Carter AFB flow ratings

[carbking]

Some time ago, I posted venturii area for Carter WCFB carburetors. Since then, I have searched the Carter documentation in my possession to do the same for genuine Carter AFB carbs.

The focus of this article is ONLY on genuine Carter AFB carbs; the modern imitations are not considered regardless of if they are marked Federal Mogul / Webber / Edelbrock / or even Carter. Yes, Federal Mogul tried to market some that were marked Carter, but Carter distributors refused to sell the watered-down versions! If you like the imitations, fine; but I will not even attempt to research them.

I have totally given up on trying to place a formatted article here, and the site would time out if I attempted to type an article of this length. Thus, I have placed the article on my website with a link:

https://www.thecarburetorshop.com/AFB_flow_ratings.htm

Comments are welcome.

I triple-checked my data, but even so, with this much data, there is probably at least one error somewhere. I have no ego. If one finds an error that can be documented, I will be more than happy to correct the article.

This will be the last of these articles on Carter 4-barrels. Virtually all of the thermoquads have published data, as do the AVS units.

Jon.

[Cadillac Fan]

Thank you

[junkyardjeff]

How can you tell if you got a imitation Carter AFB,I have picked one up that is tagged AFB but looks to be too new.  Do they still make the originals.

[carbking]

What is the number, AND the letter(s) following the number?

Edited December 3, 2021 by carbking (see edit history)

[junkyardjeff]

Have not looked at it on a couple years,will get ot down the next time I can get to it.  It looks like it could of been made in the late 80s to mid 90s.

[joe_padavano]

Jon, great info, thanks for posting that. I do have a question. The article talks about the 3636 carb that flows 939 CFM at 3 in Hg, and 1128 CFM at 1.5 in Hg. Wouldn't that be the other way around? A higher pressure differential across the venturi should increase mass flow rate.

[carbking]

Thanks Joe, you get the award for finding the first error!

 

Wish I could say I was testing you, and you passed; but it wasn't that way. I transposed the data from the flow test.

 

Jon.

[carbking]

I acquired a few surprises when analyzing the data for this project:

 

(1) I normally think of the AFB as having primary venturii almost as large as the secondary venturii. However, some of the AFB's are kind of a hybrid between a square-bore and a spread-bore. Consider the Studebaker 3540s and 3726s. Primary venturii are 1 1/16 inch, secondary venturii are 1 9/16 inch. So if one applies the 107 CFM/square inch of venturii area; then one arrives at approximately 190 on the primary side and 410 on the secondary side. These were used on 289 CID engines, and the smaller primary venturii would significantly aid in idle and low RPM throttle response.

 

(2) Lots of engines used smaller, often MUCH smaller, carburetors than modern enthusiasts would try to use. This wasn't a total surprise, but the magnitude of the difference was a surprise.

 

Jon.