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sage dated 96-02-29 12:02:18 EST, you write:

>
>Jeremy, et al,
>>Is there such a thing as TOO MUCH pressure at the INTAKE MANIFOLD?
>
>Yes.
>
>That's the weak link, as chipped 944 turbo owners can probably attest too.
>
>> Will that cause too much mass to enter the
>>cylinder and blow the head gasket on the piston compression stroke?
>>Plus, the compressor won't compress too much unless the turbine is
>>receiving too much energy from the exhaust.  It's a direct mechanical
>>correlation.  And the prevention of this occurence is the sole task of
>>the wastegate, is it not?  So why (Linus) would you need a valve to bleed
>>pressure from the INTAKE manifold AFTER the throttle body?
>
>I think this thread started when someone mentioned  a description
>of Al Swakhammers(sp?) 4000 quattro which he turbocharged maybe
>10 years ago, and they said he installed a pop-off valve to control boost
>pressure. Some background. The first version(and maybe others) of the
>engine didn't have a wastgate. In place of a wastegate, Al installed
>a pop-off valve on the intake manifold to control the boost. So the
>turbo would just spin up and when the intake boost pressure
>exceeded the control pressure(probably a spring) of the
>pop-off valve, it would vent to atmosphere.
>
>If I am not mistaken, indy cars used a pop-off valve on the intake
>manifold as their sole means of boost control. In years past,
>CART would keep all the valves and distribute them to the teams
>before a race. All the valves are set to have the same max boost
>pressure. They all have tamper resistant hardware, so if a team
>decided to alter the pop-off valve max boost setting, the authorities
>would know.
>
>
>That's the function of the turbo bypass valve, which has been sufficiently
>described in earlier notes. You can either dump the extra boost to
>atmosphere or re-route it to the compressor inlet.
> -
>Dave Lawson
>
>
......  The pop off valve is different than a turbo wastegate in that there
is no restriction or slowing of the turbo, it just bleeds boost air to
atmosphere, it doesn't bypass exhaust gasses, therefore turbo is spinning
full bore all throttle openings......  Taking a race theory of gas or brake,
nothing in between it;s a great thing for a race car.....  Hardly practical
for a street car, or for turbo longevity......  The invention of the
wastegate was a good thing....  And really, unless you have a full bore indy
car, not sure I'd do it at all.....  Even audi racing cars used the
Wastegates, so pop offs are old hat so to speak....

Scott