RE: 5kCSTQ boost Q

["Sheffield Corey" <SHEF@omdc.gso.uri.edu>]

>> ... same hoses with a built-in dump valve available. Scott, of PDQSHIP 
>> fame, said this was a dangerous set-up, but never explained why vented 
>> pressurized air at that point was dangerous to the engine.
>
>It's not.  It's the turbo.  Think of the exhaust manifold.  Large quantities of 
>hot gases are appearing and want to go home.  There are two routes - wastegate 
>against spring pressure, or turbo against vane back pressure.  Allowing for 
>some minor diameter issues, vane back pressure on the exhaust side is the same 
>as vane forward pressure on the inlet side.  Releasing the inlet back pressure 
>allows the compressor side of the turbo to spin up against no resistance.  This 
>suddenly diverts the entire exhaust flow through the turbo.
>
>There are two sources of danger - one is that the turbo can reach _much_ higher 
>speeds than in normal use, and another is that the loading on it is very light, 
>shifting the wear pattern in the bearing.

Hi Again Phil,

	It would seem to me that the purpose of the "dump" valve would be to
dump the hi-pressure spike that occurs when one abruptly and completely lifts
throttle from a full boost situation. There is still energy stored in the still
spinning turbo, but no where for it to go, so this hi-pressure spike happens
and the turbo is then stalled by the back pressure. I'm not sure what the
difference in exhaust flow is wide open throttle vs. closed throttle, but I
would think it would have to be considerable, and doesn't the dump valve close
as soon as the pressure spike has been vented? I am not a forced induction
specialist/expert and I obviously am missing some important point in
turbocharging theory. But it would seem to me that the "dump" valve vents
pressure above a certain set amount, ie. close throttle spikes, and that this
same closed throttle situation would cause a marked decrease in the amount of
exhaust gas available to the "hot" side of the turbo and that inertia in the
turbo wheels themselves would be the major remaining energy source for
rotation, thus minimizing gross overspin. Where have I gone astray/missed the
boat/gone t*ts up on my understanding of how the go pump works??

	All help/explainations/advice gratiously appreciated:-)


					Happy motoring,

							Shef