quadrajet - The 1947 - Present Chevrolet & GMC Truck Message Board Network

BowTieMan · 10-30-2005, 09:26 PM

How much cfm do you think the stock 4 barrel quadrajet on my 70 with a 350 is? Is there anyway I can find out?

*FUNMUDDER* · 10-30-2005, 09:32 PM

I think it would be a 750. They made 750's,795's, and MAYBE 850's for Cadillacs. Thats from looking at old Summit catalogs, what they have listed for replacements. They are big though.

624whldr · 10-30-2005, 09:45 PM

most are 625cfm for the 350s

matthufham · 10-30-2005, 10:18 PM

750's i read the "monster garage, how to customize damn near anything" book, and it said 750 more than a couple of times.

Robert1970C20rstbukt · 10-30-2005, 10:29 PM

Yep, 750. They're dam good carbs when tuned right.

BowTieMan · 10-30-2005, 10:39 PM

My dad said there is no way chevy put a 750 on a 350 with an automatic stock. He said it would bog down to much. And my friends dad said 500-550 stock. Are there any numbers I can look up?

stllookn · 10-30-2005, 10:50 PM

Sorry BowTieMan, Both John Lingenfelter and David Vizard state in Building and Modifying Small Block Chevys that the standard Q-Jet is rated for 750 CFM. The motor does not have to use the much cfm but the carb can flow that much if and when required.

BowTieMan · 10-30-2005, 10:55 PM

So is mine on there now flowing 750 cfm? Or is it that they can flow that much? Thanks for the help guys I will just keep this and get a better manifold instead. Man I love this board!

Robert1970C20rstbukt · 10-31-2005, 12:07 AM

When you're just cruising or whatever, the front 2bbls are flowing about 500cfm, and when you hammer the throttle and the 2 big back barrels open up, then it's pulling 750.

**Stocker** · 10-31-2005, 12:32 AM

The front throttle bores on a Q-jet are tiny (for fuel economy) - dunno what they are rated at but it can't be much. Most of the flow rating comes from the huge secondaries, which (at least on our engines) are vacuum operated, and tha't why the engine doesn't tend to bog at WOT - they only open as much as the engine can handle.

BowTieMan · 10-31-2005, 12:50 AM

Thanks for the help guys. Is there any way to tell if my secondaries are opening or not? It never seems to kick in like my dad's 79 suburban did, and if I rev it in neutral they never open and I don't want to rev it too high for too long to see.

OLDCHEV4X4 · 10-31-2005, 07:26 AM

Get a buddy to push the pedal down while you look down the carb. Not running of course. You'll have to push the choke open to see the secondary plates.

70 shorty · 10-31-2005, 07:48 AM

You should really be able to tell when the secondaries on a quadradjet kick in. It makes a very distinctive sound, very different from when just the primaries are open.

Blazer1970 · 10-31-2005, 07:49 AM

The air flapper valves on the secondaries will never open when you are just revving the engine in the driveway. The engine has to be under load (low vacuum). There is a vacuum actuator on the carb that holds the flappers shut under high vacuum.

10-30-2005, 09:26 PM	#1
BowTieMan Registered User Join Date: Dec 2003 Location: Glendale Arizona Posts: 747	quadrajet How much cfm do you think the stock 4 barrel quadrajet on my 70 with a 350 is? Is there anyway I can find out? __________________ "A true hotrodder wouldn't be content untill he had created a car so violent, so hairy, so totally sick that the very act of dropping the hammer would result in instant death. Anything less results in the need to go faster." - Tony DeFeo

10-30-2005, 09:32 PM	#2
FUNMUDDER Account Suspended Join Date: Jun 2005 Location: Maine Posts: 427	Re: quadrajet I think it would be a 750. They made 750's,795's, and MAYBE 850's for Cadillacs. Thats from looking at old Summit catalogs, what they have listed for replacements. They are big though.

10-30-2005, 09:45 PM	#3
624whldr drive it not trailer it Join Date: Feb 2005 Location: So Jersey Posts: 64	Re: quadrajet most are 625cfm for the 350s

10-30-2005, 10:18 PM	#4
matthufham Registered User Join Date: Feb 2005 Location: Carl Junction, Missouri Posts: 2,061	Re: quadrajet 750's i read the "monster garage, how to customize damn near anything" book, and it said 750 more than a couple of times.

10-30-2005, 10:29 PM	#5
Robert1970C20rstbukt Registered User Join Date: Jun 2004 Location: Beeville, Texas Posts: 1,943	Re: quadrajet Yep, 750. They're dam good carbs when tuned right. __________________ RJD 1984 Chevrolet C30 welding truck 454/SM465 1977 GMC C15 pickup 350/TH350 1970 Chevrolet C20 pickup 350/TH400 1970 Chevrolet K20 Suburban 454/TH350 1969 GMC 8500 478M V6/5 spd winch truck

10-30-2005, 10:39 PM	#6
BowTieMan Registered User Join Date: Dec 2003 Location: Glendale Arizona Posts: 747	Re: quadrajet My dad said there is no way chevy put a 750 on a 350 with an automatic stock. He said it would bog down to much. And my friends dad said 500-550 stock. Are there any numbers I can look up? __________________ "A true hotrodder wouldn't be content untill he had created a car so violent, so hairy, so totally sick that the very act of dropping the hammer would result in instant death. Anything less results in the need to go faster." - Tony DeFeo

10-30-2005, 10:50 PM	#7
stllookn Saving 1 truck at a time! Join Date: Aug 2001 Location: Kent, WA Posts: 6,465	Re: quadrajet Sorry BowTieMan, Both John Lingenfelter and David Vizard state in Building and Modifying Small Block Chevys that the standard Q-Jet is rated for 750 CFM. The motor does not have to use the much cfm but the carb can flow that much if and when required. __________________ '68 C20 Longhorn 50th Anniversary 400/TH400 '68 C20 Longhorn 50th Anniversary 468/TH400w/buckets '72 C20 Halfhorn (Longhorn w/o cab and front clip) '69 Flxible Cruiser (look up ugly in the dictionary)

10-30-2005, 10:55 PM	#8
BowTieMan Registered User Join Date: Dec 2003 Location: Glendale Arizona Posts: 747	Re: quadrajet So is mine on there now flowing 750 cfm? Or is it that they can flow that much? Thanks for the help guys I will just keep this and get a better manifold instead. Man I love this board! __________________ "A true hotrodder wouldn't be content untill he had created a car so violent, so hairy, so totally sick that the very act of dropping the hammer would result in instant death. Anything less results in the need to go faster." - Tony DeFeo

10-31-2005, 12:07 AM	#9
Robert1970C20rstbukt Registered User Join Date: Jun 2004 Location: Beeville, Texas Posts: 1,943	Re: quadrajet When you're just cruising or whatever, the front 2bbls are flowing about 500cfm, and when you hammer the throttle and the 2 big back barrels open up, then it's pulling 750. __________________ RJD 1984 Chevrolet C30 welding truck 454/SM465 1977 GMC C15 pickup 350/TH350 1970 Chevrolet C20 pickup 350/TH400 1970 Chevrolet K20 Suburban 454/TH350 1969 GMC 8500 478M V6/5 spd winch truck

10-31-2005, 12:32 AM	#10
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,595	Re: quadrajet The front throttle bores on a Q-jet are tiny (for fuel economy) - dunno what they are rated at but it can't be much. Most of the flow rating comes from the huge secondaries, which (at least on our engines) are vacuum operated, and tha't why the engine doesn't tend to bog at WOT - they only open as much as the engine can handle. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay

10-31-2005, 12:50 AM	#11
BowTieMan Registered User Join Date: Dec 2003 Location: Glendale Arizona Posts: 747	Re: quadrajet Thanks for the help guys. Is there any way to tell if my secondaries are opening or not? It never seems to kick in like my dad's 79 suburban did, and if I rev it in neutral they never open and I don't want to rev it too high for too long to see. __________________ "A true hotrodder wouldn't be content untill he had created a car so violent, so hairy, so totally sick that the very act of dropping the hammer would result in instant death. Anything less results in the need to go faster." - Tony DeFeo

10-31-2005, 07:26 AM	#12
OLDCHEV4X4 Registered User Join Date: Apr 2000 Location: Chester County PA, USA Posts: 1,228	Re: quadrajet Get a buddy to push the pedal down while you look down the carb. Not running of course. You'll have to push the choke open to see the secondary plates. __________________ 72 k/10 short wideside 72 k/5 Blazer 72 K/5 Blazer - obstacle course racer 72 custom/10 8' bed 70 K/5 Blazer 67 C-30 GMC Flat bed 86 GMC K2500 Plow Truck 85 GMC K3500 84 M1008 85 M1008 77 G30 Motor Home 72 Sears 12hp Tractor Glenmoore, Pennsylvania

10-31-2005, 07:48 AM	#13
70 shorty Where's my beer? Join Date: Oct 2004 Location: Glen Allen, VA Posts: 1,768	Re: quadrajet You should really be able to tell when the secondaries on a quadradjet kick in. It makes a very distinctive sound, very different from when just the primaries are open. __________________ 70 C10, shortstep, .30 over 250,Hurst shifted 3 spd parts hauler. Holley 390, Clifford Intake, Header, ported cylinder head, unknown bigger cam. 79 Corvette L82, 4 spd, black, red interior, headers, flowmasters, and unkown bigger cam. '03 Grand Cherokee Laredo, 4.0 4x4 daily driver. 165K miles, and runs great '08 Tahoe LTZ 4x4 155K

10-31-2005, 07:49 AM	#14
Blazer1970 Registered User Join Date: Dec 1999 Location: Old Mission, MI, USA Posts: 2,510	Re: quadrajet The air flapper valves on the secondaries will never open when you are just revving the engine in the driveway. The engine has to be under load (low vacuum). There is a vacuum actuator on the carb that holds the flappers shut under high vacuum. __________________ Tim

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