CPederse

When looking at this side muffler cut through, I can't help but wonder how big the difference is in terms of sound and performance, between a car with the exhaust valves open compare to a car with side muffler delete? Is the primary/only benefit from the side muffler delete the weight savings?

I also added cutaway pictures of the center muffler for those interested.

Thanks!

Serge944

The sound difference is significant. Despite the valves providing a nearly straight-through bypass, the side mufflers do their fair share muffling the sound. With bypass pipes in place of the mufflers, it's quite a bit louder throughout the entire rpm range.

Dr. G

Forgive me for asking, but do the valves open at 0 RPM in sport mode, or do they open at a specific rev count?

OCturbo

Valves open around 3,800 rpm when sport exhaust button is pushed (not pdk sport)

MileHigh911

AFAIK, I thought the valves were open at idle when exhaust button pushed. That is, the mechanical valves are opened, and you can hear that. Sounds nice at idle. BUT, as soon as you accelerate, they close, in theory for low end torque reasons, then re-open at 3800-4000 for better flowing exhaust pulses.

OCturbo

That's how I understand it as well, but I didn't share that little detail about idle.

Serge944

The valved exhaust has nothing to do with torque. It's the only way to pass EU noise tests and still have a nice loud exhaust at high rpm. EU testing is generally done at 50kph at wide open throttle.

Just take a look at the EU test method below:

1.4. Method of measurement
1.4.1. Measurement of noise of vehicles in motion (for type approval)
At least two measurements shall be made on each side of the vehicle. Preliminary measurements may be made for adjustment purposes but shall be disregarded.

The microphone shall be situated 1-2 metres above ground level at a distance of 7-5 metres from the path of the vehicle's centre line, CC, measured along the perpendicular PP' to that line (Figure 1).

Two lines AA' and BB', parallel to line PP' and situated respectively 10 metres forward and 10 metres rearward of that line, shall be marked out on the test track. Vehicles shall approach line AA' at a steady speed, as specified below. The throttle shall then be fully-opened as rapidly as practicable and held in the fully-opened position until the rear of the vehicle1 crosses line BB'; . the throttle shall then be closed again as rapidly as possible.

The maximum sound level recorded shall constitute the result of the measurement.

1.4.1.1 . Vehicles with no gearbox
The vehicle shall approach line AA' at a steady speed corresponding to the lowest of the three following speeds :
— an engine speed equal to three-quarters of the engine speed at which the engine develops its maximum power ;
— an engine speed equal to three-quarters of the maximum engine speed permitted by the governor ;
— 50 kilometres per hour.

1.4.1.2 . Vehicles with a manually operated gearbox
1.4.1.2.1. — The second gear in the gearbox must be engaged if the vehicle is fitted with a two-speed , three-speed or four-speed gearbox ;
' 1.4.1.2.2. — The third gear in the gearbox must be engaged if the box has more than four gears ; ,
1.4.1.2.3. — if the transmission has a double gear ratio (transfer gearbox or two-speed rear axle assembly), the gearbox must be engaged in the ratio allowing the highest vehicle speed.
The vehicle shall approach line AA' at a steady speed corresponding to the lowest of the following three speeds :
— an engine speed equal to three-quarters of the engine speed at which the engine develops its maximum power ;
— an engine speed equal to three-quarters of the maximum engine speed permitted by the governor ;
— 50 kilometres per hour.

1.4.1.3 . Vehicles ivith an automatic gearbox
The vehicle shall approach line AA' at a steady speed equal to the lowest of the following two speeds :
— 50 kilometres per hour ;
— three quarters of the maximum speed.
Where there is a choice, the 'normal' selector position for town driving is to be used.

More here: http://eur-lex.europa.eu/legal-conte...0L0157&from=en

fugeo

Finally, someone makes sense of how ridiculous this valve functions.

Dr. G

Interesting. On my Aston there was a fuse that could be pulled that would result in the valve being open all the time. Being that the valve has nothing to do with torque (as cited above), is there a fuse on the GT3 that would do the same if removed?

Serge944

There is an electrical connector in the engine bay that disables the valves. On the 997 GT3s, it could be reached in seconds with the engine lid open; unfortunately, for the 991, the process isn't as simple and requires some removals. If you search this forum, I'm sure you'll be able to find some info on how to accomplish it.

Of course, there is also the lazy way that some have used, simply by unplugging the vacuum hoses that go to the valves and plugging them. The valves default to open when there is no vacuum.

IMO, disabling the valves is the best (and cheapest) exhaust mod. I can't stand the on/off nature of the valves.

Serge944

I can't claim that there won't be any changes at all to the power band (not like it matters anyways at such low rpms to be honest, especially with PDK). I was just asserting that engine torque/power was never the reason for developing a valved exhaust.

Mike in CA

Serge, it very well may be true that sound management is part of the bypass valves function. However, it's also a fact that the valves work in conjunction with the ECU to improve low end torque and Porsche claims that it improves performance in this regard. It's not just about making more noise as is the case with PSE in other models. The interaction of the valves and ECU is new on the 991 GT3 compared to the earlier GT cars.

Tapeworm

Sounds like Porsche designed the software and exhaust valve control to deliberately cheat the noise test - oppps, where have I heard something like that before.....

I find the exhaust switch at ~3,500 rpm quite annoying in daily driving, enough so that I won't enable it normally.

The regular 991 has a much better transition from valves closed to open and a nice burble on overrun. The GT3 engine sounds great and the exhaust at high rpms sound good, but not at or below the switch point. I think the GTS / GT4 Boxsters and Spyders have the best Porsche stock exhaust sounds.

meaker

Can you supply link to where Porsche says the valves work in conjunction with the ECU?
I've always leaned toward it was strictly to pass highway cruising speed sound restrictions of some countries.

Serge944

The valves are controlled by the DME in the 991, just like it was on the 997. The DME tells the valves to open and close at specified engine speeds - that's it.

From Porsche Literature:

Like its predecessor, the 911 GT3 (991) has a function for increasing torque in the

middle rpm range. This function is not activated by pressing a SPORT button as was

the case for the previous model, but instead using the button with the twin tailpipe

symbol (1) in the center console. When this button is pressed, the torque is

increased in the middle rpm range. The exhaust backpressure in the variable

exhaust system is additionally reduced and the gas cycle is improved, resulting in a

noticeable boost in torque in the rpm range between approximately. 3,000 and

4,000 rpm. The torque levels are increased by between approximately 22 and 26 ft.

lb. (30 and 35 Nm). The maximum torque of 325 ft. lb. (440 Nm) at 6,250 rpm

remains unchanged.


The switching point of the exhaust flaps depends on the load and engine speed. As

on the 997 GT3 II, the switching point from closed to open at full throttle is 3,200

rpm (exhaust flap button (1) pressed) and 4,000 rpm (exhaust flap button (1) not

pressed).