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In a message dated 96-02-28 04:51:40 EST, you write:

>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.  

That is the wastegates job...  A pop off valve or bypass valve are the same
thing....  the pop off valve can be routed to the intake side of the turbo or
to atmosphere, it's function is the same AT CLOSED THROTTLE, to 1) let the
turbo freewheel and 2) to prevent pressure spikes at the IC.....  Venting it
to atmosphere is the easiest and usual racers way of installing the valve
(big farts), the return to intake of turbo is the more quiet and sane method
used by most production cars.....  The same valve can be used for either set
up.....   BTW, most atmosphere pop off's I've seen have a K&N filter attached
to them to prevent oil blow and the possibility of sucking crap in the hose
during transition....