For the benefit of others suffering from this affliction, I wanted to circle back with an update . . . and probably call this solved for me. As discussed, getting the car tuned and having it gear down from 6th to 5th a bit quicker is a nice fix.
To start with, with just the tune, the issue was mitigated for me; it pulled so much more effortlessly in 6th gear anyway, it really didn't have an opportunity to go into 'flutter' very easily. I know this is the opposite of what some others experienced with tunes.
But there were some conditions in which it still fluttered. To rid of it completely, it took several iterations from Torrie on the downshifting. He tells me the downshifting is a function of output shaft speed (OSS) to throttle angle. In the end, I had to data log the car with these PIDs, along with those necessary to show the flutter, e.g. boost and wastegate. Torrie was able to see the OSS to throttle angle ratio when the car fluttered, and make it so the car would downshift directly before that event.
Even then, the first stab using this technique had an issue -- the car wouldn't go to 6th until 62 MPH. That was too high for me, as I do way too much highway driving to have the car hung in 5th at 55MPH. So, he made some more changes, and the car will go into 6th if at 54+ MPH. Even when completely off the throttle coasting from higher speeds, it will downshift to 5th when it hits 54ish, which I can live with.
And, I have been unable to get the car to flutter no matter how hard I try. As I start up an incline, if I am on the throttle light enough to avoid a downshift, it is not enough to make the car flutter. If I get on the throttle enough that it used to flutter, the car gears down to 5th instead. I'm sure it is costing me a little gas mileage. But, on my commute going 65 MPH over mountains, I am still approaching 29 MPG, so I won't complain.
Still data logging and working through a couple more quarks with the tune, but at least this bit is solved.
Side note on the cruise control: I can confirm that cruise uses the same shifting tables. I know because if I set the car on cruise control at 53 MPH, it will stay in 5th gear.
But, interestingly, the computer does everything it can to avoid downshifting to 5th when I set the cruise to 54+. Even if I set the cruise on 56 MPH and go up a mountain, the cruise is on the gas so light that it won't gear down. If I am driving without cruise, it is really, really tough for me to not progress the throttle at that speed (to compensate for the hill) without making it gear down. But, the cruise somehow does it. I can't complain though -- it still manages to keep speed (thanks to the power from the tune) while not hitting the throttle hard enough to flutter.
Bottom line, it is my theory that cruise uses the same shift tables, but the computer adjusts its throttle usage accordingly as to avoid downshifting if at all possible. Just a theory.
