If both the 650 and the 850 are flowing the same amount of air, and if each is jetted properly to deliver the same air-to-fuel ratio, then both will also use the same amount of fuel. If the 850 is really flowing more air that the 650, then yes, it will also use more fuel.

One point I'm trying to make is that the carb is not the tightest restriction in the induction system. The intake valves are. The total throat area of one primary bore and one secondary bore of a Quadrajet is larger that the throat area of the four intake valves it feeds (once you factor in the fact that each intake valve is open only ~300 out of every 720 degrees of crank rotation for a performance cam). On top of that, the carb's throats have almost zero restriction in them, whereas the intake valves' throats are seriously blocked by the valve themselves.

So if your big concern is that your engine is starving and you need to lessen the restriction and give it more air, then the first place you should look is to the valves and heads. And any racer will tell you that. At 5000 RPM the 650 carb will restrict the air slightly more than the 850 (ie, have slightly greater pressure drop). Higher RPMs will be worse yet. But that restriction isn't much compared to the restriction of stock valves and heads.

Maybe I got a little sidetracked here. I just don't want people thinking that if they slap a 1000 cfm carb on their 455 that the carb is suddenly flowing 1000 cfm. If will flow whatever the engine demands from it, which is mostly a function of displacement and RPM. I'm not advocating that everyone put a 650 carb on their 455. That slight extra pressure drop will reduce performance (all else being equal). I just want you to understand that the CFM rating of a carb is only a rating as measured under standard conditions. Your engine is not one standard condition.

 

After removing old motor and installing a new 455 and getting a new cam for it from Mark,i was looking at my old motor and cam was severely out of time due to bad markings on the crank gear.New motor in sounds totally different and loves the 850 quick fuel.Wife came in while breaking in new cam and she says that sounds totally different.Cant wait till it warms up here.

 

That's why you should always degree the cam. Doing that makes sure that everything matches what the cam card says. If something is WAY off then you know that something is up either in the timing set, or somehow they messed up when they cut the cam.

Now wait for some warm weather and go tear it up!!