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Cubic inch or blower size trade-off?

 
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armyofuno
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Joined: 19 Aug 2007
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Location: Northern Lower Michigan
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1972 Buick Skylark

PostPosted: Tue Sep 23, 2008 9:38 pm    Post subject: Cubic inch or blower size trade-off? Reply with quote

I am considering a 6-71 blown BB chevy (because I have a 6-71)

Now my question is: Considering the heating of the intake charge due to the relatively small blower, should I consider a lower revving, larger intake charge (496 or larger) c.i. motor, or the higher revving, smaller intake charge 427 c.i. version and what would the pros / cons be?

I would not be using any intercooler and would be dual carbed. Mostly street, minimal strip.

Thanks in advance,

Steve

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clay
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Joined: 24 Nov 2002
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Location: South Carolina
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1972 Chevrolet Nova

PostPosted: Tue Sep 23, 2008 10:22 pm    Post subject: Reply with quote

Given the choice and starting from scratch, I would never chose the smaller size. If you look at it this way, the blower is capable of moving a fixed amount of air - period. It doesn't care what is below it. A higher rpm smaller cubic inch engine would move the same air that a lower rpm larger cubic inch engine. Blower rpm would be the same, just at a different engine rpm. I know this is a different type of supercharger, but centrifugals are rated by the cfm of air they can move. Companies can then relate a cfm rating to an approximate h.p. figure it can support. Now this is grossly simplifying the situation and centrifugals are a little different since they aren't positive displacement like your 6-71, but hopefully I am getting my point across. The 496 will be a lower rpm setup and on the street, this means longer life. Clay
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af2
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Joined: 01 Sep 2003
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Location: grassvalley, ca
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1933 Willys Coupe

PostPosted: Tue Sep 23, 2008 11:16 pm    Post subject: Reply with quote

6X71 =426 and that is spinning 3000 rpm? My Uncle runs one on his 433 and gets 30 lbs boost/manifold pressure at 9400 rpm.
I say manifold because of the reversion! Which we have no way of telling!
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Big Dave
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PostPosted: Wed Sep 24, 2008 7:37 am    Post subject: Reply with quote

A lot of the heating with a roots blower is in that the higher the boost, the tighter the seal has to be to keep the air from leaking back through the middle of the blower heading up and out through the carbs. If it is tight enough to seal at high pressure they don't last but a couple of minutes (several races).

Lower the boost pressure and a Roots is happier; that is why every one racing runs a 14-71 not a six. As Clay said it only moves so much air, and if you want pressure under the blower it will have to rev it higher to move more. That to me inplies the shorter stroke motor.

Big Dave
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clay
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1972 Chevrolet Nova

PostPosted: Wed Sep 24, 2008 9:23 pm    Post subject: Reply with quote

Dave! Say it ain't so - you actually suggested building a smaller cubic inch engine!!!!!! Very Happy Very Happy . I think the size he has would make a good low boost setup on the 496 and should be good for gasoline. Once you start running alcohol however......spin it to death Twisted Evil . Clay
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af2
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Location: grassvalley, ca
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1933 Willys Coupe

PostPosted: Wed Sep 24, 2008 9:40 pm    Post subject: Reply with quote

clay wrote:
Once you start running alcohol however......spin it to death Twisted Evil . Clay


Clay, that is so true!!!! Laughing Laughing
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armyofuno
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Joined: 19 Aug 2007
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Location: Northern Lower Michigan
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1972 Buick Skylark

PostPosted: Thu Sep 25, 2008 5:26 pm    Post subject: Reply with quote

Thanks for the advice. I am now leaning towards the 496 (or larger) Although I do have to say I have interested in the blown alcohol street car idea........ But that is another topic.

Anyways, that discussion pretty much mirrored my thoughts of not needing a 9K RPM motor for the street (427 / 433). Besides, what is that old saying? "No replacement for displacement"??

I will keep everyone advised as to the progress on my never ending project.

Thanks,

Steve

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clay
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1972 Chevrolet Nova

PostPosted: Thu Sep 25, 2008 6:44 pm    Post subject: Reply with quote

I have a feeling a 5000 rpm 496 with a blower will be downright stooopid on the street and you'll be waaaayyyy happy. Clay
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10sec.et
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1969 Oldsmobile Cutlass

PostPosted: Thu Sep 25, 2008 8:09 pm    Post subject: Reply with quote

clay wrote:
I have a feeling a 5000 rpm 496 with a blower will be downright stooopid on the street and you'll be waaaayyyy happy. Clay


exactly. you wont be able to get the power to the ground. my Olds is all done before 6000 and its nearly impossible to get it to hook up on the street.

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