I assume you have these two different sizes because you have them? It will require to different tops because of the different throttle plate designs.
What RPM do you think you will be running? How will you be running it mostly?
There's more to it than an Internet opinion. Two referenced 850's would be easier to tune IMO.
GT
Hi GT
I'm sure you where just about to ask him "what drive ratio are you running the blower ?", right ?
Blower displacement (cubic inches), multiplied by blower RPM, divided by 1728 equals minimum CFM needed to "feed" the blower.
An 871 is capable of moving between 436 to 500 ci of air per revolution (blower displacement) depending on blower design and condition. That's per revolution of the blower. So, you need to know your projected engine RPM, then your projected blower drive ratio to calculate blower RPM, then you can calculate how much air the blower needs.
So lets say, 6000 RPM max engine speed, blower driven 1 to 1. We'll go on the low side of the range for the blower and use 436 ci of air per revolution. Unless its some super trick, tight clearance alcohol blower your not going to get close to higher number.
436 (blower displacement) x 6000 (blower RPM) = 2,616,000 ci of air per minute that the blower is trying to move.
2,616,000 divided by 1728 (ci of air in one cubic foot) = 1513.88 cubic feet of air per minute.
So a minimum of 1514 CFM of carbs would be needed.
Same engine RPM but blower driven 10% over = 6600 RPM at the blower, and 1665 CFM minimum of carbs needed.
Its all about blower RPM, just plug your numbers into the equation and it will show you what you "need". Then round up to next larger size carbs, because those are MINIMUM needed numbers.
Chances are with a 540 and an 871, your going to be driving the blower close to 1 to 1. So the 660s (1320 CFM) are way to small.
