BOV's, DV's and the Audi closed loop turbo system.
The Audi turbo system is a closed loop system by which all unused boost that is spooled during off throttle and gear shifts must be re-routed within the system; it cannot be vented to the atmosphere due to the dependence on the MAF and ECU to properly read and interpret air flow dynamics within the intake tract. Therefore BOV's (Blow-Off Valves) are not used because they create a rich air/fuel condition (i.e. less air to fuel ratio) which will damage the catalytic converter, create soot on the exhaust pipes, and cause occasional misfiring (due to the purged air) which will result in ECU miscalculated timing adjustments and other errors.
For Audi closed loop turbo systems, a BPV (By-Pass Valve) or DV (Diverter Valve) is needed to recirculate the purged air and keep it within the intake tract.
As of July 2004, the following valves were available on the world market:
AmD Viper = Polished chrome version of Bailey DV with AmD logo, $$$ (see "Bailey" below)
Bailey = Probably the most popular, serviceable, $$$
Evolution Motorsports = Cosmetic, 4 color anodized choices, not serviceable, $$
Forge = Large (may have fitment problems), serviceable, $$
Forge 007 "A Line" = Same as standard Forge, flat top for better fitment, seviceable, $$
Forge Race = Larger than others (see above), adjustable, serviceable, $$$
GFB Stealth FX D-BOV* = Exclusive unorthodox design, externally adjustable, tight fitment, not serviceable, $$$$+
HKS SSQV BOV** = Dual stage pull-type relief valve, highly competitive for open loop systems, not for Audi turbo sysytems, $$$+
HKS Racing BOV Type II** = Single stage aluminum pull-type valve, same as SSQV but able to handle up to 43psi (3.0 bar), $$$$+
Hyperboost = Cosmetic package, not serviceable, $$
Hyperboost Adjustable = New, cosmetic package, may not fit S4, adjustable preload, not serviceable, $$$
Hyperboost HX = Newest, very cosmetic package, new internals, not serviceable, $$$
OE Bosch TT 710A = THE STOCK DV FOUND IN ALL (1997 AND UP) AUDI TURBO MODELS, Audi part # 034-145-710A OR 06B-145-710, $
OE Bosch TT 710N = Basic, popular & low-budget upgrade, maintains stock look, not serviceable, Audi part# 06A-145-710-N, $ (some dealers charge more)
OE Bosch Sport 110 = MTM upgrade, same as 710N with "better" internals, not serviceable, $$
$ = under $50 USD
$$ = $50 - $150 USD
$$$ = $150 - $250 USD
$$$$ = Over $250
* The GFB D-BOV is currently an unproven design; it is a hybrid diverter and blow-off valve; it is intended to act as a diverter valve while providing the cosmetic audible benefits of a blow-off valve; it has yet to be determined/proven whether this design is effective for use on the Audi closed loop system or whether it may be causing long term damage.
** The two HKS BOV's are mentioned here because they have shown up on some highly unorthodox applications on the S4; they were not used properly and eventually removed due to poor recirculating feeds and other ecu errors.
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Either lead, follow, or get out of the way...
Audis don't get out of the way and they never follow.
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