On Dec 15, 2005, at 3:03 AM, Lancair Mailing List wrote: This annalysis draws me to a trivial question I can no longer rember the answer to. Back in the days of big lizards, when cars (fun ones anyway) were powered by engines exceeding 400 cubic inches and fed by carburators, the typical carburator size was 750 to 1050 cfm. (refering to street cars here) These engines turned twice the 2700 rpm in a typical aircraft power plant. Volumetric efficiency would reduce the actual flow rate slightly below the calculated value but not a whole lot. So why weren't the carburators rated about 10 times larger than they were? Anyone remember the reason? Rob
The CFM rating of a carburetor is the air flow when the pressure drop reaches some standard value - if my memory serves correctly the value was 1.5 inches mercury, a fairly high pressure drop. I calculate a 455 turning 5,000 rpm (VE=1) would require 658 CFM. Doubling the rated flow of the carburetor reduces the pressure drop at a given flow by a factor of 4, so using a 1300 CFM carburetor in the same engine would result in a pressure drop of only 0.375 inches (pressure drop is proportional to the square of air flow). The off-idle operation would be expected to suffer.
Gary Casey |