Does CFM matter on 4 barrel carb?

[davette59]

I am replacing the carburetor on my 78 Avanti as I mentioned in another thread. I was told by the mechanic that it had to be a 650 CFM carb. When I spoke to two companies that sell rebuilt carbs, they said they didn't go by that, they simply send a rebuilt carb for that year and engine which in my case is a Corvette 350. Is this something that is important?

[bezhawk]

Too large of CFM reduces the velocity and fuel atomization, that in return reduces effiency of how the fuel air mixture burns.

[t walgamuth]

Too large of CFM reduces the velocity and fuel atomization, that in return reduces effiency of how the fuel air mixture burns.

This may be a true statement but is a bit oversimplified. A four barrel carb is normally also set up as a progressive action with normal operation being on two of the barrels. the secondary barrels open if you push the pedal down all the way to give more air and fuel flow. In this way the primary barrels often offer better fuel atomization than a large (but smaller overall than the four barrel) two barrel carb. The secondaries on newer carbs are usually operated by vacuum to avoid stumbling if the secondaries are opened before the engine is turning high enough rpm to use the extra carburation.

I once adapted a q jet with 750 cfm to a bmw straight six that was 183 CI. All reference books said it was too big but if I did not open the mechanical (gigantic) secondaries until it was at 3000 rpm it simply screamed up to the redline and past if I was not careful. I used a four barrel manifold that came from Europe and the q jet was a perfect fit with slight modification of two bolt hole locations.

That said in your case using the 650 cfm makes sense unless you are seeking more power.

[Gunslinger]

600-650 CFM is all that's necessary for a 350 engine unless it's built up for high rpm running. Anything bigger than that will likely create stumbling at take-off unless it's been worked on.

A Quadrajet has a somewhat different design. I've heard it referred to as "air on demand"...the small primaries act like a smaller CFM carburetor, giving good low end response and when increased air is required the large secondaries open up.

[PackardV8]

I am replacing the carburetor on my 78 Avanti as I mentioned in another thread. I was told by the mechanic that it had to be a 650 CFM carb. When I spoke to two companies that sell rebuilt carbs, they said they didn't go by that, they simply send a rebuilt carb for that year and engine which in my case is a Corvette 350. Is this something that is important?

Yes, if you're the engineer choosing a carburetor and changing a stock 350" for a performance built 383" or 400", CFM is an important factor to be considered.

No, if you are just wanting to replace like with like, as the carb rebuilder said, they don't give a rat's arse about CFM numbers and they're not always readily available on OEM carbs. The rebuilder focuses on the exact model number/list number stamped on the carb base/airhorn/brass tag to get you the right carb to be the exact replacement for your original.

Maybe, just give them the model number off the tag on your original and don't worry about details which aren't important in a stock rebuild.

jack vines

[bob1944]

try the Holley carb cite they give cfm based on max rpm and cubic in. engine. example 350 cu.in. 7000 rpm = ?cfm(650). most of us over carburate our engines. Larger is not always better. I've got a 450cfm for my stock 259 and I would be better at 390cmf.

[davette59]

Yes, if you're the engineer choosing a carburetor and changing a stock 350" for a performance built 383" or 400", CFM is an important factor to be considered.

No, if you are just wanting to replace like with like, as the carb rebuilder said, they don't give a rat's arse about CFM numbers and they're not always readily available on OEM carbs. The rebuilder focuses on the exact model number/list number stamped on the carb base/airhorn/brass tag to get you the right carb to be the exact replacement for your original.

Maybe, just give them the model number off the tag on your original and don't worry about details which aren't important in a stock rebuild.

jack vines

My only problem is that there is absolutely no model number on the carb. I looked all around the base, on the sides and everywhere. The only number at all is on the butterfly flap. It does say " Quadrajet manufactured by Carter for GM ".
my guess is to simply give the builder the engine type, year and cubic inches and let him give me what he feels is the proper carb. I would have my carb rebuilt but I'm not too sure that is the one that came on the car. I think someone jury rigged it. The builder did say that if there was no model number it might have been made specifically for GM for one type car only and in that case they will not take it in trade and I'll have to pay the core charge. Oh well.
there are several hoses that are not connected to anything and a wire that might have been for an electric choke??? There is also something screwed into the intake manifold that looks broken and is not connected to anything. I am going to try and take some pictures to post for some help.

Dave

[Gunslinger]

Since you have a '78 Avanti 350, depending on when Avanti Motors purchased the engine it could be either '77 or '78 specs. They supposedly purchased a six month supply at a time for best discount. Unless they got a special deal or it was a special order, it's probably an L48 engine rather than an L82. That's assuming it's a Corvette spec engine.

If it's an L48, it's rated at 185 hp. With an automatic transmission, it should use a Rochester Quadrajet model 17058202, or the Carter equivalent...for a car without a/c. With a/c it should be a model 17058204 (first design) or 17058206 (second design), or the Carter equivalent. What the differences are between first and second designs I've no idea, but you can use those model numbers to work with.

As I said, I'm assuming that's for a Corvette L48 engine. If it's an L82 I can give you those carb numbers but if it's from a Camaro or other version of the 350, you'll need to get the numbers off the pad of the block to determine exactly what you have. Once you have that you can probably find online a list showing exactly which carburetor you have.

[Jerry Forrester]

With Quadrajet carbs., there are only 2 choices as far as CFM's are concerned. 750 and 800. 800's being very, very rare.
The Quadrajets are speced to your engine size and/or needs with metering rods and jets and maybe the spring and weights for the rear shutters.
I ordered mine for a 259 engine. I think it is the same carb. (with a few tweeks) that came on the '68 Pontiac inline 6 OHV engine, 230 (i think) cubes.

Call this guy, (1 800 526 9952 tell him what you have as far as engine goes, and he'll build you what your engine needs.
His ebay handle is i5autoroca.

http://www.ebay.com/itm/350435988744?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2648

[t walgamuth]

I used the 67 Pontiac OHC six metering rods in my BMW application. The q jet is a good concept but very cheaply made.

Also the first four barrels offered on the 52 Caddy were called quadrajets too IIRC.

[Gunslinger]

A good carb guy can take most any Quadrajet and build it to suit whatever application...as long as it's rebuildable. Outside of the clones Edelbrock sold, the newest Quadrajet is over thirty years old.

[Dick Steinkamp]

A good carb guy can take most any Quadrajet and build it to suit whatever application...as long as it's rebuildable. Outside of the clones Edelbrock sold, the newest Quadrajet is over thirty years old.

True. As Jerry states, all Quadrajets will potentially flow at least 750 CFM...but the big vacuum secondaries will only allow as much flow as the engine demands.

I've used Quadrajets successfully on engines as small as a 259 Stude.