for the name droppers.........
THE AIR PUMP UNDER YOUR HOOD
By George Klass
An engine is an air pump. Based on the size of the engine (displacement) and how fast it will be spinning (RPM), it will pump out a certain amount of air. Works just like an air compressor. Of course, there are many variables to CFM requirements, such as cylinder head flow capabilities, intake and exhaust manifold flow capabilities, etc., but the basic engine block will pump a certain amount of air over a specified period of time, measured in cubic feet of air per minute.
If the engine is to be carbureted, it should be a relatively easy decision to pick out the correct sized carburetor. Carburetors are defined by CFM. If your engine calls for (or pumps out) about 664 CFM, the correct choice is probably a 650 CFM carburetor. Unfortunately, throttle bodies are rarely defined by CFM ratings. Instead, most throttle body manufacturers define their throttle bodies by the inside diameter of the unit, measured at or around the throttle blade, and usually in Millimeters (MM). Unless you have a direct correlation between the measurement in MM and the related CFM of that particular unit, the selection is going to be based on “best guess”.
And to further complicate the “guessing” process, all throttle bodies of the same dimension, do not have the same CFM rating. You might think that Brand A’s 75 MM throttle body would flow the same as Brand B’s 75 MM throttle body. Such is not the case, because all throttle bodies have a “major obstruction” in the middle of the air path, namely a throttle blade and shaft. How well the air flows over and under this obstruction will define the CFM a specific throttle body will flow. A big fat shaft with the attachment screws for the blade sticking up into the air stream will impact the CFM of that throttle body.
While most enthusiasts with EFI engines continue to purchase throttle bodies based on Millimeter size, caring little about the actual CFM ratings, the carb guys purchase carburetors based on CFM ratings, caring little about the size of the throttle plates. Score one for the carb guys.
But, back to the original question, “how much CFM do I need”?
Below is a chart with the CFM requirements, based on displacement (in cubic inches) and RPM. This chart will work for any piston engine with any number of cylinders. After you have determined the CFM for your specific engine combination, you can then choose the corresponding throttle body or carburetor size to best fit that combination.
DISPLACEMENT………….6000 RPM……….6500 RPM……….7000 RPM
280………..…………………486………………..527………………..567
290……..……………………503………………..545………………..587
300………..…………………521………………..564………………..608
310……..……………………538………………..583………………..628
320……..……………………556………………..602………………..648
330………..…………………573………………..621………………..668
340……………..……………590………………..639………………..689
350……..……………………608………….…….658………………..709
360………………..…………625………………..677………………..729
370………………..…………642………………..696………………..749
380………………..…………660………………..715………………..770
390………………..…………677………………..734………………..790
400………………..…………694………………..752………………..810
410………………..…………712………………..771………………..830
420………………..…………729………………..771………………..830
430………………..…………747………………..809………………..871
This chart should give you a general idea of the amount of air your combination will pump. Engines will pump less air because of the restrictions in the cylinder head or intake manifold design or valve lift, or all three. But, the chart still gives you a ball park starting point.
One other thing to know. A carburetor requires air speeding over the venturi to draw the gasoline into the mixture. Using too large a carburetor (high CFM rating) will usually cause derogatory performance in the lower or midrange. This is because the lower air velocity is inefficient in mixing the gasoline with the air. In general, and particularly for street use, a slightly smaller carb (less CFM) will give better overall performance.
With an EFI system, this is usually not a problem. The throttle body only controls air flow. A computer monitors the gasoline supply and the mixing of gasoline and air takes place inside the intake port, and not inside the carburetor. Using an oversize throttle body is not nearly as detrimental to low and midrange performance as is using an oversize carburetor.
So, to find the CFM ratings of a carburetor, all you need to do is to look in any catalog from Holley, Edelbrock, Barry Grant, etc. That’s how the carburetors are listed. To find the CFM ratings for a throttle body is going to be more difficult, unless you happen to choose an Accufab throttle body.
Below are the various throttle body sizes and corresponding CFM ratings for the Accufab throttle bodies. Because the Accufab throttle bodies are designed to “race engine specs”, the flow ratings are going to be greater than most of the other aftermarket throttle body designs, so don’t automatically expect a “75 MM Brand B” throttle body to flow as much as an Accufab 75 MM unit.
65 MM - 664 CFM
70 MM - 787 CFM
70 MM - 896 CFM (Race version)
75 MM - 924 CFM
75 MM - 1045 CFM (Race version)
80 MM - 1142 CFM
85 MM - 1322 CFM
90 MM - 1369 CFM
105 MM - 1550 CFM
my comment - once you have met the max needs of the engine you have, TB size then affects throttle repsonse more than anything else.
