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Posted by Dean (more from Dean) on Fri, 18 May 2001 09:25:17 Share Post by Email
In Reply to: Is a "dump valve" the same thing as a wastegate? n/m, TC, Fri, 18 May 2001 00:37:47
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A bypass valve or dump valve dumps the pressure from the turbo system when you close the throttle. This is does to prevent the turbo from loosing RPM's and to prevent shock waves which can destroy the compressor impeller wheel. So after a shift, you can get back on boost faster.

The wastegate is the boost control component that allows some of the exhaust flow to not go through the compressor wheel. The boost control valve and wastegate actuator control the wastegate. It is a negative feedback system. You can increase boost by having a stiffer wastegate actuator spring, shortening the actuator rod (which for various reasons cannot be used for major changes), or you can reduce the pressure that the actuator sees. Increaseing boost can be done via ECU changes, or by fitting a mechanical controller which bypasses the ECU's BPC valve.

But the valves used on the Saabs and many other OEM configuations are not dump valves, but are bypass valves.

If you vent a bypass valve to atmosphere instead of the return hose, it will take in unfiltered air because the valve is designed to be open when there is a manifold vacuum.

Bypass valves are closed at vacuum and typically vent to atmosphere, most often because the noise gives the owner a hard on.

If you vent a bypass valve to atmosphere or install a dump valve on a vehicle with a air mass meter, the spilled air will cause mixture control problems and may also get you some check engine lights (CEL's).

The stock valves oftern fail and/or will get musical with a modified system. I replaced mine with a piston type bypass valve to eliminate the noise.

Diaphragm valves should be avoided. That includes uprated plastic bosch oem units and the motorsport valve, which Abbott sells, unless they have changed it out.

TurboXS makes 1" by 1" piston bypass valves as well.
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