 6800, the rev limit will kick in, however, now the engine must decelerate from 1000rpm/sec and that in itself will allow the engine to exceed 6800rpm. The engine is still accelerating as the acceleration rate decreases until finally the engine is no longer accelerating. By how much? Probably a few hundered RPM. If you think I'm wrong, then you should have complete confidence in trying this on your own car in neutral.

Now, with the specific situation of cruise control, I doubt that when the clutch is pushed in, the cruise control automatically goes to full throttle. When the clutch switch was broken in my car and the clutch pushed in with the cruise control on, it would gently add throttle as the speed dropped when running on a flat road, it did not immediately go full throttle. I never tried it while going up a hill, but I imagine that it would apply the throttle more quickly, but again, not full throttle.

As another example of revlimiting, in my race car I have a pit speed limiter that is set with the MoTeC software, and it will hold the car at a specified car speed regardless of throttle position. In fact, I can go through the pits full throttle and when I cross the line I can turn the pit speed off and the traction control will take over as the car fights for traction. When I set it up, it required both fuel and spark to be cut to hold the engine at the proper RPM for the proper road speed. Using one or the other was not sufficient and the car would surge over the defined road speed, then come back under the defined road speed as the limiter went from more agressive to less agressive in the cut.

I think the test Geoff was mentioning is probably different than the way a 964 ECU is setup and programmed.

pcar964

The idea that the DME "isn't fast enough" to stop the spark in time is silly! If the DME is fast enough to provide spark at high rpm, it's fast enough to cut spark at high rpm.

Geoffrey

Noone is arguing that the DME is not fast enough. An ECU works at speeds much higher than the engine operates, nanoseconds vs milliseconds. Simply stopping spark does not stop an engine. You are forgetting about the physics involved.

Felix

Apologies if I come across as questioning anyone's experiences but I do struggle with a couple of things here.

Physics would seem to say that the engine will stop accelerating when a force (the energy from combustion) is no longer added to it. The kinetic energy (momentum) it has will keep it spinning when fuel and spark are removed but surely the complete removal of fuel and spark stops additional energy going in which makes the angular acceleration zero?

I need to read up on my various Bosch and Porsche books but I think the rev limiter is not meant to stop a no-load engine from over-revving. It expects to deal with the rate of engine acceleration you get with at least the inertia of the drive train, if not the whole car. It doesn't cut fuel and spark instantly; it does some combination of cutting or retarding the spark to a subset of cylinders and/or cutting the fuelling to a subset of cylinders, progressively becoming more instrusive if the revs don't start to drop. It can't stop a no-load engine exceeding the max rev limit at full throttle. It's a fairly soft rev limiter when you encounter it; it's not an all or nothing affair. All or nothing rev limiters tend to break engines over time.