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Exploded Turbo Diagram

Turning Up the Boost

OK, here's how it works. When you have your fuel switch set for regular fuel, your boost should only go to about 9 psi. That is the pressure your wastegate is designed to open at. If you simply ran a single hose from the high pressure side (outlet) of the turbo down to the wastegate actuator diaphragm, this is the maximum boost level that you would always have. This type of system is shown in the illustration above, one hose goes from a connector on the compressor housing of the turbo at the upper right of the drawing, and carries the air pressure to the actuator diaphragm at the lower right. All 2.3T cars without the regular/premium switch use this type of system. If you own a car with this type of system, you will need a tee from one of the systems discussed below, or an aftermarket bleeder valve or boost control system if you wish to control your boost level.

If you look at the hose(s) on your turbo, you will notice that there is a "T-connection" (two outlet connector) on the compressor housing of the turbo (if you have a regular/premium fuel switch). Besides having one hose connection to the actuator diaphragm (as shown above), there is a second hose leading to a solenoid on the fenderwell. The other side of this solenoid attaches to a third hose going from the solenoid back into the intake air before the turbo. When your fuel switch is set to premium, and the computer is happy with the signals from the engine's sensors, this solenoid opens. The passages inside the "T-connection" on 84/early 85 SVOs (and reportedly on the 87-88 Tbirds with automatics, too) are sized so that an exact amount of pressure drop occurs in the line leading down to the wastegate actuator when there is flow through the solenoid. This allows the boost to go higher before the wastegate "sees" it and opens. Because the amount of additional boost allowed is set by the sizes of the air passages in the "tee" in the early SVO (and late Tbird automatics?), the solenoid could stay open all the time (or be totally bypassed), and boost would not exceed the amount that the system is designed for, about 14psi. As long as your boost is greater when you have the switch set to premium than regular, this part of the system is working properly.

What you want is to allow the solenoid to bleed off even more pressure. The easiest way to do this is to drill out the passage in the "T-connection" that goes to the solenoid slightly larger. You want to do this one drill bit size at a time until you get a little more boost than you want. A little goes a long way! Then you buy a little plumbing valve with hose connections, and install it in the hose to the solenoid so that you can restrict the flow manually to get exactly the boost you want. Also, if you want to keep the computer from turning your boost down at any time, just bypass the solenoid.

***Note***: This was written for the early SVO (and late Tbird automatic?) version of the motor. The other version (85.5+ SVOs and late Tbird 5 speeds) uses a slightly different system. In this case, the "T-connection" is already sized for maximum boost and does not need drilled out. Boost is instead controlled only by rapid opening and closing of the solenoid, with the duty cycle (proportion of time open) doing the boost controlling. Bypassing the solenoid, and using a valve in its place to restrict the flow through the bypass hose will have the desired results, no drilling required. **Warning** Bypassing the solenoid on this type of system, without a restricting valve in place will cause an uncontrolled overboost condition.

Another note: everything I've told you is the bogus cheap way of doing things. The good way is to spend $500+ on an electronic boost controller that will do all this better, and is cockpit controlled. Also available is the Hallman boost controller (under $100) that works similarly to the stock system, but uses a check ball type valve to avoid the mechanical problem of the wastegate beginning to crack open prematurely. Mark Peters (a list member) has also come up with an idea for using the stock boost control solenoid to generate and control higher-than-stock boost levels. One more promising possibility is the APC system found on Saab turbos that control boost based on knock sensor signal.

For more information on the workings of the boost control system, see Mike Fleming's writeup on it (courtesy of the Texas chapter of the SVOOA).


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Page last updated: Friday, 28-Oct-2005 11:26:02 EDT