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Old 09-14-2011, 05:52 PM
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Another Question for the blower guys

I think I have a small bore 6-71 which would explain my low boost. I pulled the carbs and plate off the blower today and took pictures. With limited information on the internet about the differences between small and large bore 6-71's. I measured the case thickness and it was just over an inch. If this is a small bore blower then the mystery is over, if not then im back to the drawing board.
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Old 09-14-2011, 06:07 PM
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From Blower Drive Service' website....
"There are two different 671 blowers commonly used. They are the larger diameter blower and the small diameter blower. The large bore blower (case thickness 1/2"-) is approximately 11 percent larger in volume than the small bore. The small bore blower (case thickness 3/4"+) is recommended for 370 CID engines or less for street applications. The larger bore blower is recommended for 371-500 CID engines in high performance, racing applications."

How many square inches of filter area do you have for the carbs to breathe through? I would want at least a 4" x 14" element. If the motor can't breathe, it can't make boost.

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Old 09-14-2011, 06:14 PM
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Thanks alot tech inspector. Thats about all the information I could find. I also called BDS and they said a Weiand could be either one. If I did my measurements correctly then I definetly have a small bore blower. Just wanted a second opinion.
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Old 09-14-2011, 06:42 PM
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If it's a Weiand, then it's not a 6-71. It's a Roots type blower, but not a 6-71. Weiand rates their blowers by the amount of air displaced for a complete rotor turn. GMC rated their blowers by the number of cylinders and the displacement per cylinder.
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Old 09-14-2011, 08:16 PM
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by the measurement you gave and the length of case for the rotors is 15" you have a small bore . just spin it a little faster
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Old 09-14-2011, 09:05 PM
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Thanks a lot 1932 bantam. I thought it was a small bore but people usually look at me like I'm crazy when I say small bore and large bore 6-71. Anyway I will pick up another pulley and try to get a couple more lbs of boost. Thanks agian..
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