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How do you determine what size carburetor do you need? A number of factors come into play. What cubic inch size is your motor? What do you do with it? Race, street use, towing, street use with occasional trips to the track? What type of intake manifold do you have, split plenum, open plenum, tunnel ram, individual runner? How fast do you spin the motor? What is the volumetric efficiency of the motor? Do you have a manual or auto transmission? What is the rear gear ratio? Do you want to get the best gas mileage possible or do you want to develop the most power possible? Keep in mind that a carburetor is just one part of the engine combination. All of the parts need to work together. Putting a larger carburetor on is NOT going to immediately put 100 more horsepower at your disposal. The carb needs to work with the other parts you have chosen and your intentions concerning how the vehicle is going to be used.
One general rule of thumb uses a formula to determine the CFM requirements of your engine. It goes like this: You need to know the CUBIC INCHES of the motor. You also need the maximum RPMs the motor will be spun to. Finally you also need the Volumetric Efficiency (VE%) of the engine. The first two items (CUBIC INCHES and RPMs), are relatively easy to determine. The engine VE% is another matter, unless you can have your engine tested on a dyno, you must estimate the VE%. If an engine could use all of the air it ingested, it would have a VE% of 100%. Many performance engines reach this level. Certain race engines can actually exceed this and reach a VE% of over 100% at certain points in their RPM range. Most production engines and most street performance engines have VE levels below 100%. In fact, stock, production, low performance motors will fall around 75%-85% volumetric efficiency.
The math formula is: CARB CFM = Cubic Inches x Max RPM's / 3456 x VE%
As an example, if you have a stock 350 cubic inch engine and want it to turn 6000 RBM maximum and it had an estimated Volumetric Efficiency (VE%) of 75 then your maximum required carb size would be 456 CFM. 350ci x 6000 / 3456 x 75% = 456 CFM
As with almost everything, there are some exceptions to this formula. Using a dual plane (split plenum) intake manifold allows the use of a larger CFM carburetor than recommended by the formula. This is because the plenum volume is cut in half by the divider, so each cylinder only has half of the total plenum volume and carb CFM to draw thru. Likewise a carburetor with vacuum secondaries will only open the secondaries enough to feed the engine what it needs, so you can go with a larger than recomended carb. A single plane (open plenum) intake generally has opposite effects. Low rpm throttle response is decreased, but high rpm breathing is improved because of an increase in available manifold plenum volume to each engine cylinder. Open plenum intakes are more sensitive to the CFM size of the carb. In most forms of racing, most people will gladly sacrifice some low end torque for high rpm horsepower. Fuel mileage isn't a concern and the engine spends most of it's time at full throttle.
As a general rule on a street driven vehicle, a dual plane intake manifold with a vacuum secondary carburetor is the ideal setup. On most dedicated racing vehicles, a single plane intake manifold and a mechanical secondary carb is the way to go.
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