I quit posting on these forums sometime ago, as the automatic censor made one of my posts trying to help a non-Chevy owner look like it was written by an idiot. As this post does not mention the Dearborn based company beginning with the letter between E and G, maybe the censor will permit the post.
The following is part of an article done some time ago for the Pontiac community. Hopefully, it will help some understand carburetor ratings.
Jon.
CFM calculations and application
A question often asked is how large a carburetor should I install on my engine. The following information applies to carburetor application for street driven multiple cylinder (4 or more cylinders) four stroke internal combustion reciprocating engines. It does not apply to race applications, rotary engines, one, two, or three cylinder engines, nor engines with fewer or more than 4 strokes.
Most enthusiasts have seen the following equation:
CFM = (RPM x CID) / 3456
This equation will give CFM required at 100 percent engine volumetric efficiency. However, at this writing, no street engine will be 100 percent volumetric efficient. To estimate volumetric efficiency, compare your engine to one of the following examples:
1969 Pontiac RA IV engine approximately 90 percent
1969 Pontiac GTO (non-RA) approximately 85 percent
Pontiac 400 economy (low-compression) V-8 approximately 80 percent
Pontiac 230/250 6 cylinder approximately 75 percent.
These are estimates, and can change with the addition of headers, and other exhaust modifications.
Now lets break down the equation.
CFM = air flow in cubic feet per minute
RPM = engine revolutions per minute
CID = engine displacement in cubic inches
3456 = conversion constant
VE = volumetric efficiency (express as a decimal)
The conversion constant did not magically appear out of thin air. Note that air flow on the left side of the equation is measured in cubic FEET per minute; and that engine displacement on the right side of the equation is measure in cubic INCHES. As there are 12 inches in one foot, part of the constant is 12 x 12 x 12 converting cubic inches into cubic feet. This figure is 1728. Note also that the equation is listed for a FOUR STROKE engine. The crankshaft (RPM) makes 2 revolutions for each firing stroke, so a 2 enters the equation. 2 x 1728 is equal to 3456. So much for magic.
The equation is often misapplied by first time users. Note that the CFM is directly proportional to RPM. Many simply apply the equation at wide open throttle (WOT) and buy accordingly. This solution will work as long as the vehicle is driven only at WOT (the local constabulary frowns on this type of operation); but may result in an engine that is either severely over or under carbureted as normal cruise RPM. The successful engine tuner will install a carburetor which is large enough to satisfy WOT requirements, but has sufficiently small primaries to have maximum air velocity under normal cruise conditions.
Taking an example:
1969 Pontiac RA IV engine with desired 60 MPH engine speed of 2300 RPM, and maximum engine WOT of 5800 RPM
First calculate WOT requirements:
CFM = (5800 x 400) / 3456 or 671 CFM at 100 percent VE
However we stated that the RA IV engine had a VE of approximately 90 percent.
Actual required CFM = 671 * 0.9 or 604
Now, lets calculate cruise requirements.
CFM = (0.9) x (2300 x 400) / 3456 or 240 CFM
The Rochester Q-Jet from 1969 was approximately 150 CFM on the primary with a variable 600 on the secondary for a total of 750. Note that the 750 CFM is sufficiently large to satisfy moderate race conditions with open headers. The novice will now make another incorrect assumption regarding the primary. 4 barrel carburetors are rated at 1.5 inches of mercury. If this figure increases, so does the air flow. 1 and 2 barrel carburetors are rated at 3 inches of mercury. A good conversion constant from 1.5 to 3.0 ratings is the square root of 2 (1.414). To convert a four barrel rating to a two barrel rating, multiply the 4 barrel rating by 1.414. So if the primary of our Q-Jet were rated as a 2 barrel then 150 x 1.414 is 212 CFM and if the vacuum is even greater, then so is the CFM. At cruise RPM, the vacuum is significantly greater than 3.0 inches of mercury, and our Q-Jet has sufficient air flow. GM did modify the Q-Jet in the mid-1970’s to have approximately 200 CFM on the primary side for 455 CID engines.
Reviewing (and we will now include VE with our equation):
CFM = (VE x RPM x CID) / 3456
And converting 2 to 4 or 4 to 2 ratings.
CFM2 = CFM4 x 1.414
CFM4 = CFM2 / 1.414
