This is a simple calculator for determining the approximate size of a carburetor. The Size of the engine can be in Cubic Inches or Cubic Centimeters. The RPM entered should be the maximum RPM the engine is to run (you know that RED LINE on your tachometer). The Volumetric Efficiency is a bit more tricky and is at best a guess unless you have had your engine on the dyno. Some starting points might be like this- Stock Engines VE = 0.75 - 0.85 Mild Built VE = 0.85 - 0.90 Racing Engines VE = 0.90 - 1.0 Anything more then that needs a dyno sheet to prove it!
Note: These are for normally aspirated engines where the carb does not sit on a blower or otherwise is used on a draw through blown application. These numbers are typically for 4 barrel carburetors.
The last bit of confusion is the carburetor type. This selection give a bit more modern approach to the size. If you have an out of the box Holley, Carter, Edelbrock, or other replacement carb run with the STOCK setting. If you have a race prepared carburetor select the MODIFIED setting. The thinking behind the increase in CFM is that a well prepared race carburetor has a much better fuel atomization and can perform better at lower vacuum ratings then a stock carb so you can use a larger CFM rating.
Race carb info from www.pro-system.com the rest is calculated from the common CFM formula. Another good source of information is dAMBest Carburetors, they make custom billet carburetors and have some very cool racing parts and yes both would be candidates for selecting MODIFIED. As always call an expert at a tuning shop like the above they will give you more information they you will care to know about.
Remember these are ESTIMATES if you are doing stuff at the lower end of the RPM spectrum go smaller, if on the top end go larger. The graphs will help you get a visual range on what size you will need. Open plenum carbs sizes can generally be smaller then a 2 plane manifold. These numbers do not apply to IR type manifolds typical of Weber carbs. Your on your own with them, and good luck and use as a general gauge for sizing your carburetor.
Remember, Right Carb, Right Cam, Right Intake and Right Exhaust for your application will be the optimal solution as it's a package that must all work together.
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