The grainger valve is used as a manual boost controller. You'll need to install a calibrated boost gauge before you start adjusting the boost. See this site for some more info:

http://www.dawesdevices.com/

Also check out the turbobricks message board:

http://pub55.ezboard.com/bturbobricks

The Turbo+ system will only raise boost at certain times, which I believe is only under full throttle acceleration above 3750rpm. I think it raises boost from about 8psi to about 10.5psi, but that may be a little off. With a manual boost controller, you could adjust it so you get 12psi max boost, so you're not so dependent on throttle position or rpm and you'll feel a difference all the time. Even at stock boost levels, the grainger valve will improve turbo response because it will build boost quicker. At around 15-16psi, you will not have enough fuel to go any higher, so start at the stock level and work your way up with small adjustments.
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