The Motronic ECU that the AAN uses is based on airflow through a Mass Airflow (MAF) sensor. The Motronic ECU controls the fuel to the engine and it expects that all of the air that went through the MAF ends up in the engine. The air flow through the AAN is shown in this figure:


(Image courtesy of Sean D. (quattro20V))

When you back off the throttle after being in the boost, there is a bunch of air that was flowing towards the engine that is now cut-off from getting there. This air backs up to the turbo, and will stall the turbo unless one of two things happen: either the air is dumped to atmosphere via a blow off valve (BOV) or its is recirculated through the turbo via a By-pass Valve (BPV). Our cars use a By-pass valve. If you use a BOV, the engine will run rich for a short (seconds) period once you get back in the throttle (because some of the metered air that the ECU and O2 sensor were expecting gets dumped). Not good.

How the BPV functions is shown below:



The OE Part No. for the "Cut-out valve" on our cars is 034145710B. The B5 S4 2.7tt and 1.8t cut-off valve is 034145710A (Bosch PN: 0280142108)
However, the quick and dirty upgrade for the 710A and 710B is a 06A145710N ("710N") (Bosch PN 0280142114) from an Audi TT. It's supposedly close to the Bosch Motorsport "110" BPV (Bosch PN 0280142110). You can go aftermarket but you'll have to do your own research.



When they fail (the diaphragm gets a tear), they make a variety of noises including fluttering or, some cases, a goose-like "honk". This is what Hoppen Motorsports (an MTM dealer) says about these BPVs:

"Early production vehicles had a Bosch bypass valve with the number ending in 108 or Audi 710A which due to problems would eventually be superseded by a Bosch 114 or Audi 710N. This valve was far better in performance but still not completely reliable especially under modified conditions.
MTM did extensive research on various bypass valves also called diverter valves for their 400 -500HP kits and came up with the Bosch Sport Valve 110 being the best in performance and durability, even over more expensive aftermarket valves. At JHM we also tested various options and concur with MTM's findings. With the Bosch Sport Valve the boost recovery and throttle response are enhanced significantly and since it is dimensionally the same as the original the installation is simple.

As stated in Audi's technical bulletin dated Dec. 8, 2000 the bypass valves should be replaced should the following occur: "Groaning or howling noise from engine compartment when accelerating during partial load between 2500-3000 RPM," or " Rattling noise when decelerating between 2000-3000 RPM," which "may be caused by the internal diaphragm of the charge pressure bypass valve producing pulsating noises." REFERENCE


This will be on the exam.