ok i dont really know if this belongs here or not,but i thought i would give it a shot anyways
im int he process of building up a blown 383 on DD2000 using a BDS 6-71 blower.but i dont know what to use for the flow numbers and the efficiency numbers.what does a 6-71 flow??it automatically comes up with 350cfm,but that doesnt seem right seeing as how i have it setup for 800cfm intake flow.and how efficient are they?it gives a figure of 55% automatically,which seems like it could be about right.also with the belt ratio,what should i use for 10 pounds of boost?right now its at 1.00.if anyone can answer these questions feel free to hit me up on AIM at RSCruiser91.i would appreciate it

 

The efficency of a roots with about 55% is right about where it should be. As for the flow that is something that the manufacturer has done. If you call them up they should be able to give you flows at different blower and possibly engine speeds(from the pully sizes). Use the flow at the max rpm you intend to run the engine to.

 

ok thanks alot for the info man i appreciate it!!

 

What version of desktop dyno are you using? I have v3.10 and whoever programmed the roots blowers doesn't know crap about how they work.

On my version I set the flow rate of the blower at 1000 CFM, leave the pulley ratio at 1:1, efficiency at 55% and the boost limit at whatever you plan to run. That's the only way I've been able to get "sane" results from my version of DD for a roots blown motor.

Desktop dyno will NOT help you select a proper pulley ratio. That's something you'll have to figure out on your own (or with the blower maufacturer's help).

If you shoot me an email I can send you DD files for my 383ci Weiand 142 roots-blown engine. After I figured out how to tweak the blower specs I finally got a torque and HP curve out of it that almost exactly matches the actual engine's dyno performance.