I keep hearing about this type of setup people do. I know Hellion has a TT setup for Mustangs they call the "Hell Raiser," but they only mention the "possibility" of adding a s/c to it. Anyone seen this done? Does adding a turbo to a twin screw or roots style blower really help performance? If so, how?

 

its called compounded boost, the roots style blower helps creat low speed boost while the turbos work on hte big end. Not to sure about all the details, i do know that tested on a terminator it made something like 1000 hp, and torque

 

It's called twincharging, it does help performance if you take advantage of it. Running a roots supercharger along with the turbos will let you pick a bigger sized turbo(s) for even more high end efficiency. The supercharger will take care of low end, the turbo will aid on high end. The older (3 lobe/60 degree) roots have a problem with becoming inefficient at higher pressure ratios so twincharging will allow you to run higher boost #'s while maintaining a lower pr on the roots. Most kits are designed with the turbocharger feeding the roots, the wastegate line is plumbed to after the supercharger so it gets the final psi. Boost pressure will compound in this setup, so if your roots is pullied for say 8psi max & your turbo is pushing 4psi, you'll have a total of 12psi at the intake.

One builder claimed that on high end the turbo's boost actually pushed the roots rotors to put hp back into the crank, the problem with that point of perspective is that the roots is a positive displacement which means it will be compressing air to a certain pressure ratio, even with the turbo. Because of the bigger turbo you will have a very efficient turbo at high end, but the roots will still be dragging down the engine at higher rpm. The only way around this is to install a pressure activated bypass valve before the supercharger so that once the turbo spools, the valve will open to equalize pressure before & after the roots to eliminate parasitic drag (it won't completely eliminate it, it will still probably require .2 - 2hp to drive it, but compared to a normal 50-60hp, that's minimal. You will need a second bypass valve controlled by intake vacuum to retain good gas mileage aswell.

Most people wont do this because it isn't practical as far as cost goes, nor is it a very compact setup. On top of this you have added weight which will add up quickly