I know this is an old forum, but i figured I'd post anyways. There is this banks diesel sequential super turbo duramax, you'll find with google. They take a duramax, put twin turbos at the back of the block, which are liquid cooled, and that feed into a super on the top of the block, which has another liquid cooler after the super to cool the pressurized intake air. This allows this diesel to get boost at the low end, hella torque right off idle, but also build pressure as rpm's increase and keep pumping more and more power. I think they use a twin screw supercharger, which is bad ass. The setup at the beginning of this post is a nice setup when you wanna make a cheap factory charged system that will produce strong reliable mid range power, but isn't capable of using a very large turbocharger efficiently as air intake requirements of a larger turbocharger would begin to exceed the limited output of the small roots blower. But for a commuter/light sport car on the cheap, it's a good setup. However, the roots blower and the twin screw are a bit different in that a roots just forces air into the intake, as the rotors grab and push more volume of air from the ambient air pressure supply, and force that volume into a smaller volume inside the engine intake. However, the screws of a twin screw blower do the same principal, but they squeeze the air as it passes through the screws, it forces the air through a narrowing volume of space, rather than a sudden passage into a denser space from a root blower. So the root blower not only is less efficient by creating more waste heat in the intake air, but a twin screw setup would take a pressurized air charge from turbochargers and mechanically compress it just a bit further much more efficiently than feeding a turbo into a roots blower. It's a bad ass setup, gotta be like 700-900 horsepower and maybe 1400-1700 foot pounds of torque. They put their own heads on the duramax that swirl the intake air to get it to flow more efficiently, as well as their own pistons and rods on the stock crank. Their own valve system components. And it's actually a marine motor, a hell of a diesel boat motor. You could probably if you had the dime order one built to be a drag motor, not a boat motor, and they've hinted that they want to sell a drag race motor package in the future, they've got like 2 or 3 drag cars they built with this setup themselves. It's amazing how clean they get their diesel to burn too. Reminds me of fishcer troph gas to liquids process which is a technology only just beginning to be implemented in like two or three large scale plants, you take methane or natural gas from the ground and work it into a liquid with some processes, a liquid diesel and jet fuel, that's hella clean, and not too expensive, just takes investment dollars to build the plant, but the natural gas can be had for cheap.
Also, a similar setup that might be better for some applications, would be sequential turbocharging. You actually feed the exhaust pipes first into a small turbocharger that spools quick so you get boost not long after you hit the pedal, and then the exhaust from that turbo is fed into a bigger turbocharger. This bigger turbo charger doesn't spool so fast, so as you begin down the drag strip, it starts spinning just slowly enough to add maybe 2 or 3 pounds of boost to the ambient air pressure, which this larger turbo's output is plumbed into the smaller turbo's intake. This smaller turbo takes this slightly compressed air and adds some boost pressure to begin climbing the rpm power band. As the rpm's increase, the smaller turbo reaches it's peak pressure pumping, and the big turbo begins to add a little more pressure to the intake, the problem with this setup comes when the system begins to reach higher rpm's. You gotta figure out a way to have your smaller turbo not get too worked by spooling too fast and such, and it can't end up being a choke point that the bigger turbo is trying too hard to force air through the choke of the smaller turbo, so needless to say, the right size and setup for the turbo's is necessary. It think this is a setup where very good blow off valves, and possibly a setup where the plumbing into the first turbo, can have a second alternate route around the first turbo and directly into the second larger turbo, so that when you reach high rpm's, this second exhaust passage can be cracked open and then opened all the way to divert some exhaust gas around the small turbo so it doesn't over spool and melt the bearings.
This is where it gets the trade off that a twin screw supercharger instead of a small turbo can provide a more simple setup, but the high rpm power might be hampered a bit by the supercharger, and a twin sequential turbo setup might do similarly well in the low and mid rpm's, but if you can figure out how to make high rpm operation work smoothly and correctly, it would make more power than an engine who's power is being sucked into fastly spinning the super charger.
however either setup built properly would be bad ass. I personally want to build a super turbo setup using Chevy's new LSX Iron block. Iron casting is heavier, but stronger, along with head bolts instead of 4, and many other things about the block, it can handle a hell of a lot of boost and power capability. There's this supercharger, kenne bell is the company that makes some wicked twin screw superchargers, their small and medium chargers are just a little bit bigger than the other brands of twin screw chargers, and they have a gigantic charger that is considerably bigger than any other made. They have a 2.8L smaller size charger, a 3.6, and the big one is a 4.2 liters of displacement per revolution. Whipple chargers makes like a 2.6 and a 3.3, but nothing in the high 3 liter capacity. Anyways, build a LSX motor with a very low compression ratio, slap a 4.2 liter super and a big turbo, you'll have a hell of a motor that should be capable of 1500 horsepower and 1300 ft lbs at least. There are setups using this block and just big turbo's only that make 2010 and 2020 horsepower on the dyno. Absolutely insane block, it would be insane to put this super turbo motor in a Ford Gt, and take the GT motor out and build a light weight 4x4 with it's lighter aluminum block that will pump high horsepower numbers, but not quite as much or as reliably as a properly built LSX.
I ramble......
---------- Post added at 06:14 AM ---------- 6 hour anti-bump limit - Previous post was at 06:06 AM ----------
Also, the idea of disengaging the super at high rpm and letting the turbo do all the charging isn't undoable either. If you had a proper clutching system on the super's pulley, and had a way to change the intake to the turbo from feeding from the super, and then switch it to feeding just form another air intake path, and you engaged both the supercharger's clutch to stop spinnin the super, and at the same time let the turbo start breathing from the air and not the super, it would work right rather welll. At about the rpm's where the turbo charger's intake is sucking air at the same rate that the super can feed it, the super charger isn't doing any more work compressing the air, and it is thus just sucking energy from the crank shaft. The guy writing about changing the timing because the boost pressure might drastically change when you shut off the super had a good sense of a problem, however, the correctly sized and designed setup should opersate smoothly as you wouldn't shut off the super charger until doing so would actually drop the boost pressure hardly at all, as the turbo is doing at least 90 percent of the pressurization at this point, shutting off the super wouldn't cause a drop in pressure and a needed change in spark timing, there would be a slight change but it should be subtle enough to not be harmful. But there would be a subtle but slightly noticeable boost in power as you shut off the super and that drag on the crank is released, the crank would start powering the wheels a lil bit harder, you'd feel a very slight bit of extra pull in the seat for a second. you'd have a little bit of a power bump, that would smooth, and then you'd keep building that power as the turbo kept spooling higher to it's peak pressure. Be a hella fun setup.
n lotsa GREENS too!! 
hehe
