Re: [DML] Re: Turbo PRV.

[mike.griese@xxxxxxxxxxxxxxxx]

There are a couple of ways to control boost pressure in a turbo system.

The most common way to prevent overboost in a throttle open condition
on older turbo systems is a wastegate.  A wastegate is a valve in the exhaust 
stream before the exhaust gasses reach the turbo.  This valve opens and diverts 
exhaust away from the turbo, slowing the turbo vanes.  Typical ways to actuate
the wastegate is to bleed a little turbo pressure to a spring loaded diaphragm
in the wastegate.  As turbo pressure builds, the bleed air pushes on the 
diaphragm and moves the wastegate valve.  This reduces the exhaust gas
pressure on the turbo vanes, slowing the turbo and limiting the boost.  
Wastegates can also be electrically actuated.

Another option is a dump valve.  A dump valve is a spring loaded diaphragm
on the output side of the turbo.  The spring preloads the daiphragm to a 
specific pressure over atmospheric.  If the boost pressure rises above the
preload, the valve opens and vents boost to the atmosphere.  They are 
non-adjustable and are usually used in conjuction with wastegates.  The
wastegate will typically open before the dump valve, because it is more
driveable to slowly limit boost than to just vent the turbo output to the
engine compartment.  So why have a dump valve?  As Martin notes 
below, most turbo systems blow through the throttle plates.  When 
the throttle plates are closed quickly, such as when you shift or when 
you brake, there is a reflected wave of air that is sent back through the
intake system to the turbo impellor.  This wave of air can be dense 
enough and fast enough to actually damage the impellor vanes.  This 
extra pressure will also slow the turbo impellor down, adding to turbo
lag.  Since you are no longer accelerating, it is reasonable to vent the 
excess pressure suddenly.  This protects the vanes and allows the 
turbo to keep spinning.

A blow-off valve is similar to a dump-valve in that it vents turbo boost
out of the intake.  The difference is that a blow-off valve is usually adjustable
so you can vary the maximum boost, and it is more progressive in it's
action.  It will also use other control mechanisms (vacuum, boost pressure,
ECU signal, etc) to determine when to actuate.  

--
Mike





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