Since the US auto makers in general, and the Holley carb company in particular, haven't needed to cater for six cylinder perforance vehicles like ours, heres the source of the answer :arrow:
http://fordsix.com/forum/viewtopic.php?t=7015. Here's the question again:-
SuperMag asked
Recently a conversation has popped up that questions whether or not we undercarb our sixes. Since this is where the discussion started, let's look at the 300, and ask the question "Is the 390cfm Holley big enough for the job?"
Yes, we do under carb our sixes, and the 300 is one of the most undercarbed engines you can get. But thats okay, so are a lot of Ford engines.
Just for you, Super Mag, here's four empirical formulae that sorts the wheat form the charf:-
Smallest carb venturi area for resonable performance:-
Cubic Inch displacement divided by 115
On a 300 cid engine, thats 2.607 sq inches of venturi.In this case, the smallest carb should be bigger than the #2300 or #2300 350 cfm 2-bbl (2.215 sq in), and slightly smaller than the same breed 500 cfm 2-bbls. (2.970 sq in). Use the 500 cfm Holley.
Best carb for performance and economy compromise:-
Cubic Inch displacement divided by 70
On a 300 cid engine, thats 4.286 sq inches of venturi. In this case, the best carb should be bigger than the #4150 or #4160 390 cfm or 465 cfm 4-bbl (3.547sq in or 3.759 sq in), and slightly smaller than the same breed 550 cfm 4-bbls. (4.669 sq in). Use whatever carb fits a 4-bbl manifold. I'd say the Road Demon 525 cfm fits the bill perfectly!
Best carb for maximum performance:-
Cubic Inch displacement divided by 34 to 57. Use the smaller number if engine is able to rev above 6000 rpm, use larger if engine is not likely to go above 6000 rpm
On a 300 cid engine, thats 5.263 sq inches. The 4-bbl 4011/4165 650 cfmspreadbore (5.069 sq in), the old 660 Centresquirter #4160 (5.160 sq in), Fords # 4180 600 cfm carb (5.160 sq in), or he 4010, 4160 versions of this 600 cfm design.
The best independent runner carb for perfromance is (3 X 2bbl Weber, DellOrto, SKF, etc) :-
Cubic Inch Dispalcement divided by 22, if its a streeter reving to 6000 rpm, or 27 if its a lower rever than that. For a 300, six carbs with 42 mm throats are required, so nothing less than three DCO 55'S with 42 mm chokes, or some 48 DHLA Dellortos with 42 mm chokes. You could go down to DCOE 45's with 39 mm chokes for something that won't see over 6000 rpm.
If you look at the chart I prepared on all the Holley carbs made, check the engine sizes that effectivley used these carbs and relate them back to the calculated venturi area, you get a clear relationship. There are 44 carbs there, and if you look at column J. |Total V AREA|, you can find a carb that fits the formulae.
Post script. The 250 Falcon cross-flow engine, up till 1992, ran a Weber/Holley #5200 32/36 derivative carb, the Weber ADM 34/34. It was a carb of 1.736 sq in venturi area. Sized for more than 140cubic inches for every than 1 sq in of carb. It gave, in service, brilliant fuel economy, but poor power (131 hp). The same size carb was used on 97 cubic inch Cortinas and Capris. There was only 55 cubic inches for every 1 sq in of carb. So on a 1.6 liter sucker, it was a screamer that could rev to 6600 rpm, on the 4.1 liter engine, a plodder, which hated going over 4000 rpm.
If you do the calcs for the 200 cube six, you find the #5200 Holley 2-bbl is the least carb you need for reasonable performance, and the 500 cfm Holley for the performance and economy compromise, and the 390 cfm comes out as the minimum for perforamance. A real screamer might like a 600 cfm carb, if it could rev over 6000 rpm. I've suggested the Holley 465 in other posts, because it's got less signal than the 390 cfm carb. For the best independent runner carb for performance.Three DCOE 45 with 35 mm chokes if the engine is a screamer. If it is a rather milder engine, and not like Jacks alloy headed Locost or Mustangaroos 2V screamer, then 32 mm chokes in either DCOE 40,42 or DCOE 45mm carb bodies will do the trick.
For the 250 six, given good manifolding, you get the 500 cfm 2-bbl as the least carburation, the 390 cfm for the best performance/eccomy compromise, and the 465 to 550 cfm 4-bbls for the performance of a engine that won't rev past 6000 rpm. A screamer going over 6000 rpm could theoretically run an 830 cfm 4-bbl. In practice, the 600 cfm seems like the best carb for a hard charging six that might see 6000 rpm. For the best independent runner carb for performance.Three DCOE 45 with 39 mm chokes if the engine is a screamer. If it is a rather milder engine, and most 250's are, then 35 or 36 mm chokes in either DCOE 42 or DCOE 45carb bodies will do.
). The tiny primarys would give good driveability, and those HUGE secondarys would flow all the air you could want as long as the opening event was properly calibrated so they wouldn't open too soon. 
That's it mate. A rule of thumb, based on 50 engine combos with the Holley, Holley/Weber, Weber, Solex and Dell Orto carbed engines ranging from 78 cubes to 540 cubes of engine . Once you have the basic venturi sizing to the capacity of the engine, you can then play around with the level of 1, jetting, 2, cam , 3, porting 4, exhast, and ultimately, the power you need. It's all in the four modifcations that make the difference between the mild one and the wild one. Once the sizing is within 'cooee' venturi area doesn't need to change at all. This is very similar to the BSFC rating with injectors in EFI vehicles. There is a fairly wide margin. Its capped at maximum levels, and you could run much larger injectors but still have the car run well.
