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

69Chevyc10 · 04-28-2002, 08:57 PM

I bought a 350 for my 69 C-10 the other day. The engine is out of a 1976 Chevy and I was wondering how many cfm the carberautor is. It has the stock quadrajet on it. Thanks Brian Wolf

72sleeper · 04-28-2002, 10:52 PM

not to invade on your question but im sure whoever replies knows some about carbs and i dont want to make a post askin the same thing but i got a new 350 with a cam and new heads now i got a 2 barrel on i 500cfm but i waswonderin what size 4 barreli should get,600????

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Clint
72 c'10 350ci/400hp
sleeper forever

Pont · 04-28-2002, 10:58 PM

69Chevy, if you can get the number off the carb (should be on the main body running up and down, probably at least 9 digits long) I might be able to tell you what cfm that carb is.

72sleeper, IMO, 600 is just right for street use. Should be an all around good size to run. There are exceptions depending on what you want in a carb and what you want out of it for performance.

------------------
67 SWB 4x4 - 396 BB / SM420 4 spd
8" lift / 15x14 Welds & 35X14.5 SSR's
Grants Pass, OR

Longhorn Man · 04-28-2002, 10:59 PM

'72, what kind of heads you have...whats the specs on the cam?
'69, while i am not sure, I'm thinkin it is a 750-ish.

------------------
'69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man
I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it.
FINALLY got the HORNIAC...a '70 one ton Longhorn with a Pontiac 350/350 and lots of 'personality'. Check out The Longhorn Webite.
If you need a pic posted, E-mail me at longhornmail@yahoo.com
Andy, in Columbus Ohio

72sleeper · 04-28-2002, 11:02 PM

its 268 duration comp cam not really big butthe biggest i could get with a 2 barrel i might upgrade when i get a new carb and im not sure abut the heads we bought the motor built and it messed uponce at a red light (head) but we fixed em and im lookin for the most perforance gain i can get out of it

------------------
Clint
72 c'10 350ci/400hp
sleeper forever

The Blue Pig · 04-29-2002, 01:56 AM

I am sure all you guys have answered questions about CFM before. CFM is highly overrated, and most people with a dwindle of performance jump right to a 750. If you arent moving the right amount of air, fast, you just wont create enough vacumn to be able to draw fuel efficiently out of a 750. I know of one quick (and somewhat basic) formula you can use, a teacher taught it to me, and holley uses it too. Take your CID, multiply it by the high end RPM you plan on running, then divide that number by 3456, and that will give you a rough estimate of the CFM you should need. However, that equation places your engines volumetric efficiency at 100%, and most naturally aspirated engines are not running 100% v.e. So, say your rings are seated good, and there are no leaks in your system, you are probably running in between 90-95% v.e. So you would take that final number you got from the above equation, and multiply that by .9, and that will give you a rough number to go by. Just a little hint to get you off in the right direction.

Longhorn Man · 04-29-2002, 05:45 AM

Actually, to bo 100% acurate, an internial combustion engine is not even close to 100% volumetric efficiency. THe closest ones are turboed, and they only get up to about 85%. The rest of us are stuck somewhere in the 60% area.

------------------
'69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man
I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it.
FINALLY got the HORNIAC...a '70 one ton Longhorn with a Pontiac 350/350 and lots of 'personality'. Check out The Longhorn Webite.
If you need a pic posted, E-mail me at longhornmail@yahoo.com
Andy, in Columbus Ohio

The Blue Pig · 04-29-2002, 03:02 PM

Really? I thought that naturally aspirated engines could be anywhere from 60 like you said to 90 and turboed and supercharged were over 100%, and thats why one had to keep the compression down on a turboed or supercharged engine. But, hey, I never had either, so I couldn't tell ya for sure.

Blazer1970 · 04-29-2002, 03:27 PM

Those Q-Jets are 750 cfm (sometimes referred to as 795 cfm). With a Q-Jet, the primaries are relatively small, and the secondary air door is controlled by airflow demand from the engine, so there is no issue with being over-carbed as some have described above. You could have an over carbing issue with a purely mechanical secondary carb like a Holley double pumper.

------------------
Tim

dmnall · 04-29-2002, 06:14 PM

Did the engine come out of a 76 Truck or Car ?? If it came out of a truck, then it is a 750cfm 4MV Quadrajet.

------------------
Truck #1 68 Chev LWB Fleet w/ Wood Floor (Sucks Gas and Hauls Ass!) and in process of Body Work...

Truck #2 1994 GMC Sierra SL w/ 4.3L Vortec V6, 4L60E Auto!!

04-28-2002, 10:58 PM	#3
Pont I need another Barley Pop Join Date: Feb 2001 Location: Grants Pass, OR Posts: 1,629	69Chevy, if you can get the number off the carb (should be on the main body running up and down, probably at least 9 digits long) I might be able to tell you what cfm that carb is. 72sleeper, IMO, 600 is just right for street use. Should be an all around good size to run. There are exceptions depending on what you want in a carb and what you want out of it for performance. ------------------ 67 SWB 4x4 - 396 BB / SM420 4 spd 8" lift / 15x14 Welds & 35X14.5 SSR's Grants Pass, OR __________________ 67 Short Fleet 4x4 04 Honda CRF450R 89 Toyota 4-Runner 34 Ford 2 door sedan - Long term project ---------------------------------------------- There is a very fine line between "hobby" and "mental illness"

04-28-2002, 11:02 PM	#5
72sleeper []D [] []v[] []D Join Date: Dec 2001 Location: h'vegas,TN,USA Posts: 78	its 268 duration comp cam not really big butthe biggest i could get with a 2 barrel i might upgrade when i get a new carb and im not sure abut the heads we bought the motor built and it messed uponce at a red light (head) but we fixed em and im lookin for the most perforance gain i can get out of it ------------------ Clint 72 c'10 350ci/400hp sleeper forever __________________ Clint 72 c'10 350ci/400hp sleeper forever

04-29-2002, 01:56 AM	#6
The Blue Pig Thread Wizard Since 2002 Join Date: Apr 2002 Location: Fairbanks, AK Posts: 102	I am sure all you guys have answered questions about CFM before. CFM is highly overrated, and most people with a dwindle of performance jump right to a 750. If you arent moving the right amount of air, fast, you just wont create enough vacumn to be able to draw fuel efficiently out of a 750. I know of one quick (and somewhat basic) formula you can use, a teacher taught it to me, and holley uses it too. Take your CID, multiply it by the high end RPM you plan on running, then divide that number by 3456, and that will give you a rough estimate of the CFM you should need. However, that equation places your engines volumetric efficiency at 100%, and most naturally aspirated engines are not running 100% v.e. So, say your rings are seated good, and there are no leaks in your system, you are probably running in between 90-95% v.e. So you would take that final number you got from the above equation, and multiply that by .9, and that will give you a rough number to go by. Just a little hint to get you off in the right direction. __________________ --70 C-10 Longbed --71 C-10 Longbed --72 K-20 Longbed --74 C-10 Shortbed --86 Toyota Tercel (His Driver) --05 Chevy Trailblazer (Her Driver)

04-29-2002, 03:02 PM	#8
The Blue Pig Thread Wizard Since 2002 Join Date: Apr 2002 Location: Fairbanks, AK Posts: 102	Really? I thought that naturally aspirated engines could be anywhere from 60 like you said to 90 and turboed and supercharged were over 100%, and thats why one had to keep the compression down on a turboed or supercharged engine. But, hey, I never had either, so I couldn't tell ya for sure. __________________ --70 C-10 Longbed --71 C-10 Longbed --72 K-20 Longbed --74 C-10 Shortbed --86 Toyota Tercel (His Driver) --05 Chevy Trailblazer (Her Driver)

04-29-2002, 03:27 PM	#9
Blazer1970 Registered User Join Date: Dec 1999 Location: Old Mission, MI, USA Posts: 2,510	Those Q-Jets are 750 cfm (sometimes referred to as 795 cfm). With a Q-Jet, the primaries are relatively small, and the secondary air door is controlled by airflow demand from the engine, so there is no issue with being over-carbed as some have described above. You could have an over carbing issue with a purely mechanical secondary carb like a Holley double pumper. ------------------ Tim __________________ Tim

04-28-2002, 08:57 PM	#1
69Chevyc10 Registered User Join Date: Mar 2002 Location: ND USA Posts: 122	CFM question I bought a 350 for my 69 C-10 the other day. The engine is out of a 1976 Chevy and I was wondering how many cfm the carberautor is. It has the stock quadrajet on it. Thanks Brian Wolf

04-28-2002, 10:52 PM	#2
72sleeper []D [] []v[] []D Join Date: Dec 2001 Location: h'vegas,TN,USA Posts: 78	not to invade on your question but im sure whoever replies knows some about carbs and i dont want to make a post askin the same thing but i got a new 350 with a cam and new heads now i got a 2 barrel on i 500cfm but i waswonderin what size 4 barreli should get,600???? ------------------ Clint 72 c'10 350ci/400hp sleeper forever __________________ Clint 72 c'10 350ci/400hp sleeper forever

04-28-2002, 10:59 PM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	'72, what kind of heads you have...whats the specs on the cam? '69, while i am not sure, I'm thinkin it is a 750-ish. ------------------ '69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it. FINALLY got the HORNIAC...a '70 one ton Longhorn with a Pontiac 350/350 and lots of 'personality'. Check out The Longhorn Webite. If you need a pic posted, E-mail me at longhornmail@yahoo.com Andy, in Columbus Ohio

04-29-2002, 05:45 AM	#7
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Actually, to bo 100% acurate, an internial combustion engine is not even close to 100% volumetric efficiency. THe closest ones are turboed, and they only get up to about 85%. The rest of us are stuck somewhere in the 60% area. ------------------ '69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it. FINALLY got the HORNIAC...a '70 one ton Longhorn with a Pontiac 350/350 and lots of 'personality'. Check out The Longhorn Webite. If you need a pic posted, E-mail me at longhornmail@yahoo.com Andy, in Columbus Ohio

04-29-2002, 06:14 PM	#10
dmnall Registered User Join Date: Apr 2001 Location: Goleta, CA Posts: 505	Did the engine come out of a 76 Truck or Car ?? If it came out of a truck, then it is a 750cfm 4MV Quadrajet. ------------------ Truck #1 68 Chev LWB Fleet w/ Wood Floor (Sucks Gas and Hauls Ass!) and in process of Body Work... Truck #2 1994 GMC Sierra SL w/ 4.3L Vortec V6, 4L60E Auto!! __________________ 1968 C10 Truck LWB Fleetside 1968 C10 Truck SWB Stepside 1994 K2500 Ext Cab Fleetside 2008 Honda Civic Si lease



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