The only reason I can think of why someone would do a Pos Disp blower and centri would be to get all that down low torque from the Pos and top end from the centri.
I can't think of a singe reason to run two centris unless both are tiny and spool up very quickly.
Agreed.
One day, I want to build a compound boost 03 Cobra (with the Eaton and twin turbos). Port the blower to the max, and leave the stock size pulley on it. That way, it would be as efficient as it could possibly be, and should help keep the IATs down. Hit it with some methanol if needed to keep the IATs down.
I remember a BIG argument over on SVTP (or might have been HPJ or YellowBullet, don't remember) a couple years back about compound boost. One argument was that compound boost is dumb because it's inefficient, and you can make more power with the turbos, and it's too complicated, and blah blah blah. The other is that the peak power isn't that big of deal, and that there will much more low end torque with the blower, and that it really isn't more complicated in the end, and that it will be more fun on the street.
Well someone actually took a twin turbo Cobra and tested it with and without the blower. In the end, it only took 4 more pounds of boost on the compound setup to make the same peak power as the turbo-only setup; in other words, it was only 4 pounds of boost worth less efficient. To me, at 30+ pounds of boost, that's not a whole lot, especially when we're talking street car. What was a whole lot was how much more torque the compound setup made before the turbos spooled. And it was a LOT. 150+ ft-lbs of torque kind of lot.
My opinion/conclusion: sure, the Heaton adds a lot of inefficiency to the setup. At the same boost level, the turbo-only setup will make more power. No one can debate that. BUT, is that slightly better efficiency worth the lack of torque before the turbos spool? I guess that's up to whoever owns the car, but to me, the answer is no.