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Carter WGD CFM?

Twitch

By Twitch
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carbking 25,000+ Points

carbking

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Carter did not publish CFM ratings for these carburetors; nor did most other carburetor manufacturers. Of the major USA manufacturers, only Zenith published CFM ratings prior to the mid-to-late 1960's. Zenith's ratings were not by carburetor number. You had to know the carburetor type, bore, and venturi size; and then look in a chart. I can probably find a copy of the Zenith chart and attach it if there is interest. I do have permission to make these copies.

Figures for many Rochester carburetors can be approximated by using the chart from Doug Roe's book "Rochester Carburetors". Again, one must know the carb type, size, and venturi diameter; the information is not available by carburetor number. This is a copyrighted publication, so I cannot offer to post a copy of this chart, buy the book.

During this period of time, most enthusiasts went by "total venturi area" and "total bore area". There are charts comparing these figures in many of the older aftermarket manifold manufacturers' catalogues. Again, copyrighted material, which I do not have permission to copy.

This method is probably much more accurate than using CFM anyway, especially given the "CFM" figures released in the last few years! <img src="http://www.aaca.org/ubbthreads/images/graemlins/mad.gif" alt="" />

Up until the last few years, there were four rating systems in effect by different manufacturers:

(1) 1 and 2 barrel carbs (3 inches mercury - wet)

(2) 1 and 2 barrel carbs (3 inches mercury - dry)

(3) 4 barrel carbs (1 1/2 inches mercury - wet)

(4) 4 barrel carbs (1 1/2 inches mercury - dry)

If one KNEW which rating system was used, one could determine the value on the other scales. Example:

Given 4 carburetors rated 500 CFM (one on each of the systems):

Using the 4 barrel scale (wet) as the scale of comparison, then the 4 barrel dry carb would flow approximately 460 CFM; the 2 barrel wet carb would flow approximately 354 CFM, and the 2 barrel dry would flow approximately 326 CFM.

I would not attempt to speculate on the carbs rated in the last few years <img src="http://www.aaca.org/ubbthreads/images/graemlins/frown.gif" alt="" />

Jon.

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Twitch 5,000+ Points

Twitch

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Thanks for thorough feedback on the subject. I haven't had any need to pull off the carb yet for any reason and have never measured bores. I have that Rochester book and a similar one for Holley. I've never owned a Carter before. I remember the days when carbs specs were discussed in venturi area which seemed to be a size comparison not performance comparison. That's they way me and my peers looked at it at the time anyhow. As I recall Euro-iron continued to spec out in carb venturi size after we switched over to CFM.

CFM seems to be a sensible way to see it- air ingestion flow rate. Where a fuel pump can be understood in GPH gallons per hour CFM is logical to me anyway.

Seems that way to guys like Roe too as he states venturi size has nothing to do with air flow. Kind of like saying one engine has 400cid and another has 327cid- yet the smaller engine has more horsepower for a variety of reasons proving CID alone doesn't automatically mean more power.

I did see the chart of bore size vs airflow baseline again though so when I do pull the carb my curiosity will be satisfied.

<img src="http://www.aaca.org/ubbthreads/images/graemlins/grin.gif" alt="" />

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carbking 25,000+ Points

carbking

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Twitch - while I agree with you in principal, I must disagree in practice with that information which is available to us. Unfortunately, most items of this type are given a rating in a pure number, ie CFM or in your illustration with a fuel pump GPH. What is rarely given (and unfortunately, most don't ask) is the rating system. Take a look at the 4 older rating systems for carburetors. Now if I tell you I have a 500 CFM carburetor for sale, what have I told you? If I tell you that I have a 500 CFM 4 barrel rated with a liquid approximating the properties of gasoline then you know what I have.

The same is true for fuel pumps. If a pump is rated at 20 GPH at 10 PSIG, then it probably will flow 50 GPH at 3 PSIG, or maybe not.

Also looking at your post concerning engine displacement and horsepower, no mention is made of the volumetric efficiency of the engines.

Back to carburetor ratings. I know of one manufacturer that deliberately downrated the CFM on one unit of a popular size, knowing that one of the magazines was going to do a dyno test with this size carburetor. Naturally, the engine produced more horsepower with this brand than with other brands. Good selling point!

And with today's ratings, Rick L.'s comments are probably more informative than the CFM rating <img src="http://www.aaca.org/ubbthreads/images/graemlins/mad.gif" alt="" /> Unfortunately, the customer does not demand more information!

I guess maybe this whole post is a plug for "more truth in advertising" <img src="http://www.aaca.org/ubbthreads/images/graemlins/wink.gif" alt="" />

Jon.

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Twitch 5,000+ Points

Twitch

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The CFM is, as you say, basically a yardstick in commom use today as was venturi bore size before it. Neither, is indicative of performance without factoring in assorted mechanical efficiencies. It 2 carbs are basically equal and one has a velocity stack implemented it should be more efficient given the application it was made for.

Yeah the disparity in CID as a measurement doesn't mean the larger engine is more powerful as there are many factors to consider beyond CID. That's what I meant. The smaller engine can have a zillion things that make it more efficient and a producer of more HP, including the carb as you mention.

I am a writer of combat aviation subjects and know from the wide variety of piston aircraft motive power combinations that it can be confusing indeed. 2 duplicate aircraft- one is set up for optimum performance below 15,000 feet and another for 21,000 feet. The plane with the 2 or 3 stage blower produced less power and has less performance at 10,000 feet compared to its twin with a single stage blower.

Or we have 2 identical planes and one uses methanol/water injection while another uses nitrous oxide for combat overboost. Which one is more efficient? For what specific combat role should it be used? Or one is set up to pull more inches of mercury than another but will have a shorter life span. Some of it boils down to whether one craft has a turbo-supercharger vs another with a supercharger and further to what specific brand and blower type each is. Lots of variables of equal thing being unequal.

We had the Daimler-Benz engine that had Bosch fuel injection on one side that didn't produce more HP that a Packard-Merlin with a carburetor if we compare HP per CID. We know the F.I. is superior to the carb but it isn't readily apparent by just looking at HP statistics alone.

Anyhow I see what you mean in auto carbs and just had to ponder it in aviation terms as an illustration inside my own warped mind. So with your experience in carbs what one spec, if any, do you feel most comfortable with? Bore diameter, total venturi area, CFM or what? <img src="http://www.aaca.org/ubbthreads/images/graemlins/crazy.gif" alt="" /> <img src="http://www.aaca.org/ubbthreads/images/graemlins/confused.gif" alt="" />

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carbking 25,000+ Points

carbking

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Twitch - I would feel most comfortable with wet CFM at 1.5 inches mercury on 4 barrels, and wet CFM at 3 inches mercury on 1 and 2 barrels; IF EACH CARBURETOR WERE RATED BY AN INDEPENDANT RATER!

Since obviously that is not going to happen, the only single specification I feel comfortable with is the carburetor identification number. I can then find out what I need to know from my library. I realize my case would be different than most, but that is the best answer I can offer.

In the case of the original question on this thread, I would guess the flow of a Carter WGD 728s to be approximately 320 CFM, and will leave you to guess which rating system I am using <img src="http://www.aaca.org/ubbthreads/images/graemlins/wink.gif" alt="" />

Jon.

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