I posted a question on an old thread****led "cfm ratings for carbs". Most of the guys who posted on that thread are dead or missing in action. I want to use two Holley 92/81 carbs with an Offenhauser Super manifold using straight linkage on a stock 239 cubic inch 59AB. Here's the link to the thread: https://www.jalopyjournal.com/forum/threads/cfm-ratings-for-carbs.327843/ I'm using the formula posted by @thecockeyedwallaby in post #3, and the cfm ratings by @CarbOtto in post #9. At 3500 RPM X 239 CI divided by 2 X 1728 (the number of cubic inches in 1 cubic foot), the engine needs 242.043 cfm. If two 92/81's give me 284 cfm which is 129 more cfm than a single 94 drawing 155 cfm, where am I going wrong in my calculations?
@alanp561 Limitations in carburetor CFM are seen at the highest ends of the RPM range, as that's when the engine demands the most airflow. Really, small differences are seen if you're too far over your engine's needs. The CFM calculation in the thread is the theoretical perfect number that your engine needs. In the real world, treat it as a minimum. Your 284 cfm to the theoretical 242 CFM requirement should be just fine. That's not much of an over-carburetion, but it's also a small engine so it should work out well for you based on the airflow calcs. I haven't played with a flathead yet so I don't have direct experience on carb selection for one. Over-carburetion will result in less throttle response. With the numbers you have, neither situation should be noticable.
I got in touch with Uncle Max and his response was if I use straight linkage on the 92/81's, I should have a pretty snappy throttle response. If my engine was larger, I would imagine those carbs wouldn't be able to feed it. I'll find out soon.
Thanks for that. Using that calculator, at 4,000RPM and a 239 ci engine, it would require 235 CFM so I'm still 49 CFM under the limit. At 4,500RPM, I'm 20 CFM under. At 5,000RPM, I'm over the limit by 9 CFM. Time to invest in some gaskets and see how it goes.
Think of it more as a number to get you in the ballpark and not any kind of hard limit. You're in the ballpark, give it a try.
Yes, I think its a starting point calculation and only experimenting will get you where its best for your set up. I'd set up one of those fuel/air ratio guages before you start experimenting.
Something that everyone overlooks. The carb will never flow more than the pump can pull. I have a highly modified 600 holley on the shelf. It flowed 660+ wet on the bench. I ran it on a 355 cube mill that takes a deep breath. I could tell by the sound of the carb that it was flowing close to potential. They howl when they are happy. Now the reason for the story. I moved the carb over to a more stock engine same displacement. The engine ran fine, but it never flowed up to potential. The pump did not pull that much air. (note same intake manifold) If you are going to run a pair of 2 bbls straight up on a small displacement motor you may have to learn how to drive it. I got an idea it will be fine, this setup has been being run for decades but you may have to feather it a little bit. Time will tell.
Alan, I am just curious, how did those dead guys post on your thread?? Yeah, I know....thats just how I am...
I had to go back and reread the site guidelines and rules. There was no mention of math Yanking ya chain, Alan. As Mentioned, the great thing about formulas is you plug numbers in and get you in the range. That’s 90% of it that takes 10% of the time. The other 10% is tuning it, which takes 90% of the time. Pretty much like bodywork and why I leave them in primer
@porknbeaner has it right, engine breathes in, engine breathes out. My engine is never going to see a racetrack and 5,000 RPM plus, I just want it to get down the road and keep up with the traffic. If I were going for performance, I'd stab the Winfield SU-1 I've got into it and go with more carburetion than the 92/81's. If it doesn't work, I'll sell the carbs to the V8-60 race car crowd.
You're right about that. Carb tuning is rewarding if it's easily solved. It's exhausting if you're chasing an issue you can't resolve.
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