I love Roots blowers, the main advantage is the Instant torque at any RPM, no need to have any sort of high stall converter. The key is you are very seldom going to need to go to WOT and maximum RPM but if you do you want some sort of boost retard which is really your safest bottom end protection.

Very good cooling is also essential, you may be surprised how much heat even an underdriven GMC style blower builds, those ribs are there for a reason. I would probably limit the boost to maybe 7 or 8 pounds which basically increases your effective displacement another 50% in terms of torque production.


I would like to do a 470 low deck with a 2.200 journal 3.90 stroke and a 6.800 BBC rod but to use an RB manifold you would need to run some sort of Stage VI head....so to run an RB I would still do a 3.90 stroke but with a 7.100 BBC rod. A girdle would be addeded insurance but for me strong but lightweight internals are essential because the RATE of ACCELERATION of the crank under load goes way up in a boosted motor.

For a blower set-up I like to run another 1/2 a thou of P/W clearance with the finish hone over the piston MFG spec with a fairly long skirt, piston rattle at TDC and excessive rock at BDC under load is what you want to try to minimize at all costs. Run a BIG radiator and aluminum heads and to save the bottom end, keep the max RPM down under 6000. The problem with boosted motors is they make so much power it's easy to overrun the safe zone of the block because the power doesn't percievably drop off in terms of the Seat of the pants, however, even though you don't feel it you are still transmitting all those high torque + High RPM stresses into your block. Methanol/water injection is also a great investment, keeps the blower cooler and the tune safer.

An 8-71 would probably run at least 10-15% under-driven. key is to incorporate some boost limiting is not as easy to accomplish with a wet (fuel through the blower case) system, if all the fuel was injected below the case (as in a modern OEM like the hellcat) it would be easier to dump excess pressure. The thing to remember is you only boost when you give it enough air to make boost so in a way, limiting the carb max CFM can also keep your bottom end safe. But I would do this with a large carb with a throttle stop rather than too small a carb for the street. But in safe race tune with race gas you can always back the stop off.

Have fun! I'd love to do another Blower motor someday, I always had a blast.