What might my 360 CR be?

Small Block Mopar Engine

  1. challenger57

    challenger57 cuda57

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    I am changing the cam in my LA 360 and noticed the pistons are .040 above the deck and it has been previously bored .030 over. Changing springs also. The pistons also have about a .250 valve relief cuts, the edges are beveled also. From what i can tell the heads have not been milled and are 71-72 J heads. Don't know if block was decked. I am guessing about 10- 10.5 CR. So far know one has been able to identify the cam i am removing. The only numbers on it are on the back end 940162. Any info appriciated!!! Thanks
     
  2. RustyRatRod

    RustyRatRod Lemmie see your b00bs. FABO Gold Member

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    Are you 100% SURE it's not a 340?
     
  3. nm9stheham

    nm9stheham Well-Known Member

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    Pretty good guess; I get 10.5 with a .039 gasket, 6 cc valve reliefs in flat top pistons, and a 72 cc combustion chamber estimate.
     
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    • challenger57

      challenger57 cuda57

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      Thats what the block shows 360 casting on the side.
       
    • 67Dart273

      67Dart273 FABO Gold Member FABO Gold Member

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      • Rapid Robert

        Rapid Robert Well-Known Member

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        fill the valve reliefs with clay till they are "dead even" flat then take em out & drop em in to your CC ing burette that has some fluid in it & see how far the fluid rises on the scale. You know the bore and stroke and deck height and your choice of head gasket thickness. all that is left is a guestimate of the head CC's you will be using. Many online calculators good for SCR. Reportedly for sure the Wallace racing one (& I'm sure others/but not all) are accurate for DCR (cam figured in). EDIT Knead the clay into a rope before you drop it in the burette so it sinks all the way
         
      • krazykuda

        krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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        First you need the clay, before you can knead the clay.... :D
         
      • Rapid Robert

        Rapid Robert Well-Known Member

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        I'm wondering if he picked his (own) handle or if someone (that knows him) picked it for him
         
      • AJ/FormS

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

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        I love math so I'll take a stab at it.
        The .030 over pistons and 3.58 stroke make a swept volume of 748.3cc
        The heads could be about 68cc to 70cc
        Eyebrows usually come in at 5cc/pair
        The Fellpro .039 is about 8.8cc
        And the pop-up comes in at -8.4cc

        So; Scr =( CV + cv)/cv ; and CV = 748cc, cv =68+8.8+5 -8.4 =73.4

        thus (748 + 73.4)/73.4 =11.19

        if the heads are 70cc, then cv becomes 75.4cc, and the Scr is 10.92
        Or... if they are 72cc, then cv becomes 77.4cc, and the Scr is 10.66
        Did I err?

        With Scr of 11.19;You will need a bit of a late closing intake to drop the Dcr, to make that streetable.
        Ima guessing an ICA of 70* should be close, on 91 octane,at sealevel. 70* will get you to 8.5 Dcr. That's a tad high for iron heads, no quench, and 91.
        But at 1000ft, that becomes 8.3, and streetable.
        73* would drop it to 8.1@1000ft, and a safe target, and easy tune.

        But if the pistons are really down in the hole, and the heads are 70cc,then cv becomes 70+8.8+5+8.4 = 92.2 and the Scr = 9.11(oops). And if te heads come in at 72cc, you're in the basement at Scr=8.9. With this low compression, you would then need a teensy cam to bring the Dcr UP!

        So the answer to your question is; I'ma guessing, cuz there's just not enough actual input.

        -------------------------------------------------------------------------------------------
        EDIT...Abbreviations; Scr is Static compression ratio...Dcr is Dynamic compression ratio..........CV is piston swept volume....cv is total chamber volume....cc is cubic centimeters.
        EDIT 2 Should have specified pop-ups are flat-tops and no dish.
         
      • challenger57

        challenger57 cuda57

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        TOOO many abbreveations. This motor was totally street driven. Removed cam because of ruff idle and low vacuum, could not use accessaries. The cam I am going to use has better street manners and a much better projected vacuum. I will also be using adjustable rockers that were in the orig. 273 that cam in the car a 65 cuda hense the cuda 57 and challenger for my favorite car and 57 my y o b.
         
      • mod5v

        mod5v Well-Known Member

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        So are them pistons custom forged units. Are you shure its not a typo and they are actually .040 below the deck. Just I have not seen a of something the shelf piston for the 360 that was above deck uncles it was a pop up. Not trying to start anything just want to make shure of what you have.

        Oh crap! Stupid auto correct..
         
      • RustyRatRod

        RustyRatRod Lemmie see your b00bs. FABO Gold Member

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        This is almost sounding like a Hoover 360.
         
      • AJ/FormS

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

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        So, 57........What cam did you choose? What numbers did you give the cam-guy?
         
      • yellow rose

        yellow rose Overnight Sensation FABO Gold Member

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        The ONLY way to calculate compression ratio is to do a .500 downfill and cc the bore just like a combustion chamber. Then, and ONLY then can you calculate the compression ratio.

        FWIW a .040 positive deck height is no big deal. If you measure the depth of the chamber it's usually about .090-.100 and if you figure .040 out of the deck and a .039 gasket you are still at .090-.100 quench.

        To do it correctly and keep the CR below 11.0:1 you would need the piston out of the deck .080-.090 and use a DISH piston to keep the CR where you want it.
         
      • pishta

        pishta I know I'm right....

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        He said the pistons were
        Beveled and pop ups, sounds like forged to me. Might want to consider zero decks for street to lower CR. Those would sell pretty easily.
         
      • AJ/FormS

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

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        Where did he say the pistons were domed? Did I miss that?
        I took them to be KB190s, Which are flat tops, quench dome of .050 and bevelled edges.
         
      • challenger57

        challenger57 cuda57

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        The cam of choice is comp cam 20-212-2 268H. I need vacuum to move heater diaframs in my 65 barracuda. While changing springs I decovered the heads look like they were milled about .030 in comparison to other 360 heads I have. So wish me luck over and out!
         
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