Carb size suggestion
 

I am in looking to install 2 carb on the top of my B&M 250 blower on my 454ci .030 oversize. What can be the best size between 2 x 750 or 2x 850 cfm carb for my engine for best max power. How to know if carb choice is not too big?

IMO i think 2 x 750 cfm carb is plenty enought but i need to know what is the effect between the right carb size and a too small or a too big carb?

Any help would be great.

I bought a marine B&M 250 setup and it came setup with 2 1000 cfm 4150 style carbs. I don't think you can over carb a roots blower. It will use as much air as it wants.

i agree,and i feel that to small a carb on a blower motor can really hurt the upper rpm performance.

Dual 750's will be plenty of carb. In reality, a single 1050-1150 Dominator carb will be plenty.

11/2/2011 – A/F with Jets of 78 & 85 6.5 PV . Wieand 250 super charger with dual Holley 750 carbs each motor. # 28 Bravo 1’s prop
382 Formula ( Non FASTEC) 62 degree day 48% RH calm water
Bravo XR 1.5 drive. Timing 29 degrees total, Idle rpm 1,000

RPM MPH Boost Vac #1 Vac #2 Stb Eng Port Eng EGT
3000 36 0 6
4000
5000

Whoops not done
540 cid AFR aluminum heads – Jets of 78 & 85 6.5 PV . Wieand 250 super charger with dual Holley 750 carbs each motor. # 28 Bravo 1’s prop
382 Formula ( Non FASTEC) 62 degree day 48% RH calm water
Bravo XR 1.5 drive. Timing 29 degrees total, Idle rpm 1,000

RPM MPH Boost Vac #1 Vac #2 A/F Stb A/F Port EGT
1000 0 0 14 13 11.4 11.8 550
3000 36 6 6 11.8 12.0 1,200
4000 58 0 0 0 11.9 12.3 1,390
5000 72 4 0 0 12.3 12.4 1,410

The size of carb(s) or CFM required for a given application can be calculated by the following formula A: {(CID x RPM) ÷ 3456} x {Boost ÷ 14.7) + 1} = CFM required. The amount of CFM required will determine carburetor size and quantity. If you try to use a carb with less CFM than required, performance and economy may be greatly reduced. Bigger is not always better when selecting carbs that are 30% over what is required, you may encounter problems in fuel distribution

That is a quote from BDS website. Using that calculation, if you wanted enough carb to feed a 460 Inch motor, at 6000RPM, lets say running 7lbs of boost. You will need 1,174 CFM. If you wanted to go 25% larger, which is fine on a roots setup, thats 1467CFM. A pair of 750's would work great(1400CFM).

BTW, in my master holley catalog, they have carbs recommended for certain setups. For example, 871 blower on a small block chevy, recommended carbs are twin 750's. 871 on a big block chevy, recommended carbs are twin 850.

On top of the blower, the carbs are a source of gas and air. the blower works better with no air restriction to draw thru. Kind of why a 1000HP roots blown setup you'll see twin 1050 Dominators feeding the engine, and a 1000HP blow thru procharger it will have a single 800 carb. You wouldnt want to take the big air inlet on the procharger, and say ''Its to big, I wanna close this opening up and restrict some airflow''. Same idea somewhat.

Thanks for the great input and great explanation.
Is that the calculation is good for all blower size?

It takes x amount of air and fuel to make x amount of horsepower.

Twin 750's on a 250 blower works great. Also, fuel distribution, and idle will be much better.

fixx
 

to make things really confusing ask what size for a pro charged application lol..