big carbys

[santaw]

a 460 cubic inc ford spinning at 6000 rpm only draws about 600/650 cfm so mour about carb tuning than how big it is

[santaw]

formula for gaging corect carb cfm cfm +(cid X rpm X cf)/(2X1728} cf= corretion factor based on volumetric effciincy  http://www.mortec.com/carbtip1.htm

[santaw]

if you have no luck whorth that link just go out to your browser and type mortec carbtip1 and that will get you ther well werth the efet lots on tuning carbs

[Dak]

Depends on a lot more things, for instance, my old sprintboat engine, 410 cube Ford, If I had a carb on it based on the formulas it would barely even run and be strangled to death.

[NickD]

Hard to go past a 750 squarebore vac with secondary metering block if it's cranky idling. Big Blocks like air. Centre pivot floats are better too for controlling the fuel cornering. the side pivots are not that great.

[Brendan]

i run a 600 cfm vac sec on my 283 chev, i was told it was too big so i tied the secondaries shut (making it 300 cfm based on the chart from holley) and found the boat would not perform and took longer to get on the plane, i also lost 300 rpm.

[NickD]

http://www.holley.com/applications/CarburetorSelector/CarbSelection.asp

[jarvis]

One major factor that affects the sizing of the carb you choose is whether you are running a split plane or open plane inlet manifold.
I did a lot of research into this when I was trying to decide what size carb to put on my Chev 383 as the old 650 Holley was about knackered, worn around the butterfy shafts and could never get it to idle properly. I looked at all the websites and did the calcs on CID/RPM etc etc and came back with 600 odd CFM for WOT at ~4700 RPM. I wrung a number of engine builders and they all said no, go with a 750 vacuum secondary and you'll be better off and probably find it runs more economically but they couldn't really qauntify why. After a lot of further searching on the net I found an article by a yank engineer that made a lot of sense talking about exactly this subject, with an open plane inlet manifold, as each pot fires it has the volume of most of the manifold as a buffer and is able to draw through the whole of the carb so the calcs are more suited for this type of manifold. With a split plane street type inlet manifold, as each pot fires, it only has about half the manifold volume as a buffer and half the card to draw through so at the moment of peak induction it is quite restricted and going to a larger carb helps reduce this effect. So I bought a 750 Holley and now run more economically than before during normal boating and have picked up another ~1000 RPM at the top end. Hope this helps.....