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-   -   991S PDK throttle hesitation from start (https://rennlist.com/forums/991/764160-991s-pdk-throttle-hesitation-from-start.html)

chuckbdc 08-05-2013 12:09 PM


Originally Posted by CarManDSL (Post 10662410)
Tonight in normal mode and a just quicker than the next guy acceleration from stop (which for the 991S is completely lame) and the delay is there.

Posters here have shown actual 1/4 mile time slips as low as 11.7 seconds for the 991S. You will something with another hundred HP to beat that.

Mike in CA 08-05-2013 02:22 PM

Not to take a stand one way or the other on whether the PDK delay is a meaningful problem or not, or should have been engineered away, but it is consistent with the fact that PDK is an automated manual gearbox with internal clutches. The clutch for the shaft with 1st gear is only lightly engaged at a stop to simulate auto transmission crawl and, unlike launch control conditions, isn't programmed to instantaneously "dump" under a sudden throttle application. It takes a brief moment to engage fully and transmit power.

For comparison purposes, imagine you're at a stop in a MT car with your foot on the brake, the clutch depressed, and the transmission in gear, the best possible scenario for a quick take off in an "emergency". I'd suggest that the time it takes to leave the brake, mash the throttle, and dump the clutch is hardly less than it takes for PDK to perform the same function internally. Many people would be on the brake, off the clutch pedal and in neutral at a stop and in the same emergency situation under those conditions it wouldn't even be a contest vs. PDK.

Just another perspective.

SanibelSpeed 08-05-2013 02:33 PM


Originally Posted by Mike in CA (Post 10663327)
Not to take a stand one way or the other on whether the PDK delay is a meaningful problem or not, or should have been engineered away, but it is consistent with the fact that PDK is an automated manual gearbox with internal clutches. The clutch for the shaft with 1st gear is only lightly engaged at a stop to simulate auto transmission crawl and, unlike launch control conditions, isn't programmed to instantaneously "dump" under a sudden throttle application. It takes a brief moment to engage fully and transmit power.

For comparison purposes, imagine you're at a stop in a MT car with your foot on the brake, the clutch depressed, and the transmission in gear, the best possible scenario for a quick take off in an "emergency". I'd suggest that the time it takes to leave the brake, mash the throttle, and dump the clutch is hardly less than it takes for PDK to perform the same function internally. Many people would be on the brake, off the clutch pedal and in neutral at a stop and in the same emergency situation under those conditions it wouldn't even be a contest vs. PDK.

Just another perspective.

And a good one.

neanicu 08-05-2013 02:59 PM

The time it takes an Auto transmission to make the car move from a stop when " mashing " the throttle should be 0s(instantaneously)! That was valid with the old single clutch tranny,why wouldn't it be true with the Double Clutch?

WCE 08-05-2013 03:16 PM


Originally Posted by CarManDSL (Post 10662410)
Take a look at my video in posting #5 to see how "gonzo-nutso" I am driving to expose this issue.

Watching your video, I think you are feeling the accumulated effect of e-throttle response, pdk clutch engagement and engine torque response.
If this really bothers you, you might be a good candidate for an electric car!

CarManDSL 08-05-2013 03:29 PM


Originally Posted by chuckbdc (Post 10662987)
Posters here have shown actual 1/4 mile time slips as low as 11.7 seconds for the 991S. You will something with another hundred HP to beat that.

That is because they used launch control, where this delay issue does not exist.

No one is suggesting that the PDK doesn't offer amazing performance or that the delay causes a shorter 0-60 time, or in some way diminishes overall performance.
This is one amazing piece of engineering!

What the delay causes is a lack of smoothness on that initial punch of the throttle. That is what is annoying.

The take-up coding needs to be tweaked to eliminate this issue. In the next few months when the GT-3 gets into members hands we will know if this issue has been sorted.

Mike in CA 08-05-2013 03:47 PM


Originally Posted by neanicu (Post 10663417)
The time it takes an Auto transmission to make the car move from a stop when " mashing " the throttle should be 0s(instantaneously)! That was valid with the old single clutch tranny,why wouldn't it be true with the Double Clutch?

No fluid coupled automatic transmission that I've ever driven has had "0sec instantaneous" response which is why most acceleration tests of auto trans cars, even before LC, were done by running up the engine against the brakes to load the torque converter. I haven't had experience with a single clutch auto manual so can't compare to a DCT. But from what I've read their operation is generally rough and abrupt around town so maybe there is a programming trade off. AFAIK, there's nothing inherent in the design of a DCT that would keep it from operating similarly to a SC auto manual, assuming the manufacturer wanted it to.


Originally Posted by CarManDSL (Post 10663480)
What the delay causes is a lack of smoothness on that initial punch of the throttle. That is what is annoying.

The take-up coding needs to be tweaked to eliminate this issue. In the next few months when the GT-3 gets into members hands we will know if this issue has been sorted.

The only time I ever noticed this issue with my 997.2 S was off the line under WOT while autocrossing (not using launch control), never under any kind of normal or even aggressive street driving. Maybe the 991 PDK programming is "softer" in this regard and it's more noticeable in everyday use. It will be interesting to see how PDK-S is programmed in the 991 GT3.

chuckbdc 08-05-2013 04:38 PM


Originally Posted by SanibelSpeed (Post 10663350)
And a good one.


Originally Posted by neanicu (Post 10663417)
The time it takes an Auto transmission to make the car move from a stop when " mashing " the throttle should be 0s(instantaneously)! That was valid with the old single clutch tranny,why wouldn't it be true with the Double Clutch?

Porsche never had a single clutch automated mechanical transmission. With its best manual transmission, attempting an instantaneous jump from a standstill more than 300 ms would be lost in slipping the clutch, spinning tires or most typically axle tramp and or engine bog, depending on how merciless one is with drivetrain mechanicals.

But thats not really the issue here- it is the apparently obvious enough delay that some are feeling and that they wish was not there. If folks are reporting it, I believe they are observing it. From the description one hopes it is sensor/software response and not clutch slipping. I bet the little sucker is very expensive to repair!

neanicu 08-05-2013 04:46 PM

Mike,there's no delay or hesitation in any automatic transmission,at least not perceivable by one...you press the gas,you go!
I haven't noticed this issue with PDK the few times I've driven one,but I can understand why this would be annoying at some point...

chuckbdc 08-05-2013 04:51 PM


Originally Posted by neanicu (Post 10663650)
Mike,there's no delay or hesitation in any automatic transmission,at least not perceivable by one...you press the gas,you go!
I haven't noticed this issue with PDK the few times I've driven one,but I can understand why this would be annoying at some point...

As I understand it, with conventional automatics, there is always delay, smoothed over by the stator slipping until the fluids are spinning at the same speed. The best of them lock mechanically after that occurs.

Mike in CA 08-05-2013 04:55 PM


Originally Posted by neanicu (Post 10663650)
Mike,there's no delay or hesitation in any automatic transmission,at least not perceivable by one...you press the gas,you go!
I haven't noticed this issue with PDK the few times I've driven one,but I can understand why this would be annoying at some point...

Nick, now we can discuss the interpretation of "no delay"..... :)

As Chuck points out, there is some delay inherent in the design of a typical torque converter automatic; it's unavoidable AFAIK. Maybe PDK in the 991 has more; don't know haven't tried it.

neanicu 08-05-2013 05:04 PM


Originally Posted by chuckbdc (Post 10663638)

Porsche never had a single clutch automated mechanical transmission. With its best manual transmission, attempting an instantaneous jump from a standstill more than 300 ms would be lost in slipping the clutch, spinning tires or most typically axle tramp and or engine bog, depending on how merciless one is with drivetrain mechanicals.

Sorry,I think you've misunderstood my point...I know Porsche used their Tiptronic in older models,which is basically a Torque converter tranny,what I was trying to say is that there's no delay in a single clutch in general,just like there isn't in any Automatic transmissions...

neanicu 08-05-2013 05:25 PM


Originally Posted by Mike in CA (Post 10663662)

Nick, now we can discuss the interpretation of "no delay"..... :)

As Chuck points out, there is some delay inherent in the design of a typical torque converter automatic; it's unavoidable AFAIK. Maybe PDK in the 991 has more; don't know haven't tried it.

Mike,I guess we can make a whole theory about one's perception of " delay ",but that's not what we're after here...
FWIW,the only time I've noticed a real delay in automatic transmissions is when driven at a constant speed,you decide to floor the accelerator all of a sudden. That's when you'll notice it,while the computer decides to downshift a gear,but never from standstill.
I haven't experienced it in the PDK,but seeing a few of the comments,it certainly seems there is more to it than just " one's imagination ".

DKC64 08-05-2013 06:57 PM

I noticed this delay before I read about this in the forum. The delay is noticeable in most throttle applications from stand still, not just at WOT (I haven't even used WOT as my car has just finished 2000miles run-in). It is certainly more noticeable if I use more throttle, but even at 1/4 throttle I can consistenly notice it when moving off from a red light.

Very annoying as I described above when I try to join a main road from a side street -- I need to leave a bigger gap as the delay is a bit variable in duration.

Noah Fect 08-05-2013 08:04 PM

Someone is going to have to document this behavior with a video that includes an accelerometer display and OBDII live data monitor. We need to see throttle position versus acceleration versus time. Until some quantitative numbers are provided that can be used to distinguish a car with the problem from a car without it, there will be no way to make progress towards fixing it.


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