Carburetor cfm for a mild 318 build

Small Block Mopar Engine

  1. Dan the man

    Dan the man Well-Known Member

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    I've checked out cfm calculator's to see what the 318 would need for carburetor size and they showed around 500 cfm. The car is going to be a semi daily driver, I will be using a dual plane intake that has a maximum rpm range of 5,500. The cam will match the power band of the intake manifold. I'm not familiar with carburetor sizing and don't want to over or under carburetor the engine. Thanks
     
  2. Bewy

    Bewy Well-Known Member

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    An Edel 500 AVS2 would be best. These have the annular boosters in the primaries for superb throttle response & mileage.
     
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    • Slantsix64

      Slantsix64 Well-Known Member

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      600 cfm edelbrock
       
    • 273

      273 Well-Known Member

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      The problem with that so call carb formula is that it does somewhat accurately tell you how much air an engine displaces at a given rpm.
      The only thing is the way carbs are rated doesn't correlate. A carbs rating has little to do with how much air will flow through it it's more about how restrictive it is. Eg. say 390 500 650 750 1000 carbs rated at 1.5 hg, but actually they all flow 500 cfm just a different vacuum amounts "restrictions". 390 will be more restrictive and the 1000 will be less. Problem is what level of restriction is suitable to your application, tuneablity, driveablity etc.. vs power. And don't really need a formula for that we have seen what works and it's usually a lot more than that formula. Will a 500 cfm work on your car yes will it give up some power yes, most would run a 650 some even a 750 is a 750 needed and will it make a lot of power over a 650 probably not. Question is there gonna be any power difference between what most would run a 650 and is there any throttle response difference compared to a 500, I'd say 650 will make more power and there would be little difference throttle response.
       
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      • Bewy

        Bewy Well-Known Member

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        The OP has not given the cam specs, only that it is mild. That means the volumetric efficiency of the engine will be low-ish.....
        The VE is highest at the peak TQ rpm, which will be well below the theoretical max 5500 rpm of the intake. VE at 5500 might only be 80%, compared to 85% at peak tq. That equates to about 520 cfm for 318 cubes @ 5500. Better to err on the small side.
         
      • 273

        273 Well-Known Member

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        80% ve would be a 400 cfm, think of it this way, that formula if we use it to pick cylinder heads, would mean the total flow for all intake ports would be 400 cfm or 50 cfm per intake port, but that don't compute. What a heads rated cfm has nothing to do with what an engine will displace at rpm, same with carb. Within reason whatever cfm your engine displaces any carb will flow it just at different vacuum level "HG"

        What it comes down to is what carb will give you the best balance of power tuneablity and driveablity. And we pretty much know cause every combination under the sun has been tried.
         
        Last edited: Sep 13, 2021
      • moparmat2000

        moparmat2000 FABO Gold Member FABO Gold Member

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        Mild teener, 600 AVS Eddy
         
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        • rumblefish360

          rumblefish360 I have escaped the evil Empire State! FABO Gold Member

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          I’m with Slantsix64 & Matt on this one. The mid size AVS Edelbrock offers will be an excellent choice.

          A few members here have tried the smaller 500 on there 318’s. While they report a gaggle of compliments, the one or two constant short comings that is noted was a limited top end power and/or that the carb seemed to be drawing from the secondary side to keep up on the Hwy speeds. That of which may very be a gear ratio issue.
           
        • toolmanmike

          toolmanmike Moderator Staff Member FABO Gold Member

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          5-600cfm. I have a 600 Edelbrock on my 273 and it works great. Not too big and still get's almost 20 mpg on the highway.
           
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          • xLURKxDOGx

            xLURKxDOGx "An angel fat, at satan's feast"

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            Holley 600 mechanical secondary.
             
            Last edited by a moderator: Sep 13, 2021
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            • Dartswinger70

              Dartswinger70 I been North I been East to the California beach

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              I'm not sure what them Carter 2 barrel CFM was...:lol:
               
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              • dano

                dano Evil Handy Man

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                650 AVS2 or 600 Slayer (or Brawler) or 625 Street Demon. Pick which ever flavor your most comfortable tuning.

                Or forget carbs and go Sniper EFI.
                 
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                • roccodart440

                  roccodart440 Well-Known Member

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                  I'm a Holley fan, but a 600 mechanical secondary carb would be my choice.

                  Don't get caught up in the whole advertised power band. It's all BS. My cam is advertised something like 4500-8500RPM powerband and makes peak power at 5,900rpm and dyno pulls down to 2500 still show big numbers down low.
                   
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                  • rumblefish360

                    rumblefish360 I have escaped the evil Empire State! FABO Gold Member

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                    IMO it is not bullshit but a matter of the “OTHER” parts being used with it.
                     
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                    • 318willrun

                      318willrun Utube channel 318willrun

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                      600 minimum. Will it run good with the 500? Yep! It would also run good if you put the factory 2bbl on it. I assume with an upgraded cam and all, you are looking to upgrade performance. 600 - 650 cfm at least
                       
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                      • Dan the man

                        Dan the man Well-Known Member

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                        I'm thinking about using a comp xe262h cam. Would like a little rumble at the pipes. Compression ratio around 9.25:1. Speedmaster cylinder heads ( bare). 1.88" intake / 1.600" exhaust valve's. Edelbrock performer intake, edelbrock 600 cfm carburetor. Gear ratio I don't know what would work with my planned combination. I haven't bought anything yet. Is this a good choice of parts for what I'm wanting to do with the car? I'm not looking for a lot of power but still I want it to be able to get out of its own way. Wouldn't throttle response and low to midrange torque be best for a cruiser?
                         
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                        • rumblefish360

                          rumblefish360 I have escaped the evil Empire State! FABO Gold Member

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                          I’ll answer all three of those questions with;
                          YES YES YES!
                          Good combo. That XE262H cam is 218 @050?
                          You can use the stock torque converter. More stall would be good but you can get away with a stock unit. (I did)
                          The lowest numerical gear ratio to use is a 3.21/3.23 with stock diameter tires. If you went with a larger diameter, I would suggest a higher stall in the converter.

                          Check out 318willrun YouTube videos. He has gotten them to run pretty good.
                           
                        • racerdan80

                          racerdan80 Well-Known Member

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                          670 holley vacuum secondary. Works great on my 318. Very tunable especially with the secondary quick change spring setup.
                           
                        • TT5.9mag

                          TT5.9mag Two atmospheres are better than one

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                          Automatic car? Holley 3310 750vs
                          Stick car? Holley 650 dp.
                          My recommendations.
                           
                        • 273

                          273 Well-Known Member

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                          Your combo picks looking good

                          Mph x 336 x gear ratio / tire diameter = rpm or rpm x tire / 336 x mph = ratio

                          tire sizes make a big difference, with a 26” tire my preferred gear would be 3.91 but you can go higher like 3.73 3.55 or 3.23 just with performance losses.

                          26” with 3.91 = 28” with 4.10
                           
                        • roccodart440

                          roccodart440 Well-Known Member

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                          I'd have a custom cam cut for that with your goals in mind. YOu have a low compression, small valve engine with specific goals. A cookie cutter cam isn't going to be your best option.

                          Gearing is so subjective and dependent on other factors. I run 4.56's with an OD. Worked great with my old engine, works like crap with my new engine.

                          IN regards to converter, you need to know the specs of the car and the engine, anything else is guessing and a waste of good money.
                           
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                          • ir3333

                            ir3333 Well-Known Member

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                            318 ... xe256
                             
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                            • 66fs

                              66fs FABO Gold Member FABO Gold Member

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                              With a good carb, size matters little. I have run 750 cfm AVS, QuadraJet,s and TQ's on 273's and 289 Fords. I prefer an adjustable Air Valve Secondary carb so you can dial in the secondary opening without buying any parts such as power valves and springs. New, that would mean a Street Demon or AVS2. I have rarely bought a new carb and prefer original high performance carbs rebuilt.
                               
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                              • rumblefish360

                                rumblefish360 I have escaped the evil Empire State! FABO Gold Member

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                                I love a good working OE AVS!
                                 
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                                • MOPAROFFICIAL

                                  MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                                  500-600 cfm

                                  Holley vac secondary or edelbrock.
                                   
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