Your air calculations are correct. Remember that the carb does not just turn off at it's rated capacity. The 2 barrels are rated with more air being sucked thru them until they have 3" of vacuum. The 282cfm calculated above - a 4 barrel carb of 282 cfm will have 1.5" of vacuum @ 4500rpm and a two barrel of 282 cfm will have 3" of vacuum @ 4500rpm. 3" of WOT vacuum is some loss of power, there's a little untapped potential there, but it does not just fall on it's face.Looking to try a different carb without a choke horn, I suppose I can always take the choke horn off as well lol. Currently have a 2 barrel holley street avenger 350cfm. I really don't need a choke, nice to have but I can go without it. According to napkin math the engine needs about 399 cfm's in 2 barrel cfm's which could probably be achieved by just lopping the choke horn off as I have seen it increases cfm. Math in public:
255 cu.in. x 4500 rpm (I'm not running higher) = 1,147,500
1,147,500 / 3456 = 332.03 cfm
332.03 x .85% efficiency = 282.22 cfm (4 barrel cfm's)
282.22 x 1.414 = 399 cfm (2 barrel cfm's)
I'm looking at these two combined to make a high performance two barrel with vacuum, I think that's how it would work anyway if the throttle body was swapped, not sure I have no experience doing so:
Holley 0-4412BKX Holley Ultra XP 2BBL Carburetor
The original Holley 0-4412 has powered thousands of racers to the winners' circle! How do you make it better? Add the proven design elements of the popular Ultra series for more tunability, more durability, more weight savings and more features! The Ultra XP 2bbl carburetors feature all aluminum...www.holley.com
Holley 112-2 THROTTLE BODY KIT
Holley carburetor base plate assemblies are designed for replacement of damaged or worn components. These base plates are manufactured from cast aluminum for durability and good looks.
Okay learning more about carbs as I go and if I want vacuum on the 4412BKX carb it does appear to have a full manifold vacuum port at the base available to just add the fitting. No ported (spark) vacuum but the throttle body can add it if I actually want it. Once the throttle is open the "spark" port is full vacuum and no different than the manifold vacuum so not sure I need it all that much since it only has no vacuum during idle and if that's smooth even with the dizzy pulling vacuum I don't care too much for it since I care for vacuum advance when out of idle and the throttle is open. All that being said, I'm leaning toward just removing the choke tower on the current street avenger 350cfm with a saw then smoothing it out with a dremel lol
I'm speaking from experience here, being one who prefers smaller carbs for cold start, low end drivability and economy. The 2V carb on my 240 is only 245cfm, that's 174cfm if 4 barrel. With a mild performance cam and intake this truck is noticeably stronger above 3000rpm than my 300
truck with a stock cam and 4 barrel. It will pull strong well up into the 4000's, and does not start showing vacuum until 3500 rpm. It could use a bit more carb at top end, but the loss is not extreme. This same carb was factory equipment on a 289 V8.
A 350cfm 2V will be sufficient IMHO. The Ultra HP carb you posted with no choke tower looks good, and it says it's 500 cfm- that's more than plenty, and is more likely to meter well with a low-top filter.
I agree there is some over-reach with the CFM estimates. With some realistic port flows, or VE numbers from actual similar applications, we could have much better estimates.
