Early 72 340, is this one of the de-tuned ones?

Small Block Mopar Engine

  1. C130 Chief

    C130 Chief Mechanical Genius

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    A lot of the difference in horsepower numbers before and after 72 was the change in the way engines were rated. Prior to 72, horsepower numbers were "gross" and could be optimized, much like you see on a dyno in a YouTube video. They went for the best case scenario, and then the marketing department adjusted the numbers to suit. Beginning in 72 numbers were "net" and were to better represent the "as-installed" performance. So the engine had to have the production air cleaner and exhaust system installed, a fan belt, timing set to sticker spec, standard density altitude, etc... The later numbers were perhaps more "honest" but even then needed to be taken with somewhere between a grain of salt and the whole shaker. A high compression 340 rebuilt to factory specs usually dynoed around 300 "gross" horsepower, and NHRA factored them accordingly. Since you'll be replacing the pistons, use the earlier style (or better modern equivalent) and you'll gain some of that back. The 72 and later exhaust manifolds were probably the biggest obstacle other than retarding the timing for a cleaner idle.
     
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    • Wyrmrider

      Wyrmrider Well-Known Member

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      block? depends on where in NM
       
    • Project_Fish

      Project_Fish I'm on a Low Budget!

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      North Central "Santa Fe", Willing to travel to have it done right.
       
    • Project_Fish

      Project_Fish I'm on a Low Budget!

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      I know of Six Pack Performance in Albuquerque, I have never been there but I will start talking to them now that I want to build the car!
      Bought it in 1990 and now I am finally ready to get it back on the road since I last drove it in 1994!
       
    • AJ/FormS

      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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      Happy HotRodding!
       
    • ir3333

      ir3333 Well-Known Member

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      a friend had a '72 340 'Cuda, 4 speed, 3.23 rear, doorstop Torquer as some call it and 600 cfm Edelbrock.
      Ran consistent 14.5's and that's about what most '70 and earlier ran w/ 3.23's
      '72 8.5:1 340's still ran good!
       
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      • justinp61

        justinp61 Well-Known Member

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        Not all had cast cranks. I had an all original 72 340 Duster that I know had never been apart before I bought it that had a steel crank, it had the lower compression pistons and 1.88' J heads. When I rebuilt it the cylinders were good so I put a set of early 340 pistons along with a MP 274/480 hydraulic cam, stock intake with a 650 Holley and stock manifolds. It ran very well with just 3.55 gears.
         
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        • ir3333

          ir3333 Well-Known Member

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          seem to remember 1972 4 speed 340's had steel crankshafts?
           
        • ir3333

          ir3333 Well-Known Member

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          i know of one '69 340 GTS, 3.23 4 speed that was clocked at 143 mph by the Ontario Provincial Police.
          Don't ask how i know, but most peaked out at an indicated 135 or so.
           
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          • justinp61

            justinp61 Well-Known Member

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            Mine was an automatic. I've seen them steel and cast over the years and just figured they made the switch to cast when the supply of forged was gone.
             
          • Professor Fate

            Professor Fate Push the button, Max...

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            '72 was a transition year. Early ones were forged/internal balance, later in the model year they began phasing in the cast/external balance motors. By '73 that's all there was.
             
          • ir3333

            ir3333 Well-Known Member

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            did the '73 4 speeds have weights on the flywheels?
            I've heard a lot of the overlapping year parts usage but have never seen it..
             
          • barbee6043

            barbee6043 barbee 6043 FABO Gold Member

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            Amount of wear that is, taper in the cylinders measured will tell the tale.
            People can hoot about anything built after '71 is a smogger, etc. So compression was down, so the hp rating went from gross to net. You will build that engine to suit your power needs.
            Some people 's egos do their hooting! LOL
             
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            • Nat

              Nat Well-Known Member

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              right , they used whatever they had they ran out of steel cranks in goes the cast , no big difference in the long run
               
            • ocdart

              ocdart Inland Mopars Car Club FABO Gold Member

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              Just for comparison purposes, in 1970 my '69 Swinger 340 w/727, non-sure grip 3.23 rear, ran 14.28 @ 98+ mph at OCIR here in SoCal. Tires were G70-14 Goodyear Polyglas.
               
            • Nat

              Nat Well-Known Member

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              that is a decent time for the day most the 340's i knew ran 14.4 or so depending on driver of course
               
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              • MoparMike1974

                MoparMike1974 FABO Gold Member FABO Gold Member

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                True, some 72's came with forged cranks but most had cast. I dont think it was a 4 spd vs auto thing, just early production using up the forged cranks they had on hand.
                I daily drive a bone stock 73 (except air gap intake, avs2 carb) and I can assure you the low compression 340's are still strong runners. Almost the perfect engine for a street car....as far as being practical.
                All things equal, if you bump the compression up to 10-10.5 the 340 becomes a whole different animal.
                I love 340's. In my opinion its the best small block ever made.
                 
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                • RustyRatRod

                  RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                  Perhaps better street motors than their high compression counterparts.....especially with today's ethanol laced gas.
                   
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                  • Project_Fish

                    Project_Fish I'm on a Low Budget!

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                    Can you elaborate on that. Why might I prefer a lower compression motor for my car?
                     
                  • RustyRatRod

                    RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                    Do you like sitting in traffic during heat soak and then detonating badly on acceleration? If so, go ahead and run 10.5:1. It's not my car. You're gonna hear all the forum heroes talking about quench and this and that.......but the safest thing to do is run a compression ratio safe for pump gas for a street car. I think the formula is for every 1 compression point, you gain 3% in power. IMO, that's not enough to risk engine damaging detonation to what could be an otherwise nice build.
                     
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                    • C130 Chief

                      C130 Chief Mechanical Genius

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                      So all else being equal ethanol is more resistant to detonation than legacy gasoline. So if you mixed straight 87 octane with 10% ethanol, your resultant anti-knock index will be higher, like 90 or so. But there's no free lunch here. The energy density of ethanol is less than straight gasoline. So even though an E10 blend may be less prone to detonation, the resultant fuel will produce less power if no other change is made, or you'll need more fuel or compression to make the same power. Tune the engine for the fuel that you intend to use. More annoying is the fact that ethanol is more "volatile" or has a lower vapor pressure then straight gasoline. This will make hot restarts on a carburated engine a bit more hassle as the fuel boils at a lower temperature, and it will evaporate out of the tank slightly quicker. On a typical Mopar engine, the actual compression ratio was somewhat less than advertised as the deck heights were generally taller than nominal. Add a modern composition head gasket and a 340 with earlier pistons is probably closer to 9.5:1. Right where you want it. I've had more engines where you could advance the timing to the point of power loss without detonation on 87 pump gas (E10) than I've had engines that really needed premium. But then again I'm at a higher altitude.
                       
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                      • 65 Cuda 340

                        65 Cuda 340 FABO Gold Member FABO Gold Member

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                        One more vote for some 72's having forged cranks, 4-speed or automatic. The forged crank in my 65 Barracuda came out of a wrecked 72 Charger with 340 and automatic.
                         
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                        • MoparMike1974

                          MoparMike1974 FABO Gold Member FABO Gold Member

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                          Hot restarts are problem on my car with ethanol. When cold it starts in less than one revolution. If I drive and then try to restart after sitting 10 minutes it takes about 5 seconds of cranking. There is a window of time and it seems to be about 15-20 minutes after shutting down. Let it sit longer than that and it usually starts up easy like normal. Yeah ethanol sucks for carburetors.
                          Anyone else notice it tends to grow some sorta mold around the gas caps?
                           
                        • barbee6043

                          barbee6043 barbee 6043 FABO Gold Member

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                          Gas in my slants would boil on a hot day till I would re route the gas line.
                           
                        • Nat

                          Nat Well-Known Member

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                          since i added an electric fuel pump never had any starting problems
                           
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