I’ll make an attempt at this one.
Actually, the question was answered by Mark Twain many years ago; or at least the quote “figures don’t lie, but liars figure” is attributed to Mr. Clemens.
To answer the question, it is important to clearly understand the question. Wherein lies the derivation of the referenced equation?
An engine, if one removes the spark and the fuel, is an air pump. Do determine the size of the air pump, we simply need to write an equation about the air pump. So we write that:
Airflow is equal to displacement multiplied by frequency
Adding specific units of measurement we obtain:
CFM = (CFD x RPM)
Since very few engines are rated in cubic feet displacement, it is necessary to convert CFD to CID, thus:
CFM = (CID x RPM) / (12 x 12 x 12)
Or
CFM = (CID x RPM) / 1728
BUT
In a 4 stroke engine we have an intake and a compression cycle, so we must divide by 2 thus our final equation:
CFM = (CID x RPM) / (12 x 12 x 12 x 2)
Or CFM = (CID x RPM) / 3456
Now historically:
CFM should be considered a “vector” number, rather than a “scaler” number.
I do not know who published the first carburetor CFM charts, nor does it have any real bearing on the discussion. It is enough to know that charts were published at least by the early 1930’s. The state of tune of engines at this time pretty well established that at WOT (wide open throttle), the vacuum of the engine would be approximately equivilent to 3 inches of mercury. Thus it became the norm to “flow” and thus “rate” the airflow capacity of carburetors at a vacuum equivilent to 3 inches of mercury. FLOW RATING NUMBER 1!
Fast forward to 1952. Three carburetor companies offer 4 barrel carburetors in 1952. Up until 1952, automotive carburetors (for the most part) had been 1 and 2 barrel units. It was found that, under WOT with the 4 barrel, engines of the period would exhibit vacuum approximately equivilent to 1 and ½ inches of mercury. Thus it became the norm to rate 4 barrel carburetors at 1 and ½ inches of mercury. FLOW RATING NUMBER 2!
The difference between scales 1 and 2 is the square root of 2.
During the 1960’s one carburetor company began to flow carburetors “dry” (historically, ratings had been done “wet”). While seemingly obvious, wet ratings are done with a liquid with similar properties to gasoline mixed with air; and a dry rating is air only. FLOW RATINGS NUMBER 3 AND 4!
The difference between a wet rating and dry rating is approximately 8 percent.
So to summarize ratings which stayed pretty much the same up through the 1980’s:
(1) 1 and 2 barrel carbs wet
(2) 4 barrel carbs wet
(3) 1 and 2 barrel carbs dry
(4) 4 barrel carbs dry
How do these ratings compare? Lets take 4 different carburetors rated at 500 CFM (one from each scale) and re-rate them ALL using scale number 2:
The carb from scale one would be 500 / square root of 2 or 500 / 1.414 or 353.6
The carb from scale three would be 353.6 / 1.08 or 327.4
The carb from scale four would be 500 / 1.08 or 463.0
I picked scale two, as it is the scale I feel is most accurate; others may have differing opinions.
Summerizing the above, our 4 carburetos, each rated 500 CFM:
(1) 353.6
(2) 500.0
(3) 327.4
(4) 463.0
“Figures don’t lie, but liars figure”
Enter the 1980’s.
A very old carburetor company is purchased by another company. The survivor company finds that the 600 CFM carburetor is the “standard” by which many carbs are sold, and they don’t have one (they have 500’s and 625’s). The 625 is given a second part number and rated 600 CFM (guess how these compare in horsepower ratings to the competitors 600).
“Figures don’t lie, but liars figure”
A new carburetor company comes to the conclusion (probably true) that racing engines do not produce a vacuum equivilent to 1 and ½ inches of mercury at WOT, and thus rate their carbs at a lower figure (producing higher numbers).
“Figures don’t lie, but liars figure”
An old established company issues a new line of carburetors with ratings similar to the ratings in the previous paragraph.
“Figures don’t lie, but liars figure”
Another established company not making carburetors purchases the rights from other carburetor companies to produce similar carburetors. Historical CFM ratings on these similar carburetors with the same venturi area are now increased.
“Figures don’t lie, but liars figure”
Personally, I pay absolutely zero attention to CFM ratings published in the last 20 years. I use (and for the most part recommend) carburetors produced BEFORE 1980. If I were going to purchase a newer carburetor, I would DEMAND to know the test criteria used to rate the flow of the carburetor. If the vendor/manufacturer could not (or would not) provide this information, I would look elsewhere!
And remember Mark Twain’s words. Amazing how much he knew about carburetors!
Jon.
