The Stovebolt Site Forums The Shop Area Engine Shop CFM - single barrel carburetors

A question was asked on another thread concerning if there were a formula or equation to calculate CFM based on throttle diameter and venturi diameter. The answer is no. Why?

In addition to the two criteria above, other variables that can effect airflow:

(A) the diameter of the throttle shaft
(B) the diameter of the choke shaft
(C) the distance of the minimum diameter of the main venturi to the choke shaft (the choke butterfly straightens air eddy currents in incoming air)
(D) the type of venturi booster(s). Some early carbs came with a single venturi, no booster; others have a single booster, and many have a double booster.
(E) the thickness and length of the booster(s)

Carburetor companies published data that I have seen:

Carter - none (Carter offered specific carb models, rather than universal carburetors).
Holley - none (Holley offered specific single barrel carb models, rather than universals; obviously quite different than the two and four barrel Holleys)
Rochester - chart for Monojets only, no data for type B Rochesters
Stromberg - none for downdraft carbs, again, Stromberg offered specific carb models ALONG WITH MODIFICATION INSTRUCTIONS FOR DEALERS ON UNIVERSALS!
Zenith - chart for a very few universals, by carburetor type, and main venturi diameter. If the enthusiast is familiar with both Stromberg and Zenith, one can sometimes correlate this information to Stromberg

I have posteded in earlier threads on this forum the specific Carter and Stromberg models for Chevrolet.

So if none of the above gives the answer one wishes, if one wishes to install a non-stock carburetor, what does one do?

It was mentioned in the other thread to pick a carburetor from a donor engine close to the same size as the engine to have the carb replaced. This is certainly good advice within limits. One needs to confirm that the rated RPM of the donor engine is similar to the RPM of the "donee" engine. If the rated RPM's of the two engines are vastly different, then the carb sizes will also differ.

One can ask questions on forums such as this one, as to what others have done with the same configuration, and their results.

And, of course, hopefully, there are professionals that hopefully have the necessary data to offer a reasonable carburetor AFTER listening to one's questions

Jon.

Excellent, thanks

Thanks, Jon. Here’s the reason I was asking the other day:

Been looking for an upgrade from the B for my 235. The size 3 Carter YFs tend to be quite a bit more scarce on EBay than the size 2. Missed out on a couple auctions that went higher than I was willing to pay.

I have the Chevy-specific Stromberg numbers. But they seem to almost never come up for sale (as I say that there’s one on eBay right now). Lots of Dodge application stuff. And some universals.

The specific carb that spurred my question in the other thread was a BXUV-35, tag # 1-99c. It has a 1 5/32 Venturi and 1 9/16 bore and adjustable main. Was trying to reverse engineer what sort of displacement it was intended for.

(Hopefully, this is all academic now for me. Won an auction for a YF 2100s last night. But I’m still interested in learning more. And it might be of interest to others as well!)

JW - glad you found the 2100s.

On your question, MUCH easier and much less time consuming to go through the books and determine what carb or carbs might work for you, than trying to do it the other way.

Jon.

Always appreciate the insight, Jon.

Just for future reference....what is your overall opinion of the adjustable-jet "universal" Stromberg and Zenith one-barrels? (Assuming the literature is consulted to select the "right" one for one's engine).

JW - both are excellent in a single carb environment. I do NOT recommend the adjustable main jet in a multi-carb environment.

Jon.

A bit more information on Zenith and Stromberg:

First, I am amused at how many try to put any carb on any engine and then tune it Why?

I have posted many times about the S.A.E. carburetor sizes, and I think by now most that read these forums are familiar with these. BUT STROMBERG AND ZENITH FELT THESE WERE NOT ENOUGH SIZES!

So, both Stromberg and Zenith have "in-between" sizes.

If you look at the Zenith type 228, you find the size 2 might be a 228AV10, whereas, the size 3 might be a 228AV12. (A (B) replacing the (A) is possible in both cases, showing the throttle and choke shafts are perpendicular rather than parallel.

But Zenith also provided "in-between" sizes as did Stromberg. So a Zenith 228AV11 would have a size 3 bolt spacing, but a throttle bore about halfway between the size 2 and size 3.

Stromberg also provided in-between sizes, and these are specified with an "O" or a "U". The "O" is an oversize throttle bore, the "U' is undersize. So a BXUV-3 carb will flow less than a BXV-3 carb with the same venturi. Conversely, a BXOV-2 will flow more than a BXV-2 with the same size internal venturi.

And while the internal combustion engine is incredibly flexible, there really is an optimal size carburetor for each engine.

Jon.