lromanosky

I'm trying to sort out what I believe to be a problem with a 2008 GT2. It is stock with 5,000km. When upshifting the revs blip up to 500rpm despite the lifting off the accelerator in the usual fashion.

This happens under load and with boost. It does not happen at low engine speeds. It does not happen unless there is a large throttle opening. It does not happen without significant boost (.5bar). It is linear, repeatable and consistent. Temperature does not affect it. It happens in all gears at all road speeds. It is incremental - the harder one accelerates, the higher the boost and the higher the load, the more the car 'blips' the throttle. The affect is identical to one leaving ones foot on the accelerator and 'speed shifting' only the throttle is completely released.

It just doesn't feel right. With the added blip to the throttle on the up-shift the RPM's are too high, and when the next (higher) gear is engaged the clutch slips unnecessarily, and it takes away from the satisfaction of properly matching your revs on upshifts and downshifts.

I tried driving another 2008 GT3, and it did exactly the same thing. I've driven a 997 GT2RS and it absolutely did not do it. Other GT2 owners report their cars behave normally in this regard.

My feeling is that these 2 GT2's just can't be right - but what are the odds that they both have the exact same problem, and various internet searches don't reveal any known issues that I could find.

Before somebody says it is the driver, I've had 5 different drivers/owners driving one or both of the 2008 GT2's comment on the same thing.

Can anybody tell me what is going on with these cars?

CRex

You seem to be describing the case of a slow-to-close throttle.

Take off the airbox and check the pathway to your throttle butterfly. I'd make sure it's clean and free from grime/dust first...

lromanosky

We actually replaced the throttle body with a new part, and no change.

Since we have 2 stock GT2's that both have the exact same characteristics, I would be very interested to hear from another owner of a stock car.

One hypothesis is this is designed in as an emissions control, like some cars that 'hang on to the revs' to ensure more complete combustion of raw fuel when the throttle is snapped shut. In this case could it go one step further and actually raise the revs to avoid unburnt fuel from entering the exhaust and cats? The older cars would actually spit flames.... Is this designed to clean that up? Seems a very unsatisfactory solution to me, and I'm having a hard time believing it is supposed to be like that.

If it is, then the question would be if it could be eliminated by an ECU flash from an aftermarket 'chip'...

Igooz

I have a '09 GT2 and an '11 GT2RS both for many many years and have never experienced what you are describing. Maybe I am not driving aggresive enough.
I wonder if you are feeling the flywheel? I am getting rusty, but if I am not mistaking GT2 has a heavier flywheel than GT2RS and would maintain inertia.

lromanosky

Than you Igooz. This is definitely a symptom that is obvious during moderate driving - a shift at 3/4 throttle, 4,500rpm will produce the affect. We did think of flywheel inertia, but the affect seems to drastic to be ascribed to it, and wouldn't explain why different cars behave differently. It really feels like you are leaving your foot on the accelerator...

Perhaps the next time you drive your 09 you could see if you could detect any *trace* of the affect, or if in fact, when you de-clutch and lift off, the revs immediately fall as they should...

Thanks for your comment

nwGTS

Flywheel inertia will not accelerate a car. That's a misunderstanding of Newton's second law. There's no force being applied to accelerate the flywheel once engine power is reduced.

The only way for a car to continue to accelerate after throttle pedal release is if the signal to close the throttle butterfly is delayed or the throttle butterfly does not close immediately thereby continuing combustion. Simultaneously the signal would ALSO need to keep the fuel pump sending fuel to the combustion chamber. AND it would also need to continue to send an electrical signal to the ignition coil. You need all three in an ICE. (Different in diesel obviously) So this can't be just a throttle body issue.
My guess is your throttle pedal is the issue if what you're describing is actually happening. A potentiometer plunger or spring is sticking.

My first step would be to hook a PIWIS or durametric up to record throttle pedal position, fuel pump voltage, throttle body voltage, ignition coil voltage and RPM. Only once you plotted those will you know where the cause lies.
One hangup here is that the throttle pedal position will likely rely on that potentiometer to record a signal. So if it's sticking but the physical pedal is raised, you may not see the anomaly. Throttle pedals are relatively inexpensive so i'd just cave and get a new one... or swap with someone else without this issue and see if that fixes the problem.

Good luck.

Igooz

nwGTS, the way I read the problem description was the engine RPM was overshooting and not that vehicle accelerating. If you are under full power and accelerating hard and push in the clutch hard, are you saying that the engine will not over-run for a little bit? I am betting that it will over-shoot a bit since (at least me) I can push in the clutch fater and cut power faster versus the throttle return spring cutting off power. I am guessing that the rate of change on the heavier flywheel in the GT2 will make things roll a little bit before slow down? No? good stuff!

Someone should chime in...I think that the GT2RS has a lighter flywheel than the GT2...I guess I can go and start boths cars and listen for clutch noises!

My head hurts now, I will go and sit under a tree now and maybe an applie will fall on my head.

nwGTS

RPM is an engine metric. It has nothing to do with the transmission or flywheel with respect to throttle input and power output thusly. So, disengaging the clutch by depressing the clutch pedal will only reduce the load on the engine since it is decoupled from the transmission. In that scenario the RPM can increase at a higher rate (since less load) if the throttle is still depressed and the transmission decoupled.... but that's not a problem with the car. That's a problem with a driver who isn't releasing the throttle quickly enough.

To add, you're also under the assumption that the power inputs (fuel, spark, air) are not being cutoff quickly enough when releasing the throttle pedal. This is a drive by wire car. Not a 70s carb'd car. I promise you that if everything is functioning correctly (which you can test in what I described above with the dura) then the cut to power is 'immediate'.

***Once caveat i just realized is that if Sport or sport plus is on then 100% throttle is an input at 100-50% throttle pedal travel. So even if you are at 51% throttle pedal angle then the car is at 100% throttle. That may be a short distance but could factor into the discussion about not releasing the pedal quickly enough. Again though, this is a driver input error and not something wrong with the car's systems functioning.

lromanosky

Thanks for the comments. I considered the throttle switch or other issue with the throttle-by-wire. But the car will only do it under boost/load. At 1,500rpm it displays no symptoms with WOT. Only with boost registering about .5bar do I see the 'blip'. It if was a switch issue, it would be triggered by position and not boost/load on the engine one would think.

I'm not a physicist but I think if the initial acceleration on an object is strong enough, that object will continue to accelerate even if the propulsion is cut. A bullet would be a good example. The theory could apply to a flywheel, but I don't believe that is the case because 1) the affect is larger than what one would think reasonable for inertia and 2) only 2 of the 5 sample cars appear to do it.

I'm thinking that maybe there is something in the Turbo plumbing/Vacuum lines/breathers/wastegate that is causing the problem - but I don't understand enough of the way it works to come up with a hypothesis....

lromanosky

I'm not sure that we tried changing to sport/sport + to see if had any affect. It is worth a try. It is positively not the driver keeping their foot on the accelerator - though the effect is identical.

lromanosky

went for another drive.. doesn't have sport/sport +... but this time it was driven more aggressively, keeping the RPM's up and boost up over 4k. Much less noticeable. When we brought the car back down to 2,000 rpm, wot, and waited for it to spool up, then shifted at 4,500rpm, the affect was very noticeable. Perhaps something sticking in the turbo plumbing, wastegate, or turbos themselves?

nwGTS

This is not physically possible. Bullets do not accelerate past the barrel. Golf ***** do not accelerate after the face of the club. Proven many times. Scientifically proven. no increase in force no acceleration. I'll stop speculating on what the problem is since we don't have anymore information, but i think you've gone as far as you can on a forum. You need to take the car to an expert and hook it up to a diagnostic reader to plot the points I mention above. This is the only way to find out of this is a car problem or a driver problem.

lromanosky

Sorry you are right of course, accelerate is the incorrect term in this context. It has already been bounced back once from Porsche Aftersales as 'normal', and the reason for my post is to remove any doubt that this is the case. We've lined up a 3rd GT2 to drive next week, and if that car behaves the same then I will be forced to agree with them.

nwGTS

I think the best part about this thread is you have access to so many GT2s

lromanosky

Yes everybody seems to have one