4 barrel cfm

I'm trying to find out what the CFM rating was on the original 4 barrels that came with the 364's in 58. I believe they were Carters or Rochestors?

I have an Edelbrock 750 on my stock 364, and it runs ok, ( after downsizing the jets and rods and much re tuning) but I'm wondering if I would get better general performance, throttle response etc from a 600 cfm or even 500? Some say 750 is too big, others say its about right. I figure whatever the CFM size of the original 4 barrels that came on the car in 58 are probably what I should aim for.

As I said, it seems to run ok, but now I'm thinking, could it be even better going a bit smaller? Its somewhat of an experiment at this stage.

 

Hi,

Believe either a carter afb or a Rochester 4 Jett were available. Both were aprox 625 cfm I believe.

Never had a 364 but most of my nailheads ran better with more cfm. Had a 750 and went to dual quads on a 64 401.

Have heard Edelbrock are not jetted the best out of the box.

Regards,
Jim

 

Brilliant, thank you..so I dont want to go lower than 600 then. Really its a toss up between the 1406 (600 cfm) or my current 1411 (750) , and see which runs best. As I mentioned, the 750 runs ok now after much messing around, going a stage leaner with the jets and rods, advancing the timing also helped and using manifold vacuum. But I'm curious to see how the 1406 would perform. Could be an expensive experiment where I discover I should have just left it as is.

 

750 is good no more if not modified The Q-jet is 750 Carter may have used a 600 on there or 650 and the 650 is a good all around carb better than the Edelbrock.

 

I've heard people say this about the original Carters being better, i wanted something newly manufactured that would be easy to work on and get parts for etc, the Edelbrock seemed like an obvious choice as it bolts straight on. I do have an original four barrel for a 57 Buick, think its carter or rochester, its suposedly been rebuilt, i tried it once some years ago now and had no luck with it. I have since discovered a lot of the problems i was having were due to the distributor. Since installing an HEI and Edelbrock I havent tried it since. Its just sitting in a cabinet in my shed. What I didnt like about it was the fuel inlet pipe, its at a strange angle, so the hex shape of the inlet pipe fouls against the carb body and you cannot screw it all the way in, so you get leaks. I think the thread in the casting wasnt great either. Also It means having to use a standard rubber fuel line, and I wanted to use braided AN6 style lines.

 

The first thing that needs to get changed on any 750 carb be it a AfB model or the newer AVS model for a motor in the CID range of yours are the power piston springs .
Making jetting and rod changes when the rod might not be all the way down in the jet to start with, or lifts out of the jet too fast ( both of these caused by too stiff a spring ) once vacuum drops is a unrewarding effort.

a 650 cfm 4 bbl is fully capable of feeding a 400 hp motor which I do not think your 364 is.

a bigger then 650 cfm carb on a motor of your cid WILL help the hp made to not nose over as fast above 5500 rpm, but then your far more important average torque and HP numbers below 4500 rpm will suffer.

 

So would you reccomend I switch to the 600 cfm size carb instead?

I checked the springs while it was runing, they were firmly seated, until I blipped the throttle quite hard then they bounced up.

 

the stock cfm flow on the wcfb carb was 400 cfm. if I had a 364 size engine I would use the newer AVS2 500 cfm or 650 AVS2 cfm depending if you are going to do more mods over stock. they have annular boosters. the boosters give you great off throttle responses and power.

 

I had found a very reputable listing of AFBs somewhere, and my '64 Pontiac AFB came in @ 586 cfm.
I found that interesting as I upgraded to Tri-P and my specific carbs there went to 883 cfm.
Were carb's flow rates 'tweaked for marketing'?

 

need to multiply the 2 barrel by .75 to equal a 4 barrel. 883 x .075 =662 cfm.

 

if the 364 has a Rochester it could be a 486 cfm. then a 650 cfm would be good. would anyone know if Carter and Rochester used the same cfm flow ratings ?

 

All 4 bbl carbs industry wise are tested at 1.5 Hg
Which is equal to a test pressure of 20.5 inch’s on a flow bench.

2bbl carbs are rated at 3 Hg which is then over 40 inches as used on a flow bench which is why the cfm ratings stated for 2 bbls seem so out of proportion for there size.

 

I was put off of using the AVS2 after having seen some people say they were complicated and difficult to tune..is this not the case?

 

i have them on 2 of my cars not harder to tune. you can feel the power off the line compared to the old AVS. the size of the carb depends on how many RPMS you are going to rev it to.

 

Oh ok. I am seriously thibking of the AVS2. What CFM size did you use on what application? I still dont know if I should aim for 500 or 650

 

went from 750 edelbrock to the 800 AVS2 on my 446 mopars. big difference off the line but not much difference after that. i always lean out the cruise mod 1-2 sizes on all edelbrock carbs if there is no surging at steady cruising your good.

 

I'm not looking to make it go crazy speeds, its scary enough at fairly low speeds, handles like a boat, and our roads here are tiny and not very straight hahah. But if I can get it to a point where its got a lively throttle response and drives smooth enough at all speeds..i'll be happy. Its almost there I think, but I am just curious to know if a smaller carb would push the overall performance to that perfect sweet spot. I am deffinately thinking to go with the AVS2, but 500 or 650cfm, those are my options. I am struggling to decide what would work best.

 

Edelbrock sold AFB clones, and AVS clones. The AVS were a superior carburetor due to the easier tuning of the secondary air valve. (The Q-Jet and the Thermoquad were far superior to either the AFB or the AVS.)

Carb Sizing: carburetor size = engine size × max rpm × efficiency ÷ 3,456 X a correction factor for intake manifold/porting efficiency. The more efficient your intake and porting, the less correction factor needed. Typical single-plane intake manifold correction factors range from 1.1 to 1.3; typical dual-plane intake manifold correction factors range from 1.3 to 1.5.

I suggest you take the number provided by this calculator (which does not include the manifold/porting correction) and multiply by about 1.5.
https://www.inchcalculator.com/carburetor-size-calculator/

Example:
At 5000 RPM and 364 cubes at 85% "volumetric" efficiency, I see 447 CFM. 447 X 1.5 = 670 cfm.
But maybe you aren't going to spin your engine that fast.

 

If your not planning on constantly stop light racing the car then go with no more then a 600 cfm 4 bbl unless the Carb your looking at has high gain boosters on its primary side.

A 600 cfm Carb will provide far better throttle response at any rpm below 4000 to 4500 which is where with your car spends 90% of the drive time on the street.

There is for certain nothing worse then having too big of a Carb on a motor, and or too big of a Cam, BTDT !!

 

My 425 has an Edelbrock/Weber/AFB. When it arrived there was a pamphlet that was either enclosed or that you could get from Edelbrock that said just that, and how to set it up correctly. It worked well. Some people get nervous around carburetors, but IMHO both AFB and Quadrajet carburetors are easy to do. Holleys are great carburetors, but to me they're a PITA to work on. Too many small parts. I had a Holley on my circle-track car, but that's because the floats are hinged from the side and are not as finicky as the others about high-G turns.