Re: Down on the Dumps

[PDQSHIP@aol.com]

In a message dated 96-09-05 21:52:19 EDT, you write:

<< All this is foreign to me - the hose from my intercooler to inlet manifold

 is large diameter heavy-duty _smooth_ rubber.  It's slightly C-shaped to 
 accomodate the movement you describe.  No rings, no metal fitments of any
kind.
 
 > ... 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.
 
 -- >>>>>

Not the turbo at all Phil....  I'm not sure after reading this over a coupla
times I understand what you are saying at all...  Pressures on the two sides
of the turbo are EXACTLY different...  Unless you are spinning the same fan
hot and cold sides and have the same restriction on both sides, the pressures
are by definition massively different...  There are 4 parameters to change in
a turbo basically: 1) the size and design of the hot side wheel 2) the size
and  design of the cold side wheel 3) the size and configuration of the hot
side housing, and 4) the size and design of the cold side housing....   Any
and all changes (and on a k26 the matricies are pretty high, btdt) dictate
the characteristics of the turbo output given the same engine and rpm
input.....  The definition, in fact, of a turbo MAP....   The turbo will NOT
overspin with a pop off valve, at most it will freewheel, you need MORE
pressure on the hot side to increase turbo speed, a dump valve does exactly
the opposite....  The two sources of danger are misleading....  Turbos do not
reach a higher speed that normal with a dump or a pop off, and actually a
dump valve decreases the LOAD on the bearing and backpressure spike on the
cold side fan, there is no relation to the "wear pattern" with something that
spins at 100k rpm....   A full race car might see some benefits from dumping
to atmosphere, a street car won't....  Flames shooting out the exhaust is
pretty neat in a race car, but into an expensive O2 sensor or melting
honeycombs are side benefits street cars might want to avoid, not to mention
the transition to boost prollums.....   I saw the TAP hose setups at the
vendor table at Waterford, like the hoses, forget the valve.....

Scott