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RIP (12/27/60-7/18/15)
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767 Posts
Discussion Starter #1
Hi gang.
CFM, as it pertains to carbs...................does this mean "cubic feet per minute"?
if so............and 850 cfm carb would fill a 8.5' x 10'X 1' deep water bed in one minute?
excuse my ignorance..................but I DONT THINK SO!!!
 

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RIP (12/27/60-7/18/15)
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767 Posts
Discussion Starter #4
It just dosnt seem possible.
the pressure would have to be incredible.
 

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64-67 Expert
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8,561 Posts
Reminding me of the Chevron F-310 commercials from about, oh, 40 years ago. Filling up the bounce house sized bag with exhaust....!!! Yech.
 

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RIP (12/27/60-7/18/15)
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767 Posts
Discussion Starter #8
the light bulb just came on.............:cool
air,not fuel
silly me ...lol
 

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Super Moderator
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5,741 Posts
the light bulb just came on.............:cool
air,not fuel
silly me ...lol
Yeah, it's cubic feet of air. Also the ratings can be a little deceptive because they're taken at some standard 'vacuum' state - 1.5 inches of mercury for 4bbl carbs (meaning that the "engine" is pulling on the carb hard enough to create some pretty good vacuum, so therefore by definition it's not able to get 100% of the air it wants.) That's why, up to a point, an engine's performance will improve with a carb whose cfm "rating" is larger than what the motor theoretically needs. For optimum performance you don't want the engine to have to "suck" at 1.5 inches of mercury to get air. In fact, the closer to 'zero', the better. (It can't ever be actually zero because then there'd be no air movement at all :) Sooooo... if we go back to that theoretical 400 that needed 850+ cfm at 7360 rpm, at 100% efficiency --- let dial that back to say, 85% efficiency --- that puts us at around 720 cfm needed. To get 720 actual cfm through a carb without forcing the engine to pull 1.5 inches of mercury to get it might require a carb "rated" at 900 cfm or so.

So, why not just put the biggest honkin' carb on it you can and be done? Because for all the circuits to work properly a carb needs to "see" the air moving through it at certain minimum velocities for each circuit, so on a motor that's not "big" enough to move that much air it's just not going to work right.

Everything must be in balance, Grasshopper --- not too big, not too small :D

Bear
 
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