CFM does not do anything. But does represent an amount of volume. In the case of 600 CFM, that is equall to 600 Cubic Feet per Minute of fuel and air that can be passed throught that carb with the throtle wide open.
A carb rated 600 cfm has a MAX airflow of 600 cubic feet per minute thru it. That air flow has to feed all eight cylinders. It will only flow as much air as the engine needs up to it's max capability. The airflow rating is taken at 1.5" mercury, and is with all 4 barrels wide open.
So, if your turning 1000 rpm, and the engine only needs 300 cfm, that's all it will flow, and it will meter fuel in relation to the airflow, assuming it is correctly tuned.
A 360 cid engine displaces 360 cubic inches per revolution, and since the crank has to rotate 2 times to complete a full cycle (4 stroke engine). Since air intake is controlled by the cam, and the cam spins at 1/2 the crank speed, you will have to divide any rpm by 2 in your calculations.
A cubic foot is 1,728 cubic inches, so your 360 cid engine displaces 0.2083 cubic feet.
Since the 4 stroke principle has to be accounted for (see above), at 1000 rpm your 360 cid engine is using 104.17 cubic feet per minute. That's the whole engine.
Let's take it to 5000 rpm, and it is only using 520.83 cfm. Again, the whole engine.
You can break it down even further if ya want, because theoretically, only one cylinder is filling at any given time, so the carb only has to really supply 1/8 of that number at any given instant.
So, you can see, a 600 cfm carb is more than adequate for a 360 cid engine spinning at 5000 rpm. At 6000 rpm, it only needs 625 cfm, and that is assuming a perfect engine.
The short formula is cfm=[rpm*cid}/3456 * % volumetric efficiency
IE: cfm = [5000 x 360]/3456 * .85 (.85 being normal for a street driven engine)
cfm= 442.71 for a normal 360 engine
Most go too big on CFM. Going too big on CFM will make the carb less sensitive to changes in engine demand for air at part throttle. This can be more or less true depending on the design and brand of the carb you get.
I have an Edelbrock 1406 (600cfm) carb for my 318 and I would have gone smaller on the CFM if Edelbrock made a 500cfm that had all the street legal connections, but they do not. (I almost got a 500 Demon)
Before you buy a carb you should ask yourself what it will be used for. How will you use the vehicle that it is on? Is it an everyday driver, or a quarter mile runner that you will trailer to the track? Etc…
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