996TT rev limiter

Guest · Mar 27, 2005

I know all you 996TT drivers are nice and restrained, but I'll ask anyway:
Have you managed to run up against the rev limiter?

I've noticed a couple of times (in 2nd) that I have strayed into the red area. There is no coarsening of the sound, or vibration or anything that you normally get in other cars, just an easing of the power and the acceleration slows.
Officially the limit is at 6750, but I noticed I had almost made it to about 6900.

Is this what others see? The cutoff seems so gentle, I can't see it being a problem if it gets used occaisionally. On the normally aspirated cars it is more abrupt, and you are not supposed to make extensive use of it since it sets up imbalances in the engine.

Guest · Mar 27, 2005

I have hit mine a few times while driving within the speedlimits but enjoying the acceleration [

] but i concur with what you see and it is very gentle I have had cars that have been much more violent when hitting the limit but it's a non-event change gear and carry on. Like everything so far with my 996TT ownership everything it does it does just that much better than any other car I have ever owned and I could go on and on about just how pleased I am with it, but that's a whole different story.

Guest · Mar 27, 2005

I should add that I don't think it causes any damage but now I have had the car a little longer I don't hit the rev limiter as much as did in the first couple of weeks of ownership.

garyw · Mar 27, 2005

I have hit the rev limiter a few too many times[

] more worried what the OPC will ever say on the error codes readout during a service.
It just seems to dip the power on the limiter in and out, normally I'm half way towards my gearchange and just catches me out....must drive it more to get used to it....as with yourselves normally in second [8D]

garyw

glenn mcmenamin · Mar 30, 2005

I've hit mine a few too many times aswell !!!

My problem is, that i am getting used to the 8200rev limiter on my GT3, that thing
just revs forever, but when i get into the turbo, when things get going, it all seems to be
cut a bit short sometimes (reletively speaking of course)

I believe that the turbo engine is loosely based on the GT3/GT1 engine, so
i wonder why it can't rev as high ???

G.

Guest · Mar 30, 2005

I don't think there is any real technical reason why it can't rev much higher.

The main point, as I've mentioned before, is Porsche's philosophy to get the boost up nice and high low down the rev range. This makes it quicker in the real world and means you aren't bogged down waiting for boost during normal road based driving, where you aren't always using the top end.
The result of this is that the boost is tailing off, the torque declining and power dropping off after 6000, and by 6700 it is all over (relatively speaking). Since the car is set up like this, they decline to fit the titanium con rods, which means reving over, say, 7500, would be more difficult.

If you look to the more extreme tuning options, they go for increased rev limits, titanium con rods and 8K rev limits.
You need to modify the turbos to work at the higher revs. This means you will have less boost and torque/power lower down.
If you make the turbos work well at the bottom end, they then become more and more of a restriction at the top end, stifling the effective flow of the exhaust gases, increasing back pressure and reducing power.
It is a trade off in the basic design.

They are also conservative with the boost and I believe the standard turbos are capable of delivering more boost than they do. But then they have probably set the car up so that it delivers good boost even at 45 deg C, so in our temperate climate they are a bit over specc'd.

glenn mcmenamin · Mar 31, 2005

Understand all that Stuart,

But.........

Aren't the turbo's offset slightly, in that one comes in earlier than the other ??

If this was the case, would it not just be a case of spreading them a little further ??

Or, am i talking total C**p ????

G.

Guest · Mar 31, 2005

Cr*p I'm afraid.

The 959 had 2 turbos that worked in "compound" mode.
The exhaust from all 6 cylinders was combined below 4000 rpm to go through one turbo. This meant that the turbo spooled up more quickly. Flooring the throttle at 2500 rpm in 3rd gave full boost (1 bar) in 2 secs, instead of 6 secs for 2 turbos.
Above 4000 rpm, the gas was split and each bank of 3 cylinders had its own turbo.
This gave the best performance across the rev range. Remember that this was 1987.

When the 993TT was developed, technology had moved on. The turbos were better and electronic engine management was better.
The 996TT copies the 993TT. There are two turbos, one for each bank of 3 cylinders, located as close as possible to the engine, where the exhaust header pipes combine from 3 to 1, to give the quickest response. If you crawl underneath, you can see. It is symmetrical on both sides and both turbos are the same spec and give the same boost.
The air flow is combined (after the intercoolers) to go in through the throttle body.
There is no need for compound turbo charging, since they can get the responsiveness without it.

At low engine loadings, the boost is kept as low as possible to allow the throttle to be as open as possible. The improves the engine efficiency.
Since water cooling was added on the 996TT, the compression was raised. This is the main reason (along with the variocam system) for a 22% improvement in fuel economy over the 993.

BTW - the 959 has titanium con rods, and revs to 7500. The valve system is good for up to 8400.