so does anyone know where to find the cfm ratings for carburetors, or have a formula to calculate this info? the application.....wondering what my old rochester b powermatic moved as compared to some of the carbs i am looking to replace it with. in theory if the rochester moves X cfm, than ANY MULTIPLE CARB COMBINATION, regardless of make, model or size that equals X cfm should run the engine adequately. the cater carbs are my main target, especially the yf's and the yh's. but any info on any manufacturer is helpful and appreciated. big ed
no more than twice the amount cfm to engine cubic inches,when indoubt go smaller for the street per holley manual 350cubic inches X 2 =700cfm good choice 650
I guess it depends on the engine. Flatheads (239ci) came with carbs approx 150 - 200cfm. I did find this information, which was in the context of flatheads which may not be your application, but I think the information might still apply: "Carb size Determining the optimal carb combo involves figuring out your engine's airflow capacity, which is measured in cfm (cubic feet per minute). The formula to determine airflow is rpm multiplied by displacement divided by 2 (because the engine displaces half its overall capacity each intake stroke). The resulting number is then converted from cubic inches into cubic feet by dividing by 1,728 (cubic inches per cubic foot). It looks a lot simpler on paperrpm x displacement) / (2 x 1,728) = cfm or more simply(rpm x displacement) / 3,456 = cfm Let's say your 239 flathead displaces 275 ci. At 4,500 rpm the arithmetic would be: (4,500 x 275) / 3,456 = 358.07. Your engine's theoretical maximum airflow capacity at 4,500 rpm is 358 cfm. Unfortunately, airflow is not airflow. Street engines operate below maximum capacity. The ratio of how much air your engine pump to how much it pump is called volumetric efficiency (VE). As a general estimate, VE hovers around 75 percent (0.75) for stock engines, 85 percent (0.85) for modified street engines, and 90 percent (0.90) or above for race engines. To find the actual airflow for that modified street Flathead, multiply the theoretical airflow by the VE estimate of 85 percent (358 x 0.85 = 304). That Flathead is flowing approximately 304 cfm, so a pair of 155-cfm Stromberg 97s is just what it needs. Which might be the long way of saying that the manifold hanging on the wall may have been the right choice after all."
maybe i need to rephrase this. i need to know the cfm of a particular carburetor so i will know if moves enough volume to meet the cfm requirements of the motor that i am using/building. for example: the rochester (7015013) that came stock on my 261, moved X cfm. that is why it was used for that engine. i need to calculate, lets say, if a pair (2) carter yh siedrafts will flow the same cfm as the original rochester. if they do then the pair should be a usable combination on this motor. hope this clarifies things????
even if you dont know the cfm you could figure the area of the carb butterflys and do it that way. bore x bore x .7854 = area. figure out the area of the old carb and make sure the area of the new carb(s) equals or exceeds it (within reason). im sure some genius will tell you its not that simple (and it's not) and that the venturi, throttle blades etc make a difference but dont confuse yourself with the gobbledegook. find a carb you know the cfm of and use that area to guage your other findings.
ok, i can buy that logic. anybody know the bore size of a 53-55 carter yh sidedraft? would have been used on the 53-5 vettes.
Here is a bit from The Carburetor shop. http://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm I run three Model H Rochester's from a Corvair on a 250 inline six. Each one is about 70 cfm and one OEM Mono-jet is about 220 cfm. Mine runs really well just driving around, but lays over at top rpm even though the cfm is about the same. At the drag strip, the three ran just the same ET and MPH as the single. Joe
enough volume to meet the cfm requirements of the motor There's no such thing. Carbs too small will flow higher CFM with increased vacuum. Missing from the "formula": 1. manifold type 2. CFM at what pressure drop? 3. how many cylinders?
This has been here before, many times, so I'll copy-paste it here again; Small base Rochester 278cfm stromberg 48 175cfm stromberg 81 135cfm stromberg 97 150cfm stromberg LZ 160cfm holley 92 142cfm holley 94/59 155cfm holley 94/8ba 162cfm holley egc 185cfm Hopefully this helps...
In answer to the Original Question specifically regarding Carter YH carbs. I've also been searching for the CFM rating for Carter YH's and found a reference today. It's from a Corvair Site that builds TBI units to replace the Carters. (Black Hawk Engineering). The web site states that the Corvair YH's were rated at 128 CFM. I measured some of my various versions of YH's last weekend & found a 1-1/2 bore at the base and an approximate 1-5/16 venturi. By figuring out the area & comparing to known CFM of 94's (approx 150 CFM) the venturi areas (sq inches)are almost exactly the same so I figure the YH probably does flow 128-135 CFM. I'm using 6 YHs on a custom Latham Supercharger Plenum I'm building. Movin/on
I've heard that 2 and 4 barrel carburetors were rated differently and after reading the Carburetor Shop.com info now I am wondering if the commonly known ratings on Strombergs and Holley 94 2 barrels like the ones Carb-Otto listed are 2 barrel ratings or have they been converted to 4 barrel figures?
Rochester 4G cfm Ratings Model /CFM Venturi Diameter/ Bore Diameter primary secondary / primary secondary 4 Jet/ 486cfm 1 1/8 1 1/4 / 1 7/16 1 7/16 4 Jet/ 553cfm 1 1/8 1 15/32 / 1 7/16 1 11/16 4 Jet/ 692cfm 1 1/8 1 15/32 / 1 9/16 1 11/16 My blog on how to modify one of these. click
Let me correct something that comes up frequently. CFM is based on demand - not carb size. Low demand (750 on 144" Ford L6) = less than rated CFM. Very large demand (1 Stromberg 97 on 392) = more than rated CFM. This means that if your 1 bbl. flows 200, 2 of them will NOT flow 400, it will always be less.
I think I understand what you are saying but isn't CFM just a manner of comparison/measurement and generally reached by a set rate of conditions of some sort?
Not exactly a response to the original question, but here's a pretty cool carb CFM calculator you can use to see how much CFM you may need... http://www.usaimports.co.uk/Carb_Calculator.htm
I belive you can get a close cfm with the size of the venturi, there are a lot of other factors too though, i dont know the equation off the top of my head, i have it in a '57 sbc book by don francisco
just a manner of comparison/measurement and generally reached by a set rate of conditions of some sort Exactly - but the test conditions are 3" H2O WOT vacuum for 1 & 2 bbl. and 1.5" H2O for 4 bbl. How much vacuum does your motor pull at WOT? If it's not the exact figures shown, the carb flow rate is not the same either. published ratings of different brand carbs can even be compared Yes, but it only shows air delivery, not the mixture curve, response, or fuel droplet size, so the actual power may vary quite a bit even if the jetting is equally accurate in both. even in the related Holley families like the so called "500 cfm 2 bbl" is NOT rated at the same vacuum as their own 4bbls, and so does NOT flow as much. I believe, compared to a Holley 4bbl, the "500 cfm" 2 bbl is really around 390. True. To convert: multiply 2 bbl. rating by 70.71% to get flow at the 1.5" (lower) vacuum 4 bbl. rating. Multiply the 4 bbl. rating by 141.4% to get the 3" 2 bbl. rating. The formula is simple: New flow rate = (new vacuum level ÷ old vacuum level)^.5 × old flow rate 500 2 bbl. flows 353.55 CFM at the 1.5" vacuum level.
The CFM is always reduced by any obstruction in the air path. The notorious air cleaner support bridge on the Model B is an example, as are secondary air valves, chokes, venturi clusters, discharge nozzles, accelerator pump squirters, and casting irregularities, flashing, mis-matched core seams, exposed threads, bolts, etc. In addition, the length of the carburetor matters - a very long barrel with the same venturi will flow more. Larger transition from throttle disc to venturi (1-3/4" throttle, 1-1/4" venturi) flows more per square inch of venturi area than small transition (1-3/4" throttle, 1-1/2" venturi), but it's not linear. The linked "calculator" is just another of the same old D × VE × RPM ÷ 3,456 = CFM, and it still doesn't work. If you already know what vacuum your engine pulls at WOT and peak power on a dyno... you don't need a calculator. If you don't, it won't help you.
The whole commercial use of the concept of CFM reminds me of the quote attributed to Mark Twain - "figures don't lie, but liars figure"! Jon.
i'm confused w/all the numbers, i'm building a 34 with a stock 1955 ,331'' cad eng w/ 3 97's progresive linkage, 3 spd od trans, 4.11 rear, 29'' tires, i don't race, just drive. will this carb set up work???
Bringing this back up because I'm totally confused. Using the chart in post #9, I would like to use two 92/ 81's with straight linkage and an Offenhauser Super on my basically stock 59AB. Is this going to work or am I better off selling the 81's and using a pair of 94's or 97's with progressive linkage?
The Jalopy Journal
