920 heads on a 340

Small Block Mopar Engine

  1. Confusedcuda

    Confusedcuda Well-Known Member

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    Has anyone ever done this? My ports on my 920 heads are way bigger then a set of 675 heads I have. Thanks
     
  2. toolmanmike

    toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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    I think a 340 head would work better. Small chamber and small valves on both those heads will slow a 340 down.
     
  3. abdywgn

    abdywgn dismantler

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    but would really boost low end torque if using small cam and dual plane intake. but as mentioned, would probably choke the engine at 4500 or so.
     
  4. cudamark

    cudamark FABO Gold Member FABO Gold Member

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    Someone must have done some porting work if what you say is true. Stock, they should both have the same size intake and exhaust ports. What manifolds are you using? Have they been port matched to those 920's?
     
  5. MOPAROFFICIAL

    MOPAROFFICIAL FABO Gold Member FABO Gold Member

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    Like others have said the ports are about the same size it's just the port windows that could be cast a little smaller and that's just from being messy at the factory. Also the small Port volume intake runners will choke it around 5000rpm, it will just stop making any torque... and that's experienced with even ported 318 heads
     
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    • 273

      273 Well-Known Member

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      It basically just makes your 340 into a 318 power wise. This idea it will produce more torque cause of velocity maybe at the lowest rpms like idle to say 2500 rpm but I really doubt it would be huge over the 340 J or X heads. And not worth the overall power you’d be given up.

      Engines Torque is basically engine size, VE% and CR and engine that high lbs-ft per cid are usually big flowing heads solid roller cams big carbs headers High cr etc.. But yes that stuff tends to move the powerband higher up in the rpm range especially on smaller displacement engines.

      For a truck or purely street engine you want idle to 4500 rpm power, for a more muscle car streetable .9-1:1 hp:cid you want more 1500-5500 rpm.

      So yes you can used them but it’s a step backwards in the power department. The trick for a street hp engine is to try to keep bottom end hp 1500-2500 rpm while Extending the useful rpm from 4500 to more in the 5000 to 5500 rpm range.
       
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      • 67Dart273

        67Dart273 FABO Gold Member FABO Gold Member

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        "In a previous life" when I sold the old RR it ended up with a stock 360 short block, stock 340 cam, and 273 Commando heads. The thing was a torque monster until about ??4K or so and then is was just like you shut off the key.
         
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        • RustyRatRod

          RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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          I thought the 920 was a big block casting.
           
        • MOPAROFFICIAL

          MOPAROFFICIAL FABO Gold Member FABO Gold Member

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          Just to be clear, cant deny air flow dynamics..but the 340 was rated 340 tq iirr. The 340 I put ported 315 heads on w/1.88 I take 1.50 exh made 380ft lbs. Tq., just at a lower rpm. Heads were 221cfm@142 port volume w/1.88 .

          It's the demand for flow 'volume' at higher rpm the smaller port cannot move/support...so it falls off at high rpms. it's mathematical the port volume you would need for x amount of cubic inch at x amount of RPM.
           
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          • toolmanmike

            toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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            #2658920 is a late 66-67 273 and 67 318 head. Closed chamber with 1.78/1.50 valves.
             
          • RustyRatRod

            RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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            Gotchya.
             
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