Carb cfm with ported heads?
The formula people use ( cid x rpm / 3456 = cfm ) doesn't give carb size but how much volume an engine is at rpm.
If we assume 100% VE an engine only has intake stroke once every other revolution so a 410 would be 205 cubic inches per revolution or 0.119 cubic feet. At 6000 rpm it would be 1,230,000 cubic inches per minute or 714 cubic feet per minute.
Since air is compressible any carb can pass any amount of air, so a 650,750,850 etc.. would all pass the 714 cfm just at different velocity and vacuum (restriction) just like different header sizes.
On a pure race engine you can give up velocity for less restriction, opposite for pure street and street in between.
On your engine your 750 should be slightly under a vacuum level of 1.5 if the formula is correct, which is pretty conservative. But like I said measure the vacuum level and decide from there if its worth going larger and by how much. Or see if you can borrow some larger carbs and try it out.
To whom it may concern if anyone :) know im beating a dead horse, but I think now I know how to make my position clearer.
The carb rating and the cid x rpm / 3456 = cfm seems like an apples to apples but it isn't.
Take a 440 that turns 6300 that formula says this 440 can pass a displacement of 800 cfm or a 100 cfm per cylinder.
Now we wouldn't use the formula to pick a cylinder heads even though there both rated in cfm but neither have anything to do with one another. That engine would need over 3 x's the 100 cfm.
As for carbs, lets imagine we cut a 1" hole in a plate of steel. Now that hole can flow zero or 1000's of cfm depending on vacuum source. Same with carb they can flow anything we want. Now lets take a plate and cut a 2" hole. The 2" doesn't automatically flow more cfm then the 1" just needs less vacuum (power) or in the case of an engine pumping loss (hp). The 1" vs 2" both will flow the same air at different vacuums level.
Same with carbs, the vacuum level with determine the cfm of the carb.
So the trick is to figure out where you want the vacuum level to be. Then convert the cfm of carbs to that level then the formula will work.
So say you figure 1.2" hg gives you the best compromise of streetablity to power.
Say the formula says your engine will displace about 500 cfm of air and say a 650 @ 1.5" hg is a 500 cfm @ 1.2" hg then thats your carb or say its more of a strip terror and decide vacuum should be at 0.8" hg that might be a 850 cfm @ 1.5" hg.
Thats why I think carb should be rated at more the one vacuum drop.
Done beatimg the deadhorse.