I'm building a 264 nailhead engine for my '36 Olds coupe and am trying to decide on what carb cfm I need. Im planning on going with a 4 barrel Edelbrock (until I can save my $$$ for a 3x2). The cost difference between a 500 cfm and 750 cfm is only about $30 Are there reasons not to go with the 750 cfm carb? Or would the 600 cfm be better? Any advice on this would be appreciated.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o> <o> </o> Thanks - Todd<o></o>
750 is too much carb for a 264. I'd go with a small 450 or 500 cfm. -Lee Atomic Radio www.atomicpinup.com
Thank you for the quick responses! What do you mean by "too much carb"? Would it flood out (or O2 starve) the engine?
In short, the carburetion capacity exceeds the motor's flow rate. Remeber, engines act like air pumps. While you may get a 750 cfm to work at high rpm, over carburetion will hamper your low to mid range performance. As suggested, I think you'll find a 400 - 500 cfm carb works the best on that size motor. My friend has a 500 Edelbrock on his 322 Buick and it runs great. -Lee Atomic Radio www.atomicpinup.com
Like he said go with the 500. Fuel economy and performance will suffer with a 750. You wouldn't want anyone smoking near the exhaust. Flame on!
Correct answer: you must have small primaries, after that the total CFM doesn't matter very much especially if the secondaries are vacuum controlled (Holley, AFB, Edelbrock, QuadraJet, ThermoQuad). The extra CFM won't do anything, because the air valve won't open all the way.
Here's a good rule of thumb for carbs: Engine displacement (264 in your case) times RPM you hope to hit (say 5500 for a Nailhead) divided by 3456 = recommended carb CFM. So....(264 x 5500)/3456= 420.13 I'd go with a 450 CFM.
Just an update on this - I saved some $$$ and found the coolest nailhead intake out there - an Edmunds 2x2. Now the 4bbl is out and I've got to find a pair of 2bbls. One problem solved, more created.
NICE find on the Edmunds 2x2! Ebay buyers bid them up like they were cast in gold. It's a good looking intake for sure. I find it funny that they were a 2x2 intake, when Buicks came with a 4bbl from the factory. I'd love to put one on a dyno and see what it'd do against the other vintage Nailhead intakes out there. -Brad
The information I have is that Buick didn't produce their stock 4bbl intake until 54. My Edmunds intake is stamped "53 Buick" on the underneath side. I'm thinking once the stock 4bbl intakes came out from Buick the demand for a 2x2 went way down - therefore Edmunds stopped making them - making this type of intake much rarer. 2x2s and other Edmunds intakes seem more plentiful for other engine types - Buick seems to be the hard one to find.
Any of the '53 Supers I've seen had 322 2 bbl engines, but the '53 Roadmasters had either a Carter WCFB or Stromberg Aeroquad 4 bbl.
FWIW and ignore this if you want but I don't believe that formula.... For example according to that formula a 350 chev wants a 557 (say 600) cfm carb. A 4.3 liter (262 CI) Gm wants a 417 (say 450) cfm carb But each of those engines has 44 CI cylinders -- and each cylinder has to **** fuel through the carb on each intake stroke. So my opinion (again, FWIW) is that both of those motors want the same size carb. So if a 350 runs fine on a 600 cfm carb, use that size carb on the 4.3 L engine.
Straight out of the book, I would listen to that one. The 750cfm on there would be like having a 3ft pecker and not enough blood to get it hard....might look impressive, but pretty useless....
http://www.holley.com/applications/CarburetorSelector/CarbSelection.asp Here ya go, SAMEish formula. Called for a 570cfm A smaller properly sized carb will slap wide open and I mean now! Big over sized ones take a sec to work themselves up to it due to manifold vac***e being insufficient. When the people who make the extra $30 bucks off you going bigger, tell you NOT to go bigger, that really says something.
Also with that final figure allow for the fact that a normal OEM engine is only 75% efficient. So actually would be closer to a 350CFM.
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