Re: popoff valves

["linus d. toy" <linust@interramp.com>]

At 04:19 PM 02.27.96 -0800, Glen Powell wrote:

>pop-off valve = relief valve for excess boost pressure on the output of the
>compressor side of the turbo/intake manifold/throttle-body that vents the
>excess boost to the atmosphere -- more big noises!
>
>'turbo bypass valve' = the same as the pop-off valve, but, it vents the
>excess boost back into the compressor inlet, creating a closed-loop system -
>quiet
>
>All can limit maximum boost but in different ways and at diferent times. No
>combinations are mutually exclusive.

and then...
At 05:13 PM 02.27.96 -0500, PDQSHIP@aol.com wrote:

>No wastegate operates under boost, pop off valve when throttle closes, no
>boost air available to bleed......  The pop off valve opens only during
>vacuum of slam shut throttle, closed during boost ALWAYS

Agree that the WG and PO valve are both designed to control maximum boost.
However, I don't see the turbo bypass valve as a boost control
device...perhaps I'm using these terms differently, or totally confused in
my understanding here :(  otoh,  the two definitions above tell me i may not
be alone :)

I've always thought of a pop-off valve as a pressure-relief valve that opens
if manifold pressure rises to a certain point--which means it operates on an
OPEN throttle and only if it's relief point is reached.  A closed throttle
would quickly drop manifold pressure and keep the PO valve closed.  

but that closed throttle would lead to two more problems--1) big pressure
spike between the throttle plates and the compressor (typically leading to
blown intercoolers) and 2) big time drag on the compressor.  this is where
the turbo bypass valve comes into play.  The bypass valve DOES vent excess
pressure, but instead of from the intake manifold, from the chamber between
the throttle plates and the compressor--on high manifold vacuum (closed
throttle) the valve opens, allowing pressure to vent from between the
throttle plate and compressor, 1) minimizing the pressure spike and 2)
feeding the vented pressure back to the intake side of the compressor,
helping keep the turbo spooled up.  At least this is how I see it set up on
my '91 200q 20v (it does look awful small to let much get through there, tho)

As for retrofitting the turbo-bypass valve on the earlier cars (anyone know
when they were first introduced?) although the '91 200q and S4/S6 engines
are close (10 hp difference) the intake systems are very different and might
make for more custom fabrication for a retrofit (perhaps someone can
elaborate?).  On the '91 20v turbo, the intake plumbing is "sorta" like the
earlier 10v turbos--primarily found on the passenger side (LHD cars) of the
engine compartment.  The S4/S6 intake system is a bit more convoluted,
running from the turbo (passenger side) to the intercooler across the front
of the car, then routed to the intake manifold on the DRIVER'S side of the
car.  A comment from Ned some months back was that this more extensive
routing has less HP potential than the earlier motors.  FWIW.

--ldt
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