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post #1 of 9 (permalink) Old 10-03-2011, 07:25 PM Thread Starter
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Cfm

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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post #2 of 9 (permalink) Old 10-03-2011, 10:33 PM
 
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LOL, funny stuff. Yes, CFM is... Are you asking what carb size for your car? Quadrajet is the answer.

Burning rubber since 1982!!
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post #3 of 9 (permalink) Old 10-04-2011, 02:53 AM
 
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A 400 cu. in. motor would ingest 851.8 CFM at 7360 RPM at 100% efficiency.

Larry

2005 GTO Quick - 1984 Honda V65 Magna Quicker - 2004 Z06 Quickest
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post #4 of 9 (permalink) Old 10-04-2011, 06:24 AM Thread Starter
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It just dosnt seem possible.
the pressure would have to be incredible.
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post #5 of 9 (permalink) Old 10-04-2011, 07:52 AM
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A motor is nothing more then a glorified air pump. So, yeah, hook up the exhaust to the air bed, crank it up and voila!
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post #6 of 9 (permalink) Old 10-04-2011, 08:21 AM
 
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Imagine your 400 running at 7 grand....then hook BOTH exhaust pipes to your air mattress.....



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post #7 of 9 (permalink) Old 10-04-2011, 08:49 AM
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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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post #8 of 9 (permalink) Old 10-04-2011, 01:04 PM Thread Starter
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the light bulb just came on.............
air,not fuel
silly me ...lol
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post #9 of 9 (permalink) Old 10-04-2011, 02:09 PM
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Quote:
Originally Posted by Topkat View Post
the light bulb just came on.............
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

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