Posted for 13brv3@mchsi.com:

Single point failure mode leading to immediate engine failure. A blow
off valve in a car is for instanteous relief when shifting and off the
gas, but the wastegate is also working with it, just slower. A blow off
valve is an emergency device, no more no less.
[Marc Wiese] ------------

===>  Technically, you're talking about two different things.  A blow off valve is not the same as a pop off valve.  It is true that a pop off valve is not supposed to be used as the primary means of controlling boost, but we're not suggesting that at all.  The boost is being controlled by the pilot's use of the throttle, and the pop off valve is only there for protection against operator error.  It's reasonable to think that we will be able to control the boost via throttle, since we won't be slamming the throttle open and closed rapidly like you would do in a car. Rusty [If you'd like to get some real world experience flying a turbocharged engine whose boost level is controlled solely by the pilot's operation of the throttle, get yourself some time in one of the early Turbo Arrows.... they didn't even go so far as providing a popoff in that airplane (at least I don't think they did), instead they lit up an overboost light on the panel when you opened the throttle too far or forgot to back it off during descent.  I've logged a reasonable amount of time in these kinds of airplanes and have never damaged an engine or forgotten the fine points of operation.  The popoff, as you've mentioned, is there for protection... kind of a hardware turbocharger insurance policy.         <Marv>                    ]