Carb. sizing

I had answered a member question on another site, and thought I would post that answer on this site as well,,,, It went like this,

John, for future recommendations on carburetor size, the easy tried and tested rule of thumb for carburetor sizing is two times the engine cubic inches.(Thanks Don.) Or there is the long formula that goes like this,,,, desired top engine RPM divided by .5 (a 4 stroke engine only draws in combustion air every other rotation), that number is then multiplied time total engine displacement. that number is then divided by 1728 (1728 is the cubic inches in a cubic foot) to this number I like to add about 10% to allow for flow quality of the components being worked with, here is the math for a 318 engine that would be run at a maximum of 6000 RPM.

6000 RPM (X) .5 = 3000

3000 (X) 318 = 954000

954000 (/) 1728 = 552

552 (X) 1.1 = 607

Using the long formula it tells you to use a 600-650 CFM carburetor, where as the easy formula comes in at 318 X 2= 636

LOL,,,,,which is easier,,,,,,,,

Will the engine run on a smaller carb, absolutely, but not to it's full potential

Will it run on a larger carb. absolutely again, but with the oversized carb you need to get into the finer tuning and rejetting for optimum performance handling qualities,,

In a message dated 10/27/2011 2:33:40 A.M. Central Daylight Time, jmellelieu@xxxxxxxxx writes:

Of course those days I only had the local hot rod club for input (pre-tinternet) and they all reckoned at least 600cfm on an egine making around 280 brake - I ignored that advise and went 450cfm (might have been 500) from book research and the thing pulled like a train.