Alright, so I recently found out that 4 barrel and 2 barrel carburetors don't use the same standard of flow... Basically, take the flow rating of a two barrel carburetor and divide it by 1.414 to get the 4 barrel flow. So a Six Pack, for example, does not actually have 1,350 CFM of flow potential, it has about 955 CFM. My question is, does everyone use this? Holley for example. Does a 500 CFM "race carburetor" actually only flow 354 CFM, or is it 1/2 of a 1,000 CFM 4150? It would make more sense for it to be 500 CFM by the 4 barrel standard to me, because 354 CFM isn't what I would call a "race carb", even for a 2 barrel. So does anyone have any info on this?
OK, I'm in, I have had this explained to me before, it seems I got it a little different though, and still don't completely under stand CFM in 2 and 4 barrels. So any one who can or will help clear this up, I'm in.
I heard it initially from http://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm. I also have a program on my computer from Comp Cams called Cam Quest, which is a virtual dyno of sorts.. Whether or not it's accurate in power numbers is unclear, but it's supposed to be pretty close for curves and proportional power differences between builds. Anyway, it lets you chose from 2 barrel or 4 barrel when you enter the flow, which I never gave a second thought to before. So I punched in 1,350 CFM under two barrel in a 460 build I had already set up, then I went to 955 under 4 barrel, and the power numbers were identical to the tenth, which is as accurate as the system goes. So apparently it's true.
yes they are rated differently because the cfm is derived from pulling air at a different atmospheric depression for 2 barrels vs 4. as to why- i haven't the foggiest... rick
It appears to an arbitrary standard. Better explanation at http://www.candsspecialties.com/ratings.html
Also note that different carb companies use different standards to rate cfm. For example, Holley vs Edelbrock/Carter. Holley uses "wet" flow to rate their carbs, so, air+fuel. Carter (and now Edelbrock) uses "dry" flow to rate theirs, so, just air. What does that mean? Well, it means that a 770 cfm Street Avenger Holley actually flows about the same as an 825cfm Edelbrock, because "wet" flow ratings aren't as high as "dry" flow ratings. And of course, you're engine won't run on "dry" flow...
It actually makes some sense if you understand what they are getting at on the C&S website... Let's say you have the same 350 Chevy (of course) with a 2bbl or 4bbl carb. Keep in mind that flow ratings are at WOT...that's important. The 4bbl is going to open a bigger hole in the intake manifold no matter what, so the vacuum signal against the carb will be lower than it would with a 2bbl. The different vacuum standards for the flow ratings probably originated from the OEM, and continue to be used to show improvements made by the carb tuning shops. C&S did point out that those flow ratings are wildly inaccurate for racing engines...
Taking this issue a step further. The original poster mentioned the ratings of a Six Pack. I have seen very few multiple carburetor setups rated, because few have the capability of doing the rating. But the ones I have seen (tripower or six pack) have derated the total another 20~30 percent less than the sum of the individual carburetors. This is because of the "join" in the intake manifold where one cylinder is fed from two sources. The dual quad setups seem to lose less in the manifolds than the trips; but there is still some loss. I have posted this before, but take a look at the metering done by the Chrysler engineers on the dual quad hemis (no, I am not a Chrysler person, but I do recognize good engineering) and you will find that the amount of metered fuel is different for each of the eight carburetor throats. Those who do their homework will always have better results than those who rely on a "formula". Still for the street, a large percent of the aftermarket carbs are too large. Racing is a totally different story. IT IS MUCH MORE DIFFICULT TO SET UP A CARBURETOR FOR A STREET ENGINE THAN FOR A RACE ENGINE! Jon.
I don't think he was referring to the original Carters, but the new Carters that are the same carb as Edelbrocks.
One could also ask what the air flow would be for the primaries of, say, a 750 carb. Would it be exactly half, 375 cfm, or some other number. Do four throats on the same carb mean more or less cfm when fully open. Wanna talk about part throttle cfm? Would it be what you think it is or what it is in reality? John
APPROXIMATELY!!! Carter 9400s, 9410s - P 200, S 200 Carter 4758s, 4761s, 9500s, 9501s, 9510s, 9511s - P 225, S 275 Carter 4759s, 9625s, 9626s, 9627s, 9635s, 9636s, 9637s - P 250, S 375 Carter 4760s, 4762s - P 375, S 375 Carter 9800s, 9801s, 9810s, 9811s - P 200, S 600 Carter 4846s - P 200, S 650 Carter 4847s - P 350, S 650 ALL of the above are for genuine Carter carburetors. These numbers may or may not apply if the carb suffix is "sa" rather than "s". Note the APPROXIMATELY in the first line! The engineers did NOT fool around with sizes until they got a venturi that flowed exactly an even number. Thus a carb rated 600 might flow from maybe 585 to 615; as the engineers built a standard size throttle and venturi, and let the CFM be what it was. Also, I KNOW of at least one company that needed a 600 for comparison, and rather than build an entire new unit, simply ***igned a new part number to a 625 (and they came out pretty good in the dyno tests). As mentioned by someone else in this thread, marketing had much to do with ratings, ESPECIALLY in the last few years! Jon.
I had one 600 CFM 2 barrel on a 406 ford with police interceptor heads and cam in a 40 Ford pickup it was real fast and got 17mpg if you kept your foot out out it. Oh when you look down into the carburator how many big holes do you see 1, 2, or 4, that's the difference in Carbs. HAHAHahahah!!!
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