Carb CFM recommendations

nm9stheham said

IMHO the smaller carb argument is just for best driveability and ease of tuning. If you want max power, then you'll still need the larger carb, regardless of altitude but I doubt your cam will pull in the extra air to need it, and big is certainly not needed for your intended use. And IIRC, at your altitudes, carbs are starting to go rich vs standard atmospheric settings, so a lean carb at sea level may be just about right up at you elevation.
And the carb size versus elevation situation goes both ways so I am not sure my thinking of going smaller is good advice. If you want the same power output, you gotta open the throttle to pull in more of that thinner air so a smaller carb will get into the secondaries, etc., sooner.

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I understand what you are saying about needing more air to produce the same power. Problem is, the only way to pull in more air is more displacement. There is nothing I can do to make a 318 engine pull in more than 318 cubic inches of air per combustion cycle. Obviously changes to cams and valves impact the exact airflow in and out of the engine, but the maximum amount of air still cannot exceed displacement on a naturally aspirated engine.

We actually pull in the same volume of air in Colorado as they would in Florida - the air just has less density. That is the reality we have to live with at altitude. So we make intakes as free flowing as possible, but we have to live with the fact that our air has less energy in it. And if we oversize a carburetor, the lower density only compounds the issue of poor signal at the boosters. Thinner air passing through the venturis has less vacuum than denser air. This is even higher than the standard air energy loss. We lose approximately 16.8% vacuum pressure at 5K feet. This means the problem of an oversized carb will only be compounded at our altitude. I think your original advice is pretty solid.