Can you change the cfm rating of these carbs if so, how?

The rating go from 400 to 750. I think the 625-750s are likely the later Edelbroch carbs?

The ones used on the C2 Corvettes were 575 cfm and variants of the 3720 series.

I would like to know if my 1965 300 h.p. AFB can be increased in cfm and how?

 

The 1967 GTX 440 used a 750 cmf AFB (switched to AVS in 68) as did a few other cars. Advertised CFM was probably dependent on a few obscure factors. But, I believe the big ones had 1-11/16 inch primary and secondary throttle bores. To try to mount this on an intake that used a smaller AFB, the primary bores in the intake would probably have to be opened up to match the primaries on the carb.

Smaller AFBs usually had 1-7/16 primaries/1-11/16 secondaries, or real small ones might have had 1-7/16 bores all around.

 

Air horn diameter was also a factor, with the smaller diameter for the older carbs............which also had the smaller CFM. And along with the various throttle bore sizes were the matching venturi sizes.

Larry

 

Plus, CFM Is a factor based on flow at 1 1/2" vacuum for the four barrel carburetor, if you change the depression you change the CFM.

 

There's little you can do to increase flow... maybe smoothing out any casting flash in the flow path might help, but the difference would likely be marginal.

I have a letter from Carter that states the OE AFB flow at 2" Hg depression. Flow varies with the square root of depression, and since 1.5" has become the defacto standard for four barrels (3.0" for two-barrels) I converted the 2" flow to 1.5" and got 550 CFM. The Holley is rated at 585, so I doubt if there would be any measurable difference in power on the same engine.

The OE AFB throttle bores are 1 9/16", primary and 1 11/16", secondary. The Holley is 1 9/16" on all four, yet it flows marginally better. Nothwithstanding the differences in venturi diameter and booster venturi restriction, I attribute the difference to the air horn design, which appears more efficient on the Holley.

Duke

 

Duke,

Jerry Stein used a trick of putting the 5-1/8" air horn on the "little" AFBs. Just an old drag racer who was very successful.
Verne

 

Just about anything can be done with some patience and ingenuity.

Larry

 

I see several different cfm rating on the Net for AFB but have also seen it stated that Carter never actually state any for it.
The site in this link says it is 575 cfm.
http://www.carburetor-blog.com/carter-afb-carburetor/

Well, the reason I am asking this question and the other last few questions I have posted in the last 24 hours is that I plan to upgrade my original 300 h.p. engine with a 350 h.p. intake and cam. I am trying to decide which way to go with the carb. According to the char rating above there is only about 25 cfm difference between the original AFB I already have on the car from the Holley used on the 350 h.p. OR the new Holley replacement. I have already changed the valve covers to the 350 h.p. aluminum ones and would like to use the better flowing 350 h.p. air cleaner.

 

How does your car run right now in stock 300hp trim? Wide box trans and 3.36 final drive gear?

You can probably guess where I'm going with my comments, WHY are you doing this, your 300hp engine will serve you well for street driving. The 350hp engine has other goodies that are not present on your base engine so keep it simple and enjoy your car.

 

The letter from Carter that I referred to earlier was in response to a Corvette owner's inquiry about the flow rate of the OE Carter AFB on his Corvette back in the sixties, so I accept the 550 CFM @ 1.5" Hg as gospel.

You're wasting you time fretting over a few percent difference in flow rating. The OE heads are the biggest restriction to flow, but "massaging" them will improve top end power by at least ten percent and add another 500-1000 useable revs. If you massage the heads and install a McCaw Special camshaft and high strength conn. rods with a CR of about 10.25:1 the engine will maintain the smooth 500 RPM idle and stump pulling low end torque of the 300 HP engine while producing L-79 type top end power and revs. In fact, with proper OE valve spring setup lifter pump up speed is 6700-6800.

Search for threads started by me and download the "Tale of two Camshafts" article.

IMO AFBs are more reliable than Holleys because the AFB doesn't have any "wet" gaskets that eventually leak. The only AFB issue is that the choke vacuum break bore/piston gets dirty and sticky, but that's easy to fix by simply removing the choke housing, disassembling, and cleaning. (Don't forget to replace the tiny O-ring between the choke housing an main body or you will have a vacuum leak and poor choke pull-off performance.) You don't have to take the carb off the engine.

Duke

 

Intended to be 3.70 gears.
I know ALL of the differences between the two engines.
How my 300 h.p. engine "serves me" is a personal choice. I always hunger for more power. I bought the 65 because it is an original rust free, low mileage, no hit body with original interior. I would have preferred it had a 350 h.p. engine. All of what I am doing to the car can be reversed so no harm done.

 

So if I compare the 550 cfm Carter and the OE holley @ 585 cfm or the new Holley replacement @ 600cfm I doubt that here would be any seat of pants difference. I agree that the Carter is a more reliable carb than the Holleys and prefer to keep my perfect running original Carter with the 350 h.p. intake and OE type 350 h.p. cam.

I don't plan on doing any work on the heads and if that was a consideration I would just go with aftermarket aluminum heads that re already worked over.

Thank for the info. I believe I will be going with the Carter provided it will fit under the hood with the intake adapter and air cleaner adapter for the 350 h.p. air cleaner.

 

As I mentioned yesterday in your other thread on this subject, the old AFB bolt pattern is different than the Holly bolt pattern, and the L-79 manifold is only drilled for the Holly. You might have to use the 327/340 HP aluminum manifold........or drill 327/350 manifold for AFB mounting.

Larry

Here is a pic of the two bolting patterns from a vintage intake. This is not what you want, but Edlebrock did have one that was similar to the factory manifolds that would work and had both bolt patterns. https://www.ebay.com/itm/VINTAGE-ORIGINAL-Edelbrock-TM-1-Aluminum-Small-Block-Chevy-Intake-Manifold/283236293477?hash=item41f232db65:g:XHwAA OSw2~ZbvKON:rk:5:pf:0&vxp=mtr

Here is the correct dual plane manifold with the two carb bolt patterns that should work. https://www.google.com/search?q=chev...vTOBQcEHhmY7M:

 

Thanks. Yes, I am aware of that. However they do make an adapter for that and I believe I will be going with the Carter provided it will fit under the hood with the intake adapter and air cleaner adapter for the 350 h.p. air cleaner. I will also be looking at the possibility of drilling the manifold so I don't loose 1/2" hood clearance with the adapter since I will loose 1/2" with the air cleaner adapter. All else fails I will have to locate a 340 h.p. intake, however I already have a 1966 350 h.p. intake.

 

I hope you did not take offense to my comments, I just wanted to point out that the expense is not worth the little gain.

You stated the reason you purchased the car was because it was rust free, low mileage, no hit body with original interior but then you state you would have preferred the L-79 motor. Seems to me you bought the wrong car, now you are taking apart a original car to change the engine and also the rear gear.

Just my thoughts, I should have kept them to myself.

 

No offense taken, everyone is entitled to their opinion.

If there would have been a like type/condition car with a 350 h.p. when I was looking I would have bought it. I really did not especially want another red car but when you are looking at these old cars the condition is more important.

I personally feel the h.p. difference will be worth it to me and I also like 3.70 gears.

All is reversible but unlikely I will ever do it and all parts taken off will be saved.

 

I read all your posts and value each and every one....

Just a bit of an object lesson....my 250hp split window came to me with a nice Carter AFB 3721SB carb perched on an Edelbrock 3CBX aluminum intake (a NICE dual plane Z-28 style intake)......(the seller included the original carb and intake separately in a box) when I had the valve seals replaced I put the original WCFB 3501 and cast iron intake back on the car... You can imagine the improved flow of the AFB/3CBX setup over the WCFB/cast iron configuration...

Even discounting the 3CBX enhancements the CFM difference from an AFB (550) to the WCFB (384) is more significant than the specs between variants of AFBs... Both carbs secondaries are non-mechanical and controlled by engine demands, to wit -- the CFM max spec is theoretical (and perhaps why Carter doesn't publish it)..

The resulting "seat of the pants" difference in performance ? Zilch, nada , bupkis...
I got more benefit from optimizing the advance curve by far.

Not to dissuade 68hemi from doing whatever he wants, but he shouldn't be disappointed when the labor to modify his mill doesn't have a payoff in a noticeable performance boost.

As noted by Duke - reworking the heads/cam swap will net the most benefit.

 

The '63 327/250 has "283 size" valves and head/manifold ports. The biggest restriction is the head flow, so installing a large port manifold and higher CFM carb would not, as you observed, produce any significant increase in power.

Duke

 

This is not my 1st rodeo with this upgrade. I did it 20 years ago on my 1966 Corvette that also had the original 300 h.p. engine that I put the 350 h.p. cam in it and dressed it as the 350 h.p. engine. However it did not have all of the challenges of doing this on a 1965 as the 1966 300 h.p. came with a small Holley from the factory so the accelerator linkage was the same and I changed the cam, intake, used a larger carb, aluminum valve covers, and used the same factory air cleaner. There WAS a noticeable performance difference.

Also not mentioned in any of this is that the dual snorkel air cleaner is pretty restrictive compared to the 350 h.p. air cleaners.

 

Not so much if some prior owner drilled nice big holes around the circumference ! Not the base I used for judging BTW...
Nobody is trying to insult you or denigrate what you're doing. I went down this road on a Ford 289...and nothing made a big difference vis a vis carb, intake, etc.. until I put 351 D00E heads on it, larger valves, smaller combustion chambers - the engine was then a "street Boss" and I estimate a 50-70 hp bolt on improvement in power. I also tried to suggest that banging CFM numbers from one carb to another is not much good unless the engine "asks" for the max amount - that is the nature of cars with secondary "flappers". To wit, there needs to be a holistic approach and not piecemeal. I think that's what all the respondents are saying...