This discussion on VE and cfm is entirely based on US V8 logic, not any practical work the US guys have had with a 250 large runner in line six with even spaced ports.
Go to
any Aussie 200/250 X-flow or 202 Holden forum, and they won't even have these low cfm carb discussions and aspirations, because low cfm carbs just don't work with I-6/L-6 engines.
One of the greatest engine tuners in Australia is Kevin Bartlett, a Gold Star F5000 driver, Ford 351 campainger and GM 350 expert.
In 1990, He said this:-
"250 Falcon Cross flow with Alloy head, go to 4150/4160 Holley 4-bbl 600 cfm vac sec, and set it up with a huge flat spot when going into the secondary barrels".
The reason for the big carbs with the open runner I-6'S?
On an I6 like the Classic Inlnes or 2V 250, or Cross flow 200/250 with 2 or 4bbl carb, a 2-bbl 500 cfm or 650 cfm, or 4-bbl 600 cfm Vac sec or 650 double pumper will provide the fuel an I6 requires.
This is due to
flow efficency problems with an I6.
Essentialy one zone of fuel supply THEN has to distibute out to six cylinders via a 22 " long intake with over 2 liers or 121 cubic inches of ruuner volume.
That needs CFM to fill quickly. If you come down on cfm, it starts to revert to 10:1 to 15:1 air fuel ratio swings cylinder to cylinder. This morbid fixation with reducting CFM does not apply to an I6 because there needs to be inlet runner volume to avoid reversion and fuel stand off, and reducing carb venturi size then produces excessive air speed which creates extra issues with calibration. 4 cylinder guys have found 4-bbl carbs run better than 2-bbl carbs on big cam engines with Holley carbs. The same with I6's.
Most 250 to 365 hp 4.1's get better results with a 650 cfm Double Pumper Holley 4-bbl.
The difference is that any
good Windsor GT40 or Cleveland US V8 or Pinto 2.0 EAO OHC or Fox/Ranger Lima 2.3 OHC are IMHO the most higly developed engines on earth, with cylinder to cylinder flow efficencies of up to 80% from cylinder to cylinder. That means air fuel ratio swings of less than 1 air fuel ratio between any cylinder. Along with that, the intake manifold and carb is only a 10 to maybee 15% flow drop from a bare intake flow CFM. That means if you have 165 cfm at 28 "H20 from one cylinder, the intake and carb flow for an 85% VE carb selection on a 2.0/2.3 I4 or 5.0/5.8 V8 the flow drop will be down to 144 cfm. So you can run a NASCAR on a 390 cfm Mech Sec Holley and get 700 hp or more with perfect cylinder to cylinder air fuel, or a 2-bbl 500 cfm carb on a 2.3 Lima OHC and get 265 hp net with perfect cylinder to cylinder air fuel.
Try a 500 cfm Holley 2-bbl on an I6, and it looses a lot of power over even a 390 cfm carb. Pump up the cfm, and power figures go up exponentially.
Its just an I6 thing, that was found out by Repco engineers in the late 60's, and that is why those old Antipodean racers always dialed up huge 4-bbl intakes or even bigger port on port triple carb systems. Going down to little 4-bbls just gave them less hp and no drivablity improvements
There is no stale, dead air in an even fire in line six with a 2 liters of intake runner volume. Its just a nice enviromnet to sit a big a$$ 4-bbl, and then dial up the air fuel until it gives a good drivablity.
4-bbl in line sixes beat V8's out of the hole shoot, and the bigger the cfm, the better.
The idelised cubic inches times peak rpm divided by 3456 formula is a great tool for V8's, not for I6's.
A final word.
In Australia, the Commodore Challenge 4.2 liter V8's used to make 290 net flywheel hp do 155 mph, and 290 hp with just one 465 cfm Holley 4-bbl, but thats a nice intake manifold of one of the best flow efficency V8's. The factory carb on 81 to 84 4.2 liter V8's was a 725 cfm Quadrajet 4-bbl, and it gave 16.7 second quarter miles with just a 155 net hp engine. When you dongraded the carb to 465, you could get even more power. It could go up to about 334 dyno hp, or 377 Engine Analyser hp with just a 465 or 600 cfm Holey 4-bbl
But an I6 is not a 4.2 or 5 or 6 liter v8, or a 2.0-2.3 liter I4
An I6 needs about 40% more cfm than a V8 to even see the same hp readings.
An ex Ford engineer
See "Restoring the American Dream:130 mph for the Masses" by Csaba Csere
built a1981 Mercury Capri 4.2 4-bb hot rod, the lamest Ford for its time V8 ever, and needed 600 cfm to make a 132 mph road burner. Doing anything less than 600 cfm of a 4-bbl I-6 will suck the life out of it...
http://vb.foureyedpride.com/showthread. ... res-needed
The Aussies found in 1 and a half hours an other 90 hp on there little league 4.2 pip squeek engines. They then wanted to go to a 500 cfm Holley 4-bbl, down from a 600 cfm 4-bbl, to get more power. But that's V8's. And its typical of any dyno check. Why do you think 5.0 2-bbl 1982 Mustang GT's ran so darn hard on a 369 cfm 2-bbl 1.21 Motocraft 2150 carb?
V8 logic is just that, for V8's.
Not i6's. tOTALLY dIFFERNT hEAD...
See this
http://s76.photobucket.com/user/Litre8/ ... 3.jpg.html
I6's don't like reduced cfm, because of air fuel ratio and intake mnifold geometery.