ipse dixit

^
Exactly.

Last thing one wants to do to the GT3 is decrease low end torque, especially in the fruitless pursuit of more noise.

Mike in CA

Steve, I suppose if all one wants to do is open the exhaust valves fully when desired the Capristo device will do the trick. But Porsche's stock GT3 ECU controlled exhaust valves do pretty much the same thing with more sophistication. They modulate the valves for optimum low end torque depending on engine speed and load, and open them completely at full throttle. If you have a stock exhaust, defeating the system by holding the valves open at all speeds does make more noise, but at the expense of some low end torque assuming, as CAlexio point out, one cares about that sort of thing. Again, if you modify the exhaust in other ways then unaccounted for variables are introduced to the factory programming and all bets are off.

As you mentioned earlier, the mod to disconnect and plug the vacuum lines is reversible so there's no harm in giving it a try if you're curious. Porsche's description of how the system works implies that you should notice a degradation in performance, but if you can't tell the difference and like the extra sound then no worries.

Chris R.

Perhaps a silly question as I am also new to the 991's and sound is something touchy for the tracks I go to...can one leave the sport button off and the car "quiet" all the way to redline? As in, is "sport" is off, the valves always stay closed?
And then, let's say I wanted something like the BBI "stretch" excuse centre bypass, which keeps the valves...would, with the valves closed, be relatively quiet?
Thanks
P.S. I have done ~60km in the car before I had to store it for winter so the one 30 min drive I didn't get to play with everything nor did I go past 4500rpm...

NVRANUF

You could do this with the 997 GT3s Chris by plugging the vacuum line at the solenoid while leaving the electrics plugged in. At Tremblant I had to set mine to "closed 100% of the time" when I had SW bypass installed and sound police were happy again. It was definitely quiet but did make the car "feel" down on power when closed 100% of the time, not sure how the 991s react.

Jimmy-D

The good thing is this mod is easily reversible. For now I am enjoying it. On my GT4 I put on an FVD Valveless sport exhaust and thought that was one of the best exhausts i have put on a Porsche and it was great having the valves always open.

One thing I would never do is delete/bypass the side exhausts like many do. I have heard you will lose quite a bit of torque but I guess if you are adding more power with the headers than I guess they off-set. But, when I was invited to Atlanta when they opened up the Center and had a chance to speak to one of the Engineers; he is all for the rear bypass but said the side mufflers play a big role in the GT3 performance

Mike in CA

Good question, Chris. My understanding is that activating the sport exhaust allows for an increase in torque, especially in the range of 3-4K RPM, as compared to leaving the system off. However, peak torque isn't affected by the status of the sport exhaust which tells me that at WOT the valves are fully open whether the button has been pushed or not. So at part throttle and lower revs the engine will usually be quieter with sport exhaust off, but regardless of the sport exhaust setting it will always be loud when you are really on it.

Mr. Turtles

Hey Chris, as "Mike in CA' said upon full throttle at higher RPM's the exhaust will open whether or not you have the sport exhaust on or not. The switch won't keep it quiet on the track with a modified exhaust. One of the few ways to put in an aftermarket exhaust and be "legal" at Tremblant for our track events is to manually close the valves during the track event. There is a way to keep them closed, I haven't done this but I know someone at track that does this and isn't getting black flagged. Can't wait get the new toy out!

Dino524

At Tremblant, with my previous car (997.1 GT3 with sharkwerks bypass) I would manually close the valves by plugging a vacuum line (reversible in one minute) and would pass. On one event, I had forgotten to bring the rubber vacuum plugs and ran without any black flags. Since then, I ran with sport button pressed on with no modifications and never got black flagged. Car sounded much louder, but never got the attention of the "sound measuring police".
Keeping the 991.2 stock, hopefully no issues..

Hex

Is there any difference if you plug one valve hose versus both valve hoses?

Hawkeye2

I've gone back and forth twice. The transition at 3800 can be annoying, but for the occasional trip on the freeway, it's nice to be able to close the valves. Switching doesn't take long, though I've had the driver's side hose come loose once, creating a vacuum leak at lower rpm levels. Local shop fixed it. No problems since.

box986

I have the Cargraphics bypass module in my .2 GT3. Remote control switches from full bypass to stock (control via center console switch).