Tom is correct by the calculations. there is alot more invloved in determining how much boost is being generated by the blower....
many things effect the amount of boost a blower is putting out....
* type of blower,teflon stripped or not, tolerances,how old the blower is.
* the way the engine is built, heads,cam etc....
* and of course the gear drive ratio.
when you speak with a Blower company like BDS etc...they will give you a ball park of what a certain drive ratio will make on a certain type of their blowers ..when they came up with these #'s they used a base line small block/Big block with their recommended blower cam with un-ported head and ported heads...and they designed their boost charts..(slide chart)
example:" when i ran my Alochol blown injected small block.with a BDS stage 4 competition blower...BDS said the drive ratio i had should make between 14-15lbs of boost....well i had extensive work done on my motor and it made a tad over 17lbs of boost on the dyno...
so you cannot know for sure what a drive ratio will make..they can only get you in the ball park with their Slide chart..
by re-reading your intitial post..you need to INVERT your question and ask yourself how much boost you wanna make ( taking everything above into consideration) then you work toward your drive ratio..
i get the feeling you have a basic small block..next to stock.....without getting into a nuch of other stuff a blower motor musy have ( cam,heads,headgaskets, etc...)...i will say no more than 7 to 9lbs of boost tops on a stock type motor
i hope this helps you...Zip you have any input here
Brian
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Bjuice..
"I'M YOUR HUCKLEBERRY ! "