Carburetor CFM adjustment - The 1947 - Present Chevrolet & GMC Truck Message Board Network

chevyc1068 · 11-20-2006, 08:40 PM

How can you adjust the CFM of a carburetor? Can you adjust the CFM of a carburetor? Is it as easy as changing the size of the jets or is it a little bit more complicated than that. I'm asking because I only need about a 500 CFM for my 307 engine but I like the Holley 4160 carb. So please help me out.

qksilver · 11-20-2006, 08:51 PM

Not really except on high dollar carbs. On those you can change the venturies

jhow66 · 11-21-2006, 12:06 AM

Holley 4160 (600cfm) wth vac. secondary will work just fine with the 307. When you floor it the rear barrels will open only as far as the engine can handle.

*Shane* · 11-21-2006, 12:10 AM

CFM = Cubic Feet per Minute which is how much air will flow thru a carb at a given velocity. As mentioned above, the only way to "change" CFM is to make the air passages/venturi either smaller or larger.

It's kind like asking if you can change a 2 1/2" exhaust pipe into a 3" pipe. Sure, it can be done ... but not by practical means.

*Billla* · 11-21-2006, 12:43 AM

Quote:

Originally Posted by jhow66

Holley 4160 (600cfm) wth vac. secondary will work just fine with the 307. When you floor it the rear barrels will open only as far as the engine can handle.

That's one of the nice things about a vac secondary carb

You might need to put in a stiffer spring, but I doubt it.

*Billla* · 11-21-2006, 12:44 AM

Quote:

Originally Posted by Shane

CFM = Cubic Feet per Minute which is how much air will flow thru a carb at a given velocity. As mentioned above, the only way to "change" CFM is to make the air passages/venturi either smaller or larger.

It's actually possible to do this with Weber carbs; kind of a cool feature

I don't have any experience with them, but I thought it was possible with the Demon carbs as well - you can just pick up and install another venturi.

gouldie · 11-21-2006, 12:54 AM

you could probably bore the barrels and get buterfies to fit. i know it's been done wih throttle bodies not sure about carbs.

*trkklr77* · 11-21-2006, 01:05 PM

the big demon carb and holley dominators, $1000 carbs can

you can change the jetting to control the amount of fuel per cubic foot of air but you are going to need to get a smaller carb.

you can theoritcialy bore to make them bigger and get butter flyies that fit, but he needs it smaller, you would have to add material to make it smaller.

11-20-2006, 08:40 PM	#1
chevyc1068 Registered User Join Date: Oct 2006 Location: Murdock, Kansas Posts: 147	Carburetor CFM adjustment How can you adjust the CFM of a carburetor? Can you adjust the CFM of a carburetor? Is it as easy as changing the size of the jets or is it a little bit more complicated than that. I'm asking because I only need about a 500 CFM for my 307 engine but I like the Holley 4160 carb. So please help me out. __________________ 1968 C-10 longbed, 307 V8, 4spd- currently under construction

11-20-2006, 08:51 PM	#2
qksilver Senior Member Join Date: Dec 2004 Location: Southern Pines NC Posts: 3,853	Re: Carburetor CFM adjustment Not really except on high dollar carbs. On those you can change the venturies

11-21-2006, 12:06 AM	#3
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Re: Carburetor CFM adjustment Holley 4160 (600cfm) wth vac. secondary will work just fine with the 307. When you floor it the rear barrels will open only as far as the engine can handle. __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

11-21-2006, 12:10 AM	#4
Shane Account Suspended Join Date: Jun 2000 Location: over yonder Posts: 14,270	Re: Carburetor CFM adjustment CFM = Cubic Feet per Minute which is how much air will flow thru a carb at a given velocity. As mentioned above, the only way to "change" CFM is to make the air passages/venturi either smaller or larger. It's kind like asking if you can change a 2 1/2" exhaust pipe into a 3" pipe. Sure, it can be done ... but not by practical means.

11-21-2006, 12:54 AM	#7
gouldie Registered User Join Date: Oct 2006 Location: daytona beach Posts: 101	Re: Carburetor CFM adjustment you could probably bore the barrels and get buterfies to fit. i know it's been done wih throttle bodies not sure about carbs.

11-21-2006, 01:05 PM	#8
trkklr77 Account Suspended Join Date: Aug 2006 Location: vacaville ca, because it as far from indy as i can be Posts: 775	Re: Carburetor CFM adjustment the big demon carb and holley dominators, $1000 carbs can you can change the jetting to control the amount of fuel per cubic foot of air but you are going to need to get a smaller carb. you can theoritcialy bore to make them bigger and get butter flyies that fit, but he needs it smaller, you would have to add material to make it smaller.



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