Small block gas mileage DD magnum/LA ?

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  1. rustytoolss

    rustytoolss Well-Known Member

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    Looking to builds a stockish 5.2/318 for a DD. If I use a 5.2 magnum, it will use the stock roller cam. aftermarket intake and a small Edelbrock Thunder carb, and elecrtonic DIST. cast iron exhaust manifolds.
    Or should I use a LA 318 (which I have 1986 truck engine) possibly small summit cam, 4 barrel intake, small AVS carb, electronic DIST, and cat iron exhaust manifolds.
    Will have dual exhaust , and have a 42RH /A500 with stock converter. Rear gearing possibly 3.23.
    So which combo should provide the better gas mileage ? I got the 318 for free...so I can walk away from it..if I choose to.
     
  2. toolmanmike

    toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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    Probably not as high as 25 but close to 20 I would estimate .
     
  3. Hellrats

    Hellrats Just another dumbass FABO Gold Member

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    What are you putting it in?
     
  4. rumblefish360

    rumblefish360 So close, yet so far away FABO Gold Member

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    Between my 360 vs the 5.9, both stock long blocks, the LA had better mileage. Why? I don’ t know for sure. I think on paper, the Magnum has all the marbles. But it didn’t pan out that way. Your results may vary.

    My LA was a ‘79 mill. Added on to it was;

    A ‘72 trap door air cleaner w/a K&N filter
    OE ‘79 cast iron intake
    625 Federal Mougal AFB
    JY distributor triggered by a MSD 6 and backed by a MP Chrome box. Because MSD stands for “May Suddenly Fail” in which they did for me, constantly.
    2-1/4 H piped dual exhaust with twin catalytic converter’s (that was the law) and thrush mufflers.

    A 904 w/the MP shift improver kit, 2.76 gears riding on 235/60/15’s. The car was a ‘79 Magnum. Best gas mileage was 20 mpg’s Hwy.

    Run as much initial as you can, limit timing to approximately 34/36*’s, open up your plug gaps to .050 unless it misfires, then close them down .005 until it runs right, tune tune that carb!

    I find the Magnum a little more thirsty. But power is much better! IMO, I think it is the cam specs.

    IMO, your use of a 318 cid engine and the overdrive, in a lighter car, I don’t have a reason as to why 22 & better mpg’s can not be seen. I’m sure elevation has a role in this. As I live on an island, it wedge shaped maxes out at approximately 400 or so feet. At best. So I looked it up for ha-ha’s. 400 feet! Highest elevation on long island - Google Search
     
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    • toolmanmike

      toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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      Good info there. I would imagine with a 3:23 gear and a lock up/Overdrive even a Magnum could pull 20's.
       
    • rumblefish360

      rumblefish360 So close, yet so far away FABO Gold Member

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      I don’t see why not. I didn’t have headers on my ride, just the stock exhaust manifolds. No overdrive. LU? Yes.
      The best mileage getter was the 4bbl. running and tuned very well, followed by the MSD. (When Working.)

      3.23’s X .69 OD ratio is 2.22. Hummm. That should work well @ 80.
       
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      • toolmanmike

        toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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        (When working) That's funny! (but true) LOL
         
      • rustytoolss

        rustytoolss Well-Known Member

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        hopefully a 62-64 B body.
         
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        • rustytoolss

          rustytoolss Well-Known Member

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          Well my plan is to find a 62-64 Bbody car. I know most people snicker when people are looking for fuel mileage...but gas cost money. I'm retired so the days of gas guzzling cars is over for me. I plan to use this car from May till October every weekend (about 80 miles round trip)
          Just was not sure if the 5.2 magnum would be a better choice....or 318 ? I do have a 1986 truck 318 (roller block engine) This will be a budget rebuild, rings, bearings maybe a small Summit cam, no expense parts or machine work. I also have a 1995 42RH/A500 for the driveline.
           
        • rumblefish360

          rumblefish360 So close, yet so far away FABO Gold Member

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          Maybe it’s the gasoline around here but I find that my 5.9 just simply likes the 89 octane and hates the 87. The curve on the distributor is hot rod like. Not to quick. For me, it is what it is. But when I go out of state, it is fine on 87.

          I do favor the LA for intake choices and the verity of available parts. Ether way you go, 20+ mpg’s should be obtained. My roadways are generally flat. Not like the desert states, but pretty good. Long Island is wedge shaped high in the north to beach sands in the south, wavy east to west.

          If I was doing this, (318-LA) it would be a simple cast iron OE intake, or the right aluminum one w/OE heads with the use of a small primary (electric choke if possible) TQ and dual exhaust H piped. A mileage cam and a maximum compression of 9.0-1 with a high energy ignition.
           
          Last edited: Oct 18, 2020 at 4:32 PM
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          • rustytoolss

            rustytoolss Well-Known Member

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            If I can get 20's and use the engine that I already have...thats a win win..Would leave me more money to spend in better places.
             
          • rumblefish360

            rumblefish360 So close, yet so far away FABO Gold Member

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            The LA 318? Yea, it’s possible. (At least in my driving environment it is. It’s all in the tune of the carb and distributor. The OD will take care of the rest.
             
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            • Canucklehead

              Canucklehead Well-Known Member

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              I’m a big fan of the 5.2 Magnum. I put one in my 72 Valiant because it was cheap (175 canadian bucks), put the Edelbrock intake on, hughes fuel pump eccentric, put a 3-speed manual out of a Volare into it, an 8.25 rear with 2.71s, with a lean Edelbrock carb I averaged 25 MPG on a 13 hour highway trip. I’ve heard the 5.9 is tough on fuel though.
               
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              • MileHighDart

                MileHighDart Well-Known Member

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                I just did my first gas mileage test a few weeks ago.
                '68 Dart, 5.9 Magnum, stock bottom end, edelbrock airgap, edelbrock 1406, Doug's headers, dual 2-1/2" exhaust. Stock mopar electronic ignition, A500/42RH trans, 3:23 gears, 225/60/15 tires.

                Also, I'm at about 5300ft elevation.
                Filled it up and drove to work all week (40 mile round trip), I'd say 3/4 highway driving using the overdrive as much as possible, 1/4 in town.
                I got barely 15mpg out of it. Was hoping it would be a little better, but the 42RH is still the best thing I ever did to the car. Cruising rpm's on the highway is just way more comfortable than with the old 904. If I remember right 65mph is like 2200rpm with the overdrive and lockup on.
                 
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                • rbkt65

                  rbkt65 FABO Gold Member FABO Gold Member

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                  i am thinking of doing same thing for my son in a short bed truck but i will use a q-jet on la on later cast iron intake or q-jet on assault carb intake if using magnum. following this to see which cam to use. probably use a518 w/lu and dana 60 w/3.55's.
                   
                • TrailBeast

                  TrailBeast AKA Mopars4us on Youtube

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                  5.9 Magnum, EQ heads with LA intake bolt pattern, Edelbrock dual plane, Edelbrock 1406, HEI with a .050 gap, NGK plugs, 1-5/8 long tubes, 214/224 Oregon cam, 42RH, 3.55 rear gears, synthetic lubes in engine and diff.
                  24-25 mpg on the hiway in OD and lockup at 75 mph.

                  18 initial, 34 total and 50 or so with vac advance.
                  Averaging 14.5 AFR.
                  Then I put a Holley 770 on and never checked the mpg after the change.
                   
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                  • rumblefish360

                    rumblefish360 So close, yet so far away FABO Gold Member

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                    That’s the way to do it! Nice job.
                    What dual plane from Edelbrock?
                    Can I assume a Hyd. roller cam was used?
                    How big was the exhaust pipes and how was it run and with what mufflers?
                     
                  • toolmanmike

                    toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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                    There you have it. A recipe we can all follow. Thanks
                     
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                    • TrailBeast

                      TrailBeast AKA Mopars4us on Youtube

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                      The intake is a 318/360 performer.
                      Obviously it could do better with a larger runner intake since the ports in the heads are about 1/4 bigger all the way around.
                      (At least the air/fuel charge isn’t hitting 1/4 inch walls if it were the intake that was larger)
                      Yes, a hydro roller with OE lifters and 1.6 rockers with Hughes Engines springs, retainers and locks.
                      1.92 intake valves and 64cc closed chamber heads.
                      Dual 2.5 inch out of 3 inch collectors and flowmaster 40’s.
                      The 2.5 inch pipes literally cross through each other and then out in front of the rear tires.
                      How it was run?
                      For checking fuel mileages I ran it on I40 at 80-90mph for 700 miles.
                      Averaged 2,500-2,600rpm.
                       
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                      • rumblefish360

                        rumblefish360 So close, yet so far away FABO Gold Member

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                        Posting a “Like” & a “Thanks” because I can’t select both!
                        But I can post it.

                        The minor intake to head difference is nothing. That is a good intake for the job at hand.

                         
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                        • MopaR&D

                          MopaR&D Nerd Member

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                          I get around 13.5-14 MPG with my stock bottom end 5.9 with ported Edelbrock heads and custom-grind hyd roller, 904 trans with 2.94 gears in a 70 Duster. I'm in Fort Collins so similar elevation around 5000'. Have you leaned out the cruise circuit on the carb very much? I've picked up 3 MPG on a 360 across the board just from leaning out the cruise AFR from ~14.3:1 to 16:1. First thing I did after installing my wideband O2 sensor lol.

                          When I installed a Edelbrock 1406 on my truck's 360 (stock 1972 converted from 2-bbl) it had factory jets, I went down one size on the jets all around and tried different metering rods to lean it out until it had noticeable hesitation when opening the throttle, then went one step richer (while also keeping an eye on running temps). Runs great and gets.... wait for it.... 10 MPG :rofl:

                          Even tried one step richer than that to be safe and nothing changed except the gas mileage dropped to 9-ish. Ran a bit better when cold but that's not how they're supposed to be anyway.
                           
                          Last edited: Oct 21, 2020 at 10:00 PM
                        • MopaR&D

                          MopaR&D Nerd Member

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                          For the topic at hand, I noticed you said the LA 318 you can get is a 1986 which means it most likely has a roller cam. In that case you could just swap in a small aftermarket roller cam (or stock replacement 5.2 Magnum cam if you don't need mech fuel pump) and reuse the stock lifters if they're in good shape and bolt on a set of Magnum heads. Probably have to get custom-length pushrods but that's something to check anyway for any build that isn't bone-stock IMO. Heck if the stock heads on that 318 are in good shape they should be 302 castings which are already closed-chamber they just don't flow as well as Magnum heads. Probably not very noticeable in a DD with mild cam. And I believe they don't have as much of a tendency to crack between the valve seats like stock Magnum heads do.

                          In an A-body with an O/D trans (42RH/A-500) and 3.23 gears I'm willing to bet you could break 25 MPG on the highway, possibly more.
                           
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