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Old 12-11-2008, 07:26 PM
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Big block hemi and boost guys

To my knowledge you can build a boost motor @ 9:1 compression. Even higher sometimes like the new vettes. But anyways I know a guy who ordered a 472 Hemi crate motor 9:1 compression and he says he cannot boost the motor because it is 9:1. My thought is that any motor can get it, just how much can be the difference. He says that he called the company he purchased the motor from and the say NO WAY.. He would have to o ring the heads and change the pistons for lower compression. All the bottom end of this motor is forged. It is iron headed and the 525 horse version.
What are your guys opinions?

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Old 12-11-2008, 08:18 PM
8-71 blown 461 Rat on E85
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I am not a big fan of forced induction atop a engine designed to operate naturally aspirated.Of course the iron head hemi @ 9/1 would survive with a few lbs of boost..but does it have a blower friendly cam,increased top ring gap,3 angle ss valves..preferably inconels on the exhaust side??
Probably not..that is why the builder is steering your buddy to a n/a setup.
yes to your question tho..a few lbs of boost from an undriven 6-71 would not be harmfull and sure as heck does not need o-ringing.
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Old 12-12-2008, 12:33 AM
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Fatblock is on the money. If you want a blown enine that performs build the engine for it. Far to many people throw blowers on engines that aren't set up correctly and then wonder why it runs hot or doesn't make any power. For an engine of that size it would need a minimum of an 8-71. Ideal compression is 7.5-8.5:1 sure, he can run a blower with 9:1 but, as Fatblock mentioned he will only be able to run a few lbs of boost not really worth the $5000 it's going to cost to put a blower on it.

O rings are not needed. To be honest these days with MLS head gaskets you very rarely would need to O ring a block/heads for any application.
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Old 12-12-2008, 01:26 PM
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The clearances are more on blown motors bearings, ring gaps, and the cam is different. Just to name a few.

Comp for a blown motor on gas runs from 6.5 to about 8 to 1 depending on the amount of boost and speed the blower is driven. Once the static compression gets over 13-13.5 to 1, oring. MLS gaskets will not hold it.

Multiply the blowers first number(s) by the 71 to determine what size the motor was it was built to feed. That was at low rpm of about 2K. IE 4X71 6X71... 10X71 14X71.......

Determine your engines air needs at the rpm level it will run and size the blower to those numbers. Larger slows the needed blower speed down and saves hp and heat levels. Just remember it takes major HP to spin a blower at high speeds....

PLUS, there are also MAJOR differences between street forged parts and blower forged parts. IE a street cheapie forged crank runs 7-800 bucks, a pro forged crank runs 2500-3800.....THAT IS JUST THE CRANK.......

The cheapie PARTS installed in a street type motor are NOT for blown applications and will turn that 14K Hemi into 500 bucks of scrap, real quick.

Last edited by OHD; 12-12-2008 at 01:39 PM.
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