Wastegate mod.....

[Robert McGhie]

At 04:22 PM 10/13/97 +1300, you wrote:
>Hi All
>
>>      Unfortunately its not (quite) a perfect world,
>
>;-) too true
>
>>      and if you do not let 
>>      the pressure get to the wastegate a few psi before the set boost 
>>      level, you will find that boost may fluctuate (oscillate), 
>
>This I know, so just to make sure I have this right your micro device is
going to 
>do the same my compariter device, which is only open the wastegate at the last 
>possible moment to achieve a stable boost set point.
>
>The 2 promo's of the boost controles I saw weren't very clear all they talked 
>about was programmed boost control to achieve more power. All I'm trying to
find 
>out is is the boost controlled just doing what I want to do?
>
>Cheers
>
>Simon

 Adrian and Simon

I have been following your discussion and am confused. I understand that all
systems that use feedback may oscillate but why is it necessary to use an
electronic device to control the waste gate? Don't waste gates work by
simply applying boost pressure against a diaphragm to open a valve against
spring pressure? The greater the spring pressure the greater the amount of
boost required to unseat the valve and vent a portion of the exhaust,
thereby reducing the pressure driving the turbo?

As I understand it, when the force of the exhaust on the waste gate valve
plus the force of the boost pressure on the diaphragm equals the spring
force, there will be no seat pressure and a slight amount of exhaust will
leak past the waste gate valve. Additional boost pressure will compress the
spring lifting the valve off its seat. This will vent some of the exhaust
gases, reducing the amount of exhaust available to drive the turbo.
Depending upon the spring rate and the area of the diaphragm, an increase in
boost pressure of 1 psi could lift the valve 1/4 inch off its seat. (4 inch
diameter diaphragm and 50 lb/in spring rate) A 1 1/2 inch diameter valve 1/4
inch off its seat would vent quite a bit of exhaust. Thus, it would appear
that normal waste gates would control boost pressures over a fairly narrow
range and that until the boost pressure limit was reached all exhaust would
be forced to flow through the turbo. Am I missing something?

Robert