The basis of this design was created by Thomas "Gus" Mahon and can be used, by itself, as a manual means of controlling boost level with close-to-maximum boost response. To see this design in its original format, see Dempsey Bowling's Turbo Performance Upgrades page. There are several designs available by Gus, so see my Turbo Performance Modifications page for details about these and other modifications. The necessary pressure switches are available from NAPA (part# 7011577).
Conceptually, this design makes use of a mechanical pressure switch and solenoid to halt boost increase when it reaches a particular level. The advantage of using this instead of an electronic version is response time. There is enough time delay in the MAP sensor output and the corresponding response of the circuit that the boost will climb over the threshold before the circuit can respond. This is partially why the stock computer starts to open the wastegate at around 3-4psi of boost; to slow down the rate of boost increase so that boost peak does not occur. The other reason is to suppress the "turbo surge" (when the turbo engine suddenly increases output torque) in order to increase ride comfort, driveability, drivetrain longevity, etc.
The easiest but least responsive way to do the installation on a stock system, or one with an electronic boost controller, is to simply wire the pressure switch in line with the solenoid being controlled by the circuit. Depending on how you have the solenoid configured, you may have to hook the switch is series or parallel with it. The hose barb on the pressure switch gets connected to the intake manifold or right on the turbo housing (available on some turbos). If you connect to the manifold, be sure you are not splicing into a part of the vacuum network that has check valves. There will be no positive pressure here. The best place to hook up is the vacuum/pressure line that comes out of the power brake actuator and goes to the speed control. Here are the two examples:
<pic showing the series and parallel configurations>
A much more responsive way to do it is to add an SPDT solenoid to the system. Use the solenoid to switch between the vacuum output of your boost controller and the wastegate actuator. Make sure you connect the solenoid directly to the wastegate actuator with no inline restrictors. It would be configured as follows:
<pic showing switch plus solenoid configuration>
this configuration, the pressure switch and solenoid will halt the boost
increase by quickly opening the wastegate. For the best response
possible, you should use 5/32 inch inner diameter (I.D.) vacuum hose
for the connection between this solenoid and the wastegate actuactor as
well as the pressure switch and manifold. This will provide maximum
air flow to the system and allow for the fastest possible response time.
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This page is maintained by Russell W. Knize and was last updated 09/01/98. Comments? Questions? Email email@example.com.
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