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Where this formula goes "off the tracks" so to speak, is that it assumes
that the CFM rating is measured at the optimal amount of vacuum for the
engine in question. It is not commonly known, but four barrels are CFM
rated at 1.5 pounds of vacuum, and two barrels are CFM rated at three pounds
of vacuum. Now the real interesting part is: engines make optimum power
closer to .5 pounds of vacuum.
This formula cannot take vacuum into account. So is that 500 CFM for a two
barrel or a four barrel? I would say a two barrel at three pounds of
vacuum. That big block will trying to suck that carb into the manifold! My
experience has been, it is difficult to over-carburete a big block. My
friend Ron and I put a 850 double pump on a smooth idle 396 in a truck and
it drove just fine.
For some reason, hot-rodders tend to be in disbelief at how Rat motors
love fuel. They can't believe a big-block wants a really big Carburetor.
They can't believe that Big-blocks need a HUGE fuel delivery system to NOT
run out of fuel. They can't believe that Big-blocks want a dual three inch
exhaust system, and so on.
Here's a better way to do the math: most people have cut there teeth on
small blocks, lets use a small-block and compare it to a Big-block. Lets
take a mild 327 small block with a 650 spread bore: We will cross multiply
to get the answer.
650 x
------ ----- = 650 times 396 divided by 327
equals 787 CFM for a mild Big-block.
327 396
So a 750 is the very smallest carb I would go with. And if you would like
some tire smoke to go with that Big-block, you will need a bigger carb,
bigger valves, bigger cam, and bigger exhaust (more compression)
I like a racing mechicanal fuel pump with a regulator to go with that Holley
carb, and make sure you have at LEAST 3/8" fuel line!
Don't let the small-block thinkers talk you down! You are building a RAT !
Rob
'67 Malibu
'67 El Camino (in a basket)
----- Original Message -----
From: "John Nasta" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Tuesday, March 19, 2002 2:21 PM
Subject: RE: [Chevelle-List] Carb sizing
>
> Replies to this message are sent to The Chevelle Mailing List
> ------------------------------------------------------------
>
> Yes, that is the correct carb if you want 80% volumetric efficiency. The
> idea is to assume that you want 100% VE, and figure out what cfm carb will
> theoretically give you that.
> -----Original Message-----
> I've had essentially the same engine), at 5500 RPM (realistic[?]) x 396
CID
> x 80% (if lucky) / 3456 = a 500 cfm carb
>
>
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