Originally posted by MIKE FREEMAN
ON 84 AND 85 TURBOS THERE IS A VACUUM LINE TO THE BYPASS VALVE ON THE COMPRESSOR HOUSING,THIS BYPASS ALLOWS THE COMPRESSOR SPIN-UP WITH OUT CREATING BOOST IN THE MANIFOLD DURING E.G.R. "ON" PERIODS.
Correct me if I am wrong. I thought the bypass valve on the 84 and 85 years is for California only. I am in California and I never have a chance to look at a non-Calif model to verify it. The bypass valve is part of the smog system controlled by an electronic module. I heard that it delays the turbo boost until ~2500 rpm. I do not know how it is related to the EGR but I know the fact that if the vacuum line is disconnected, the car seems faster.
The reason people want to adjust the wastegate because the wastegate is either below specification (due to weakening of spring) or not at the upper limit of the specification. You want to adjust it to the upper limit so you can get as much boost as possible. But the adjustment task is not easy. A work around method is to install a manual boost controller as some of our members here doing successfully few months ago. I am thinking of doing it too instead of adjusting the spring.
The wastgate on these old diesel engines (617) is just mechanical, no electronic stuff involved. I think the way the manual boost controller work is to control the pressure line and allows the pressure to reach the wastgate only when the pressure reaches the upper limit set by you, therefore, prevents the wastgate from open early. Once it is open, turbo boost stops.