Author Topic: A note about carburetor flow capacities  (Read 2730 times)

Offline rich weyand

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A note about carburetor flow capacities
« on: July 22, 2015, 01:57:45 PM »
The topic comes up often: how big a carb should I put on my XXX engine.

Bigger cfm numbers -- higher flow capacity -- is always better, right?  No.

Carburetors approximate the correct mixture for the engine.  Exactly correct mixture can only be achieved in a closed-loop system, like electronic fuel injection.  A carburetor approximates the correct mixture.  And since a too-lean mixture won't burn, carburetors err on the side of being too rich.  This is why you will always get better gas mileage with some sort of EFI.

Carburetors are also most accurate when they are running at their designed flow.  Most testing and tweaking occurs at the rated flow of the carb in CFM: cubic feet per minute.  That's where the carb is running at its designed stack velocity, and you get the best atomization, the most uniform mixture.

So, how do you calculate the correct flow rate?  Take the cubic inches of the engine, divide by two (because only half the cylinders fire every revolution), and multiply by the redline rpm.  That will give you cubic INCHES per minute.  Now divide by 12 three times (12 cubed) to convert to cubic feet.

This gives you the maximum flow rate for the engine, at a volumetric efficiency of 1.0.  That is, the volume the engine will pump in the ideal case, where the rings are a perfect seal with no blow-by, the vacuum in the intake manifold is 0", the valves open and close at the ideal time to completely load and empty the cylinders, etc.  A volumetric efficiency of 1.0 is never achieved unless the engine is being blown, turboed, or injected with an oxygenator like nitrous.

So what are the numbers?  Here they are:

305 cu in @ 5000 rpm: 441 cfm
350 cu in @ 5000 rpm: 506 cfm
454 cu in @ 5000 rpm: 657 cfm

Then why are there 800 cfm and bigger carburetors, and why would anyone do twin quads or tri-power?

A 454 at 6000 rpm will pump 788 cfm, at 7000 rpm will pump 920 cfm, and at 8000 rpm will pump 1050 cfm.

For your 350 at 5000 rpm in your truck?  Anything above 500 cfm is fine.  600 is a good choice.  Go bigger, and your performance will DECREASE as you never get to the carb's design stack velocity and its most efficient operating range.

And I'm not claiming to be a carb expert.  This is just some basic stuff I know, to help out people thinking about carb replacements.

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« Last Edit: February 10, 2016, 09:21:59 AM by Irish_Alley »
Rich

"Working Girl": 1978 K-10 RCSB 350/TH350/NP203 +2/+3 Tuff Country lift