Post by Site Admin on Jul 12, 2004 19:51:11 GMT -4
Hawked from MorTec
CARB CFM SIZING
One of the first things to determine is 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 PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPMs), are relatively easy to determine. The engine VE% is another matter. 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 divided by 3456 X VE%
So if you had a stock, low performance production motor of 350 cubic inches and you wanted to spin it to 5000 rpms max and it had a VE% of 80%, the formula would determine a required carb CFM of 405 CFM. If you had a warmed over street performance motor of the same size, but it was capable of 7000 max rpms and it had better heads, camshaft, headers and a performance intake that raised the VE% to 95%, the formula would give you a minimum required carb CFM size of 673 CFM.
In a controlled situation on an engine dyno, the amount of air actually ingested by the motor can be measured. Since most folks don't regularly have access to a dyno, the above formula will get you in the ballpark. There are some exceptions of course. Using a split plenum type street manifold allows the use of a larger than "normal" carb CFM. 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. Consequently, on a street driven vehicle a split plenum intake with a vacuum secondary carb is the way to go. The vacuum signal stays high for good throttle response at low and mid range rpms. Fuel mileage is good. An "open" plenum intake, generally speaking, 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. Consequently, open plenum intakes are a little more sensitive to the CFM size of the carb. If you are drag racing, most folks are willing to sacrifice some low end power for high rpm horsepower. Fuel mileage isn't a concern and the engine spends most of it's time at full throttle. Open plenum race intakes and "double pumper" carbs are the norm.
CARB CFM SIZING
One of the first things to determine is 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 PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPMs), are relatively easy to determine. The engine VE% is another matter. 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 divided by 3456 X VE%
So if you had a stock, low performance production motor of 350 cubic inches and you wanted to spin it to 5000 rpms max and it had a VE% of 80%, the formula would determine a required carb CFM of 405 CFM. If you had a warmed over street performance motor of the same size, but it was capable of 7000 max rpms and it had better heads, camshaft, headers and a performance intake that raised the VE% to 95%, the formula would give you a minimum required carb CFM size of 673 CFM.
In a controlled situation on an engine dyno, the amount of air actually ingested by the motor can be measured. Since most folks don't regularly have access to a dyno, the above formula will get you in the ballpark. There are some exceptions of course. Using a split plenum type street manifold allows the use of a larger than "normal" carb CFM. 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. Consequently, on a street driven vehicle a split plenum intake with a vacuum secondary carb is the way to go. The vacuum signal stays high for good throttle response at low and mid range rpms. Fuel mileage is good. An "open" plenum intake, generally speaking, 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. Consequently, open plenum intakes are a little more sensitive to the CFM size of the carb. If you are drag racing, most folks are willing to sacrifice some low end power for high rpm horsepower. Fuel mileage isn't a concern and the engine spends most of it's time at full throttle. Open plenum race intakes and "double pumper" carbs are the norm.