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Loosen the throttle cable a bit so you don't get WOT until you are at the HP you need. Then run the WOT map for the computer on a vacuum switch that kicks in at say 3 hg.
I cut the belt on back-to-back dyno runs and saw a ~7 hp increase on a '86.5. That was a one time test, not sure what an average would be.
Thanks for the data point but 7HP for the fan? I think there's run-to-run variance in there.
Check out the specs for warehouse fans. Really big ones move a lot of air with 1 to 2 HP specs. Also, consider the stock electric fans. A 12V 20A circuit can produce 1/3 of 1 HP maximum.