Squirrel cage fan performance (some sheet metal maybe)

I have small squirrel cage fan salvaged from and old disc drive from times
when they had 4 or 5 12" plus removable discs on a single spindle, about 5
mB IIRC model BK5 I think. I have been using this in a homebuilt cyclone
dust (wood) collector with quite good results.
I think I may be able to improve the fans through put. The intake area
around the motor is 5.7 sq.in. The exhaust port is 3.75 sq.in. Originally
there was a rectangular opening about half way around the spiral housing
with area 2.625 sq.in., this has been blocked. The total exhaust area would
have been 7.375 sq.in. I guess the pressures would have been different.
Should the exhaust port be opened up to equal the intake area? Something
different better?
I experimented with the clearance between the fan and the start of the
spiral housing at the close edge of the exhaust port, closing it up seems to
make little difference to the air flow, however the noise goes up
considerably as the fan is approached, a la air raid siren I suppose.
Comments, advice please, John.
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