THE AIR PUMP UNDER YOUR HOOD

What size throttle body or carburetor do I need?

Good question but the real question should be, “How many Cubic Feet per Minute (CFM) of air will my engine require?” 

To begin, think of an engine as 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.

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 an Accufab throttle body.  An Accufab throttle body is designed for “race engine specs” and the flow ratings are going to be greater than most other aftermarket throttle body designs.  Do not assume a 75mm Brand B throttle body will flow as much as an Accufab 75mm.  For a complete list of flow data research click here.

Standard Version

65 MM  -  663 CFM

70 MM  -  786 CFM

75 MM  -  924 CFM

Race Version

70 MM  -  895 CFM

75 MM  -  1045 CFM

80 MM  -  1142 CFM

85 MM  -  1322 CFM

90 MM  -  1369 CFM

105 MM  -  1550 CFM

Other

Cobra (’96-’98)  -  1284 CFM

Cobra (’03-’04)  -  1696 CFM

Lightning Pick-up (’99-’04)  -  1696 CFM