Back-filled 2011 ; all early posts are under the “Archive” link above.
Disconnected the upper wastegate hose to ensure there’s nothing keeping it from working properly but boost creep persists. I had a talk with a buddy who really knows turbo:
“You increased compression 8:1 to 9:1 (~12% more flow), right?” Yes
“You added cams that flow more air.” Yes
“You switched from a 0.82 AR turbine housing to a 1.06.” Yes
“You’re running E85, which produces a lot more exhaust gas than gasoline.” Yes
“You built an exhaust header with poor flow to the wastegate.” Um, yes
He went on to explain that the 1.06 AR turbine housing is far less restrictive to exhaust flow. That’s a very good thing but means that given the choice, exhaust gas would much rather go through a low-restriction turbine housing instead of making a hard turn into a smallish wastegate. He recommended one or more of the following:
Change the geometry of the header to provide a more direct shot at the wastegate: This is the right way but there’s no room
Add a second 38mm wastegate: There’s room, just
Replace the single 38mm wastegate with a 60mm unit: The “nuclear option.” Builders use these to regulate 1200-hp engines, but much more often, as a Band-Aid to limit boost creep caused by poor wastegate routing. Not an option because chassis tubing blocks the area.
Of the above, adding a second 38mm unit works the best short of starting over. For what it’s worth, two 38mm wastegates have the equivalent valve surface area of a 54mm unit, so it’s not vastly different than going with the 60mm unit.