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Bypass Valves for the archives
Following the Bosch Bypass Valve thread for a couple of weeks, maybe some
clarification of it's operation and testing methods might be in order:
Bypass Valve mods are good upgrades for older turbo audis. The valve allows
the turbo to freewheel off throttle, reducing pressure spikes in the intake
system (pre t-body), increasing throttle response vacuum to boost. Longer IC
life and reduced turbo stall are the main benefits.
* Sorry for the caps.... IT'S A BYPASS VALVE WHEN INSTALLED CORRECTLY, IT'S
A POPOFF VALVE ONLY WHEN INSTALLED INCORRECTLY.
* High pressure goes to 90 degrees to the turbo pressure, low pressure inline
with the diaphram.
* 6psi "cracking" pressure only gives you the vacuum spring "weight", does
not give any indication of valve operation.
* Bosch Bypass valve failure at high boost is usually attributed to a rip or
tear in the diaphram due to high boost pressures or a cocked sealing piston
due to high side boost pressure. The Sport Q uses a slightly HD diaphram, but
even that has a limited life at 18+ psi
* 2 valves in parallel will not then open at 12psi (assuming you installed it
incorrectly or as a popoff), remember, pressure in terms of these valves is
absolute, 2 valves in parrallel will both open at 6psi, you just have two
valves open instead of one.
* Hooking the valve up incorrectly is NOT a good idea, the valve takes longer
to close when reapplying boost.
To properly test the valve as a popoff, you must apply 6psi to the top of the
valve as you apply 6 psi to the bottom, not sure why you'd want to do this,
but neither measure is really relevent to it's operation (6psi on top, 6psi on
bottom = nothing will happen). Assuming correct install, the MEASURE is
vacuum to unseat the valve, and side and top pressure to create diaphram
failure (cocked or ripped seal), as well as flow thru the valve. Vacuum
(+spring pressure) should tell you how far the t-body flap needs to be closed
before the valve opens. The higher the spring pressure, the more vacuum
required for operation, the closer to full close the t-body needs to be for
Using the popoff and bypass terms interchangeably creates confusion. A bypass
valve is designed to open on vacuum, a popoff valve is designed to open at a
preset absolute boost level (ala indy cars, and non computer controlled
overboost protect). A bypass valve is BEFORE the t-body, the popoff valve is
usually AFTER the t-body (but can be anywhere in the charge air system).
Hooking up the bypass valve to be High Pressure opposite diaphram, delays the
closing of the valve, btdt. If you are hooking up your bypass valve this way,
you are missing out on it's benefits, and creating more work (read heat) for
the turbo, as it tries to overcome the pressure loss created by the open valve
on re-spoolup (vacuum to boost). Open loop bypass valves (vented to
atmosphere) create rich fuel spikes since the metered air is sensing "fuel
demand" as the valve opens. Closed loop bypass valves (vented to intake side
of turbo) have the advantage of not creating rich fuel spikes. Open vs Closed
performance gains? Well, nil really, a few of the Talon list guys did 1/4
mile tests open vs closed loop, .3 secs in time with open, if my memory
serves. Risk open vs closed? Clogged Cat, reduced O2 life, raw fuel in
exhaust system. Open loop bypass valves are NOT emissions legal devices in
audi turbos. 2 bar mods should be fine with a stock Bosch valve, but just.
Failures on the Stock Bosch valve occur in the 16psi range, and time will get
you eventually (wet intake oil + rubber diaphrams = failure).
ALL audi turbos should have this device installed. Those running 2bar and
above that haven't blown the IC end caps off yet, are on borrowed time. There
are a plethora of them available, the Bosch the simplest and cheapes to
install. The latest updated Bosch has a C suffix with a metal plug to reduce
diaphram tear problems. TAP has IC to T-body hoses of silicone WITH the 1"
valve port molded into it. 2 valves are better than one, even better than the
HKS sequential in terms of objective. The stock Bosch unit is easy to install
to ANY turbo system, the HKS (especially to the audi) is more of a challenge
(read: bux). Bottom line, a good mod with only benefits.