Again: 250 is the actual flow of a Holley 350 2 bbl. Holley 2 bbl. rated at 3.0" Hg. 4 bb. rated at 1.5" Hg. To convert, multiply by .7071 350 .7171 = 247
The factory QJ manifold is better than any 3 2 I've seen, most are either 1955 tech (too small) or cosmetic only.
So I found a guy who builds multi carb setups and is charging $1300 for a ready to use system. Using eelco 3x2 intake and 2G carbs with progressive linkage
If it is about performance, which is perfectly admirable, 2x4's is the way to go or the big single 6bbl. For me, I kinda like cool, unique, and hood up pretty. I rarely put my foot to the floor. 465ftlbs is pretty strong to me.
That's correct if you're referring to the old Offenhauser 3x2 intake. However, the EELCO 3x2 manifold is similar to their 2x4 manifold....which is a clone of the old Edelbrock 2x4 intake....one of the best designs out there. I don't recall ever seeing any dyno tests of a single 4bbl intake vs a 2x4...has anyone else? My blue car did pick up 2 tenths with a 66 Q-jet over a pair of 500cfm AFB's.... where are apparently too small.
Haven't seen any tests, but "a clone" is what the salesman thinks you want to hear (as opposed to "my kid drew it on a cocktail napkin"). Manifold development is almost as bad as cam development, and what appears to be a simple swap (2 4 > 3 2) introduces many variables - a clone is simply not possible. The entire popular wisdom of "nailheads like a lot of CFM" has little basis. A 401" engine with a 4,500 RPM peak power and 90% VE needs about 470 CFM, with a single-plane manifold to get 1.5" Hg at WOT (which is where all common 4 bbl. carbs are designed to function). Using a dual-plane manifold almost doubles that, since each cylinder can only access barrels on 1 side of the manifold (single 4 bbl.: 1 primary + 1 secondary), so 2 500 is about right (especially if either the VE or RPM go up). If this is exceeded, the carbs simply flow less (WOT vacuum drops) so it's partially self-correcting. If the vacuum drops to 2" Hg, the actual flow drops to 86.6%, so an 750 rated CFM actually flows only 650. The reverse is also true: you can't "starve" an engine with a carb that's only slightly too small, because the higher vacuum increases flow. Example: a 390 CFM 4 bbl. carb (on a dual-plane) looks too small, but if the actual WOT vacuum is 3" Hg the carb will flow 551 CFM - not bad.
So I am going to buy the 3x2 setup but I have a couple quesitons. Should I get hard lines or regular hose lines. Air cleaners or scoops
Different rating methods between 2-barrel and 4-barrel. They are not directly comparable. Two barrels are rated at 3" vacuum; four barrels are rated at 1.5" vacuum. More vacuum = more CFM. But if you re-rate those 2-barrel carbs as if they were a four barrel, you'd have "only" about 930--950 cfm. At that point, you'd need to look at the plenum volume and runner shape/cross-section to see if the manifold can flow what the carbs can provide. First Guess: A "good" 4-barrel and modern 4-barrel manifold will out-power a typical 3X2 or dual-four system. Second Guess: The best 4-barrel manifold available for the Nailhead is the '66 Q-jet unit. Maybe I'm wrong. In fact, I hope so. I'm thinking the nailhead needs some serious development in the speed-parts area.
Hmmmm, let me study this out,,, holley rates 2 barrel carbs at 3.o hg and says they flow 350 scfm......but you say they only flow 247 cfm....hmmmm,,,,, do you suppose that holley doesnt know that they are rating at 3.0..... or that the carb only flows 247....wonder why they would do that????? they just design them and build them..........
i do remember on the 440 six pack carbs they had 350cfm center carb and two 500cfm end carbs for a total of 1350cfm. in a article in the mopar magazine they said you really only have about 955cfm.when you do the math 1350 divided by 1.414 = 955
Agreed. I like something different as well. However, remember that "aggravation is proportional to the square of the number of parts in the assembly". 1 4bbl = aggravation factor of one. 2 4bbls = aggravation factor of four. 3 2bbls = aggravation factor of nine. Eventually I will install dual quads on my Buchev pickup. I have no illusions about it - it will be strictly for looks.
'Cause they started rating two-barrels first, chose 3" vacuum as a standard in part because 3" of manifold vacuum is realistic for a V-8 at WFO with "a" two-barrel. By the time they were rating four-barrels, 1) finding an air pump huge enough to suck that hard on the larger venturi area was a problem; and 2) 1.5" of vacuum at WFO was a more reasonable figure for a V-8 of that time period with a 4-barrel of that time period. As we've found out, 1.5" of vacuum is still too high for maximum power; and "fixing" that means buying carb(s) so huge that idle issues become a factor. The better solution would be rating multi-two-barrels as if they were a four-barrel so the numbers would be comparable. But hey, 1300 cfm sounds more impressive in the magazines than 950 cfm.
I built a 6 Strombergs on an offy sbc setup when I was a teenager. I spend the whole summer building it out of my Dad's parts bins when I was 16. My dad sold it when I was at work one day.ou:
I bought my setup the other day I should have it in about a week or so. It's an Eelco 3x2 intake with 3 Rochester 280 cfm carbs and progressive linkage. My question is, is this enough cfms for cruising around?
