Seems like you guys have talking this topic out pretty well.
I would say from my experience, the next setup I do will probably have a V60 gate on the second stage and a V44 on the first.
Why gate both? Because as Mike said, so you can decide your balance, aside from merely controlling manifold pressure. If you just pull fuel to control boost you will never make the same power without overboosting, and more importantly, won't have any way to alter the amount of PR coming from either stage without physically pulling the housing on the first stage and swapping it out. Even then, you will likely never find one large enough to actually control it. Bare in mind.... once the second stage pumps up the boost to the engine, that first stage charger is basically being driven from a 900+ cube engine in terms of how it reacts. Good luck finding a housing to keep that in check while allowing you to run a PR balance of your choosing between chargers.
As for sizing the stages.... the 66mm second stage seems to be a winner. It will jump up good on a 7.3 and make boost comfortably, as well as handle power output needs at rpm ranges below the point where the first stage lights off easily. And that's all it needs to do. Total power and such don't play a role in selection of the second stage charger. Make it jump up on boost to get your truck out of the hole and then gate accordingly. Almost any charger can handle the duties of the second stage on one of these engines. There is nothing to be gained from a larger unit aside from more spooltime at the power levels we're discussing here.
For the first stage, it must be sized for the boost you intend to run, at the rpm you intend to make that boost, accounting for any intercooling.
My first setup used a GT4718 with an 88mm inducer and a ~93mm turbine inducer. It would run 1:1 boost drive or better up to 60psi on the manifold. Beyond that it would fall short, with boost:drive coming in at say 80lbs drive to 70lbs boost IIRC, getting worse as you drove it harder. The GT47 turbine was TOO SMALL for the exhaust flow.
Going to a GT55 with a 94mm inducer and 112mm turbine inducer the boost jumped from 65 to 80+ on the exact same wastegate settings. The turbine is SO much more efficient. I haven't checked the boost:drive, but I know it's solid well above 70psi now. Probably 1:1 to 80 or so if I had to guess. maybe better. And the spool is almost exactly the same. You can tell that it's a bit off from the 47, but it really doesn't matter because the tires are already spinning anyway...
Sizing the intermedite pipe (between second stage turbo turbine and first stage turbo turbine) I would use a 3". I went 3" out of the 38R and then 3.5 after the second stage gate dumped in and it's probably too large. It also doesn't fit well in a T-6 flange. Just go 3" IMO. Keep velocity high.
If at all possible, cool both stages.
On Edit:
I would have added tons of pictures but Webshots died and all my pictures died with it.
oke:
