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Re: Best Carburetor

To: fd911medic@aol.com
Subject: Re: Best Carburetor
From: John Clark <clarkjc@runbox.com>
Date: Sat, 01 May 2004 12:21:57 -0400
Hi Ron,

The best carburetor size for ANY engine is given by this little formula:

Carb. size in CFM = (Displacement in cu. in.) x (Max. RPM) x (Volumetric 
Efficiency) / 3456

Street engines usually have volumetric efficiencies around 80% to 85%.  I'm 
going to assume red line on your 289 is 5000 RPM:

CFM = 289 x 5000 x 0.85 / 3456 = 355 CFM

According to this, you need at least 355 CFM.  The closest Holleys are 465, 
570, and 600 CFM.

A typical performance carburetor has four barrels.  The primary barrels are 
usually the front two.  The secondaries are usually the back two.  I would 
recommend vacuum secondaries on a street car for driveability.  Mechanical 
secondaries (like "double pumpers") are best suited for race use.  Vacuum 
secondaries open up based on engine demand.  Mechanical secondaries are 
linked directly to the throttle like the primary barrels.

If you can fit a spacer between the carburetor and manifold, then use 
one.  It will help to keep the carburetor cool.  If a carburetor gets too 
hot, fuel can boil in the fuel bowls.  This is called percolation.  It 
usually leads to bad things like a lean air/fuel mixture and stalling.  Of 
course, making everything fit under the hood is usually a bigger priority.

Original equipment on most Ford engines was a Holley carburetor.  Edelbrock 
carburetors are based on Carter carburetors.  Most carburetors come with 
either square bore or spread bore mounting configurations.  Your intake 
manifold will determine whether you need a square bore or spread bore 
carburetor.  Holleys are usually square bore, and Carters are usually 
spread bore.

That's all the time I have for now.  Inquire further for more detail.

- John

At 10:55 AM 5/1/2004, you wrote:
>Hello-What is the BEST carburetor setup for the Ford 289?  Do you need a
>spacer between the manifold and carburetor?  Is a Edelbrock taller than 
>the Holly?
>  What is the BEST CFM for a mildly modified engine?  What is the better
>carburetor..Holly Vs Edelbrock?  I do not know anything about 
>carburetors...Any
>help would be greatly appreciated.
>
>THANKS
>RON  65 Tiger Mark 1





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