My vote is for a twin-screw Positive Displacement Blower. Both the Saleen, and KB are twin screw designs. The adiabatic efficiency of a twin-screw far surpasses the Roots or the Centri style. This is the overall effeciency of the blower. Basically it's ability to make hp with less boost/less parasitic load. The only other power adder that comes close is a turbo setup.

As for the Roots style, it is also a PD blower, but an older design. A roots is a "looser" fit than a Twin-Screw blower so it has less drag under cruise, but has more "blowby" during high boost/loads, so it's less efficient under load. Anyway here are some comparisons for you:

1. Twin Screw pros:
a. Mild mannered, easy to drive fast, very predictable, linear power curve
b. Quieter in non-boost situations
c. Huge low end torque (400 ft-lbs at 2800RPM) no need for lower gears and lower MPG
d. More usable power on the street due to torque and linear power curve. On 90% of my "Street Races" the other person gives up in the first 100 ft because I "Frog hop" them sooooo Bad. I usually start at 2000 RPM in second gear and the race is over by 5500 in second!!! I never have to go over 75 MPH!!
e. Easy to install and maintain. ONE belt and oil change every 50K miles.

2. Twin Screw Cons:
a. Expensive 4-6K for basic setup
b. heavy. Adds 50-60 lbs to front end of car.
c. Harder on driveline components. Huge low end torque is a bit more stressful on stock components.
d. Hotter. Engine will suffer a bit from "heat Soak" due to extra metal casing from S/C
e. Less MPG. Expect 1-2mpg less overall.
f. Limited High HP potential. (There the law of diminishing returns)
g. "Seem" slower because of very smooth, linear power delivery.

Centrifugal S/C Pros
a. lighter (smaller components)
b. cooler. (also do to smaller components) Less heat sink
c. Less expensive ( about 3.5 -5.5K for good kit)
e. Higher HP potential. (Centi S/C is basically a belt driven Turbo)
f. Get a "Rush" when the power comes on

Centrifugal S/C Cons
a. very troublesome to install. (Many pieces, hoses clamps, fittings, brackets, etc) Look at some of the pics of a Twins screw kit and then look at a Centi kit.
b. higher maintance costs, and time. Lots of fitting, clamps, brackets, etc to get loose.
c. Lack low end torque. Have to "spool up" to get power.
d. Less driveable, more unpredictable power delivery. Harder to control especially in traction limited invironments. A twin screw with good tires will have better traction even though there is more torque because the power delivery is even and controlled, not "on -off" like a centi, or a turbo.
e. Noisy. (All around the loudest of all three discussed here)

Turbo Pros:

a. Cooler (turbo is not doing much when not in boost
b. better MPG (no parasitic draw when not in use)
c. Very high HP potential (U can really turn up the boost on these!)
d. Quiet, even in boost they are very quite
e. Driveability is good. They do have some of the on-off like a Centi S/C but can be controlled easier with boost controllers.
f. Not as hard on engine, driveline components.


Turbo Cons:
a. heavy. Usually as much as twin screw because of all the plumbing/piping involved
b. very troublesome/time consuming to install
c. Also more maintenance hungry. (pipes fittings, hoses etc)
d. No torque down low. (have to spool up).
e. Expensive. Good turbo kits are 6-8K
f. harder to tune

So there you have it. Basically it really boils down to what you want to use the car for. If you want a fire breating 1/4 mile beast, get a Centi S/C, you'll get the most bang for the buck, but loose streetability, and will work on it much more. If you want quiet, super High HP, best MPG, then get a turbo. If you want a "street killa" that will kick most people in the ditch and still provide good daily "Stock like" driveability, and be the most maitenance friendly, get a twinscrew!! I've owned turbo cars before, and I'll tell you, the twin screw is the best fit for all around daily street use with the occasional 1/4 mile jaunt. I wouldn't have anything else.