Holley published a formula for deciding carb size....Based on cubic inches, and a .8 max volumetric efficiency. I never had an issue , using their guidelines. 4TTRUK
Life just isn't as simple as the OP assumes. Just the design of the intake manifold, for example on a V-8; single plane vs open plenum vs cross ram vs individual throat for each cylinder, can make a HUGE difference in how much carb CFM is needed. And, on a log type manifold a one cylinder, two cylinder engine, and four cylinder of the same identical engine size and design would ALL need the SAME size carb. Althouh it sounds confusing, it isn't too complicated. But as I said, it's not as simple as suggested.
much as carbs cost now a days I think I will stick to the old stand by of cubs, cam, gearing etc to not make a bad decision
And, to reiterate, you cannot expect a carburetor to work perfectly out-of-the box. You might get lucky, but probably not. It might work "fine" for you, but that does not mean that it is properly tuned, or as efficient as it could be, in both fuel economy, or power output. The aforementioned 7 MPG car I got to 21 MPG, was running "just fine" for a customer, but had a little flat spot when bringing in the secondaries. It was not only not "just fine" it was as far out as I have ever seen a carburetor be, without fouling the plugs. Tuning is the key. It is not just a good idea, it is required. I have lost track of the times I have heard "I can't even believe that this is the same car/engine!"
I caught your intro on the layitlow.com website. http://www.layitlow.com/forums/25-air-suspensions/177764-skoty-chops-kustoms-shop.html You do some fantastic work. Have you posted any builds here? I must have missed them.
Technically, that's my boss, Scott, but yes, that's our stuff. I have been after him to post here more, so far... I will see if I can get more stuff up. We're never NOT busy. We routinely have 16-18 cars going at once!
It's really pretty simple, at least to start. (CI/2) * (RPM/1728) = CFM A carburetor is measured by air flow in cubic feet per minute (CFM). An engine is measured by the size of the displacement (of air) in cubic inches (CI). There are 1728 cubic inches in a cubic foot. At 100% efficiency a 350 will displace 350 cubic inches of air, but only half of that is intake because it is a 4 cycle engine, so it intakes 175 CI per revolution. (CI/2) A 350 engine spinning at 10000 RPM would displace 1,750,000 Cubic Inches per minute and therefore require a 1012.73 CFM intake flow. At 80% efficiency, that's only 810 CFM for 10,000 RPMS! So, looking back at the original post ---- if you were planning on a 350 spinning at 6000 RPMs (175x6000/1728= 607.64) , then a 607 CFM carb would be more than enough at 100% efficiency.
Most of the early reference books used the formula of twice the displacement minus 15% as a starting place.
Wow! Ha Ha...it's kinda funny just how carried away this whole thing has gotten! The simple 'formula' I posted gives a very good indication of what a typical street/strip engine may require. (That is to say, one not built for Granny economy nor Don Garlits duty...but a typical 'street machine' that sees use as a street vehicle while also being potent enough for weekend drag racing...in case anyone's forgotten that old "street/strip" term). The dude with the 500 Caddy? That engine came from GM with a ginormous Quadrajet on it already...in STOCK form. So, is it ridiculous to believe that upping it just a bit to 1000cfm if it's a modified engine in a street/strip application is out of the question?? There ARE many factors to consider if you want to bang on your calculator for half an hour determining that you think you need exactly 459.233715 cfm for your particular application...but you'll likely discover that you've under-carbed it...if you can find a carburetor that size to begin with. My method is one I've used for almost 30 years and I haven't had it fail me yet. The 396 in my old Chevelle? Oddly...it was very strong on the street and at the strip with it's 780cfm Holley, just as most of my 305 Chevys have been stellar performers with their 600fm Holleys. Crazy, Man...Crazy.
The "factory" can be all over the place. The Ford 351C2V comes with a 350 CFM 2 bl. The Aussie 302C come with 351C2V ports & the giant 351C4V valves & a 750 Carter....
That's a pretty good rule of thumb. Maybe multiply the result by 1.1 if you're going to turn more rpm. Also it depends a lot on the intake manifold. A dual plane can often take a bigger carburetor because the larger carb has a slower exit velocity allowing the mixture to turn more easily into the runners without fuel separation. That's why the 780 CFM carb on early dual plane Z28s worked so well with 302 cubic inches. And it was still large enough to service the engine's higher rpm capability.
I just sacrifice a chicken on a full moon and the voodoo gods send me visions of glorious burnouts and ancient voices whisper the correct carb I should use. It's never failed me before, I just wish they'd stop speaking in spanish.... "Utilizar un holley treinta y tres diez!"
It's never failed me before, I just wish they'd stop speaking in spanish.... Next time you call, you have to press 1 for English.
so lemme see if I got this straight... calculating the swept volume of the engine, multiplying it by the rpm, then taking that number, and comparing it to a cfm number generated at an arbitrary pressure drop ( LOTS of them arent even actually flow numbers, they are fudged to differentiate one carb model from another) is the right way to select a carb, right? So is this method correct for IR or plenum equipped intakes? Two small plenums connected by balance tubes? One large plenum? Think plenum volume might influence this?? Think the pressure drop the carb was tested at might change the "cfm" number? How about wet or dry?? I guess booster venturi design has nothing to do with this either right?? You guys have fun...
George - the engineers sat up nights to come up with the exact diameter throttle bores, venturi diameters and heights and air straightening vanes so that the carbs would flow exactly even numbers to make life easy for the sales department. Just to prove your point, when one carburetor company was asked to produce a 600 CFM for comparison purposes with another carburetor company, they simply renumbered a 625 CFM with a new number. Guess which one the magazine bench racers liked! Jon.
Heres another one for the "formula" guys to chew on. If I have a 750 and its "just right" for my combo, and I then take it and polish the casting marks off the booster venturis, put a nice 3/8 radius on the inlet side, and slab the throttle shafts, and it picks up a 135 cfm on the flow bench at the same pressure drop, am I now "over carburated"??
Per my previous posts in this discussion, carb flow requirement can be effected by the variables you mentioned, and more. I often say essentially what you posted, the less you know about something the simpler(or easier) it looks.
Yes, my mother provided 30 days worth of homemade soups. The parts to upgrade the 455 Olds have mostly arrived. And a major league stair climber and pulldown machine were scored for w a y cheap. And back to topic, I am not over carb'd on any (OT included) of the rides. Fat Hack is legend here on HAMBville.
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