Frank,
On the vacuum modulator setups...
I have a theory as to why they may cause issues.
Vacuum modulated line pressure has worked for years on the older 3 speed units without issue. I do manual/auto Th400s often that are in the 650-800+ HP range and they are living fine (with weekly abuse at the track) despite some other builders claims you must have a full manual VB at these levels.
So the principle is sound, at least for these units.
What I suspect could cause an issue on a high powered, frequently raced, 4L60e with vacuum modulated line, is that at the end of the run, most people will leave it in 3rd, letting it engine brake to decel.
When you let off, the vacuum system drops line to minimum pressure, but in 3rd you must still hold the over-runs and 3-4's for engine braking.
I think the loss of line on decel (high vacuum), causes the clutches to slip.
I haven't looked at the OEM calibration for one in depth yet on decel. I'll do that next time I get a chance.
However I can see the OEM keeping higher line in manual 3rd on hard decel.
The vacuum modulator system would be stupid in this regard because it is running things solely on vacuum, which would be maxed on decel.
So the minimum pressure with the vacuum system would need to be high enough to keep the frictions alive on decel, and yet not so high under zero vacuum condition to cause other issues.
A very good trans guy once taught me to think about the downshift and decel side of things. He said more trans failures are caused by downshifts than upshifts....
In older 3 speeds, I do prefer a full manual, if the car has forced induction, or a really large cam. The modulator system starts getting unstable with those combinations.
The things you're looking at in the vacuum modulated 4L60E's, are some good points. These are some of the instabilities I was referring to.
We could probably fill a few pages with how vacuum modulators work different than an EPC, but the last sentence will always be the same....Vacuum modulators don't work as well as an EPC to control pressure. Again, all the theory and technical points don't mean anything when you've got a bunch of burnt up transmission parts because of a vacuum modulator. It's one of those "it looks good on paper" things. It SHOULD work, but it doesn't.
Frank
CPT