Speedfox

What are the exhaust flaps?
The exhaust on the B7 RS4 features valves that can open and close when commanded by the ECU to subtly change exhaust noise. They are easily visible by looking up the exhaust pipes - the main oval pipe exterior cover two separate round exhaust outlets - one outlet is permanently open whilst the other can be opened or closed depending on the exhaust flap position with the open position making the overall exhaust note slightly louder and deeper.

When do the flaps open?
The flaps are linked to the "sport" button either mounted on the steering wheel (flat bottom wheel) or on the dash (multi-function wheel).
When sport mode is "off" the flaps always remain closed for making for quiet engine running. As sport mode resets to off on every ignition start the exhaust flaps default position is closed.
With the sport mode engaged the flaps will open in the following circumstances:
- when the engine speed is below 1k rpm (essentially at idle)
- when in neutral or gears 1, 2 and 3 *and* engine speed above 4k rpm
- when in gears 4, 5 and 6
At all other times the exhaust flaps will be closed regardless of the sport mode selected.

Do the exhaust flaps affect engine performance?
In theory the exhaust flaps being open can reduce exhaust backpressure aqnd unless mapped accordingly can reduce engine performance. Some tests have shown a small drop in engine output (approx. 5bhp) with the exhaust valves open compared to when they are closed. However given the nature of engine dynometer testing and the small difference it’s fair to say that there is a negligible effect. As such the same can be said for fuel usage - the flap position has negligible fuel usage impact. Any difference is mote likely connected with the sensitivity change of the accelerator pedal when activating sport mode.

How do the flaps move? 
The flaps are powered by the car's vacuum system by a pipe running to the rear of the car to a dedicated exhaust flap reservoir tank. A single solenoid controls the vacuum feed to both the rear vacuum flap actuators. 
Note: The vacuum is required to *close* the flaps as they are spring loaded to the open position.

What is the position of the flaps when the engine is off?
As the flaps are sprung loaded to be open and that the vacuum system is not a perfect closed system then as soon as the engine is stopped the valves will return to the open position.

What position are the flaps in when starting?
When the car is started the same sport mode algorithm is used to determine flap position.
As such whilst the idle speed is above 1k rpm during initial warming period then the flaps will be closed regardless of sport mode button settings.
The flaps will close as soon as sufficient vacuum has built up in the system and this takes approximate 1-2 seconds after starting the car.

Can the flaps be set so that they are open all the time?
As the flaps require vacuum to remain closed, simply unplugging the vacuum lines to the two actuators will ensure that the valves are open constantly regardless of the sport mode and operating conditions. You must make sure the vacuum lines are plugged and leak free and cable tied out of the way. A small self-tapping wood screw makes an excellent vacuum line plug. Be sure to be on the lookout for any tell-tale vacuum leak symptoms after disconnecting and plugging the vacuum lines (reduced performance, EPC light on, rough engine running).

What happens if there is a vacuum problem with the car?
The vacuum system of the B7 RS4 is prone to failure and has a direct impact on any system requiring vacuum to operate - the exhaust flaps are no exception.
Symptoms of a low vacuum supply include:
- failure of the exhaust flaps to close
- flaps initially closing then slowly opening when they should remain closed
- exhaust flaps rattle during cold starts when the secondary air system is active
However due to the dedicated vacuum reservoir tank it is not correct to say that there is no vacuum problem if the exhaust flaps are operating appropriately. Even if the flaps work as expected there can still be an underlying vacuum issue causing other issues and reducing performance.

What are the common problems with the flaps?
Common problems with the exhaust flaps include:
- vacuum lines to the exhaust actuators can become detached causing a serious vacuum leak. Simply re-attach and all should be well assuming no other vacuum problems
- exhaust flaps "stuck" in position. This can occur if the one or both actuators fail or seize due to lack of suitable lubricant. It is worth applying some WD40 or similar overnight to penetrate into the mechanism and help free the components. Then apply some 3-in-1 oil to replace any lost lubricant and see if this solves the problem. The mechanism may require several applications of wd40 and 3-in-1 oil to completely free up the mechanism along with a “nudge” with a tool (the handle of a small hammer works well). If this fails to work then it may be something more serious however Audi do not service these items and instead will recommend a replacement exhaust system.

Can the exhaust flaps be tested using diagnostic equipment?
Using VAG-COM/VCDS it is possible to run a vacuum diagnostic and one part of the test is the actuation of the exhaust flaps. See the sticky thread on “How to - function test of Power Flap and Manifold Flaps” for more specific information.

Do the exhaust flaps need any maintenance?
The mechanism is subject to a dirty operating environment and large temperature changes and as such requires some TLC in order to keep in good condition. The exhaust flaps should be maintained by normal servicing as there is a specific RS4 line item to inspect and lubricate the exhaust flap mechanism but this can be overlooked. Some application of 3-in-1 oil can help keep the mechanism suitably protected. Do not use WD40 or similar products as no matter what is advertised they are not designed as a long term oil and will in fact strip off any remaining lubricant/grease from the actuators making problems considerably more likely.