A simple and effective boost controller for an external wastegate (or internal, see note at end) can be constructed with simple hardware store parts. This controller installs between the pressurized manifold and the wastegate. The controller prevents any pressure from reaching the wastegate untill the desired manifold pressure is reached thus preventing premature wastegate "cracking open" before the set pressure level is reached. A 1-2psi boost spike occurs when the wastegate opens but manifold pressure quickly levels off to a constant regulated levell. This is a true manual wastegate controller, not just a bleed off valve. Sustained boost pressure regulation is not as reliable using a bleed off system and these systems tend to allow the wastegate to open prematurly. Bleed off valves do not actualy open th wastegatet at a preset level. A bleed off valve can only temporarily "trick" the wastegate into seeing less pressure than actualy exist. Eventualy the slight amount of air that does get through a bleed controller will trip the wastegate open.

OPERATION: Pressure enters from the hose barb on the right side of the drawing. A ball bearing is pressed into an airtight seat formed in the threaded end of the inlet hose barb preventing boost pressure from passing through the controller untill manifold pressure is sufficient to overcome the preload of the spring and push the bearing off the seat. To increase the boost level, just add more preload to the spring by adjusting the screw. It may seem that the bearing would serve as a check valve trapping pressure inside the wastegate once it opens. However, external wastegates are constructed with a poppet valve stem passing into the pressurized diaphragm chamber (like an intake valve and valve guide). Gas leaks by the valve stem quickly enough to allow the wastegate to quickly bleed off pressure and close once manifold pressure drops. (or install a small air bleed between valve and external sealed wastegate).

CONSTRUCTION: Use a 1/8" NPT brass 90 deg elbow as the body of the controller. Alternatively, a 3-way brass "T" fitting can be used and actualy makes fabrication simpler. A hole must be drilled and tapped into the elbow fitting to allow the adjustment screw to thread into the fitting body dead center with the bore that the spring will go in. If the hole is off center, binding of the spring may occur and cause erratic adjustment. Using a "T" fitting instead of an elbow, you can use the existing 1/8" NPT hole to center the new adjustment screw threads. Just tightly screw a male 1/8" fitting into the hole where you will place the adjustment screw, cut off the protruding part of the male fitting and drill and tap the inside diameter of the remaining threaded portion to accept the adjustment screw.

The end of the adjustment screw should be filed so as to form a "shoulder" (removing the threads) so the spring can slip over the end of the screw but is stopped by the shoulder. Be shure the spring does not bind on the adjustment screw. A pre-load spring is selected that has approx the same OD as the ball bearing diameter so that the spring can push against the bearing. The bearing should be slightly larger than the ID of the inlet hose barb (threaded end). To form a airtight seat between the inlet hose fitting and the bearing, just securly hold the fitting in a vice and place the bearing on top of the threaded end of the fitting. Hit the bearing with a hammer to press the hard bearing into the soft brass so as to form the seat. Repeat hitting if necessary untill a wide seat is formed in the fitting and test for airtight by sucking on the fitting (inlet end) with the bearing in place.

You may need to try several different stiffness of spring to achieve good adjustment range of pre-load pressure. The spring should not be so stiff that the pre-load increases dramaticly when adjusted and it should not be so weak that the spring must be"stacked" to achieve sufficient pre-load.

You must bench test the controller. You are depending on the controller to regulate the boost to the engine so enshure that it is working properly and reliably! Use a high volume pressurized air source with adjustable regulator attached to the inlet to simulate manifold pressure. Start with the regulator pressure adjusted to 0 psi and slowly increase the regulator pressure. No air should pass through the controller untill the preload on the bearing is overcome by the pressure. Make a graph to calibrate the # of screw turns needed to adjust boost by a given amount. The boost controller can eithor be installed in the engine compartment or in the dashboard for on the fly adjustment using longer lines.

Update: I found a webpage of turbo chrystler modification that describes using a similar wastegate control setup with pre-made parts. It also has instructions for using this setup with an internal wastegate by installing a small pressure bleed after the controller so to allow the wastegate to close (since internal wastegates don't leak pressure like external wastegates). Check it at Using a Grainger valve to control boost.

Update-update: I found Dawes Devices website that sells the granger valve controller fully assembled for . Not much more expensive than buying all the brass fittings like I did.