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Discussion Starter #1
My buddy just picked up a Weiand 256 to put on his 509 cid BBC which seems do be plenty of blower for this application...

A 6-71 blower is listed as a bigger blower cid wise but is generally considered too small for most any Big block.... Can anyone explain why??

Thanks!!


Doug
 

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It depends on the objective of the boost. Whomever considers the 6-71 is too small, you should ask them why and I bet their boost objectives are different.

If a little bit of boost is what he is looking for then the Weiand is OK. It does have to work really hard to boost a big block.
 

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Like Infomanic posted, it really dependeds on your requirements. The 256 is pretty popular as an add-on for the Merc 500 HP because of the stock compression ratio and stock components limit the boost capability of the engine.

The 256 would have to spin twice (100% over) as fast as the crank to get 11lbs of boost on a 502, the 6-71 only has to spin somewhere around 12% faster. Weiand only makes pulleys to drive the 256 to 112% over and 13lbs on a 502. Thats only 20% over for the 6-71. And thats if you use an unstripped 6-71. A fulley stripped blower would be even slower.



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Discussion Starter #4
He's only looking to run 5-6 psi with the Weiand.. Any more and he'll just be gernading drives (if he won't be already)..

I was looking at a 6-71 setup for my 454 this past summer but was told it was too small so I passed on it.... Guess I should've found some better sources of info. ;)

Thanks,

Doug
 

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all depends

it all depends on what you have and what you want to boost it to. i maxed out a fresh littlefield 6x 1071 trying to get 15 lbs of boost on a gas 509. now that blower would be considered big on a lot of engines, but i had to run it at 25% over to get 15 lbs. at 25% over it was making a ton of heat, and was at the end of getting power.

i didnt notice any gains at all from 20% 14lbs to 25% 15 lbs, other than a lot more heat. this same blower was great on the same engine down at 10 and 12 lbs boost. you get to a point, real quick, where more boost isnt more power, if it makes too much heat. and i run a chiller too.

i went to a 1471, before i decided to add more CI... should have got a 1671:D
 

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Boatally Insane,another thing that should be mentioned here is it depends on the build of any given engine. Boost can be looked at as the air that DIDN'T pass into the engine. So with small port heads on a relatively low RPM motor with a medium lift short duration cam its not to hard to back the air up in the manifold, thus creating boost. Nelsons 509 had some pretty decent heads and cam in it. So even with a 10-71, the motor was using alot more air than say a stock Merc 500HP or something similar.

Nelson. stop worrying about the boost number. Just remember your moving 50 cubic inches more air with every revolution of that 14. And every revolution of the 14 is only being asked to feed 1/2 of the increased engine size, per revolution. And thats if you were running 1 to 1. So you have OVER 50 cu in additional inches per rev. feeding 15 additional inches. Add in the new heads and cam:D and you'll be fine.



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no worries

im not too worried. i think ive got what i need to get where i want to go. im hoping that i can run it on pump gas and still make close to the same hp i was on av gas. so those long weekends at the river are a bit cheaper. i should have a much stronger top end / valvetrain as well, so i can just put a stick on the throttle to hold it wide open, and kick back in the back seat drinking on those long runs down to havasu.
 
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