What a bitch… is that crazy or what? In hindsight, it explains virtually everything that happened during the months of fruitless troubleshooting:
1. MAP-drop never happened with the engine cover off (when I did my 12 runs in a row and deemed it fixed due to an “rpm limit issue”).
2. It never happened with the car on the dyno because the engine cover was always off.
3. The scoop collapsing shut caused the air filter to get pushed off its mount – it was never an air filter issue.
4. Last week when run without the air filter element, there’s apparently enough leakage between the air filter and engine cover that it can’t develop enough of a suction to pull the scoop shut.
5. Upgrading the wastegate made no difference.
6. Removing the muffler had little effect – if anything it made it worse.
Before realizing what was happening, it seemed like the air filter element was getting sucked down onto the inlet bell. A K&N filter had been used before, but was upgraded to a 33-2135 unit, designed for the 403 hp late-model Chevy truck. That’s close enough in power that it should do fine, and in addition, a cross-support was added across the airbox below the filter. The K&N is roughly 1″ thinner than the enormous Amsoil unit, so that’ll improve air flow heading into the bell-mouth inlet.
I can’t begin to explain how much of a relief it is to FINALLY have this brought to light. Of the dozens of suggestion I received, not one suggested a possible filter or scoop issue. Needless to say, the air scoop is coming off until a real one is fabricated.