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  • Compression Ratio Calculator Link or an Easy formula wanted

    Ladies and Gents

    Does any one have an easy to use formula, or link to compression ratio calculator for dummies. I have been on the computer and calculator for hours and I am not getting it,

    Hello. ( I'm like a Myers Manikin, the lights are on, but know one's home).

    I have the following known values.
    64.5 bore, 56 stroke, trapped volume above TDC 26cc. Do I need any thing else (exhaust port top to top deck 29.5 mm), am I missing something.

    Also I want the reverse, aiming for a 6.6.1 ratio. Got the bore 64.5, got the stroke 56, (exhaust port top to deck is hopefully standard at around 30mm) can someone tell me the trapped volume cc's I need above TDC ( if its possible).

    Advance thankyou on the reply's.
    Kind regards seahorse

  • #2
    Compression Ratio Calculator - Wallace Racing

    Try this as i use it for my V8 builds, but you will need to cc the head volume and add it too then it should be all very close.

    Comment


    • #3
      Originally posted by seahorse View Post
      Ladies and Gents

      Hello. ( I'm like a Myers Manikin, the lights are on, but know one's home).
      with or without cloths?

      Comment


      • #4
        Originally posted by HRC 51 View Post
        Compression Ratio Calculator - Wallace Racing

        Try this as i use it for my V8 builds, but you will need to cc the head volume and add it too then it should be all very close.
        tried it I couldn't get that to work either

        Comment


        • #5
          Give me everything that the above link requires mate and i will look up another couple of sites i use tomorrow..

          Comment


          • #6
            Bore x pi x stroke + trapped vol at tdc = your volume at bdc.

            Divide this number by the vol at tdc and should be your compression ratio.

            So in your case 6.45cm x 3.141 x 5.6cm=
            108.6cc
            +26cc
            = 134.6cc


            134.6/ 26 = 5.17:1

            Comment


            • #7
              Its a 2 cylinder 2 stroke lc 350 , squish will be 1.1mm. Gasket thickness is about .8 on a 64 bore. 64.5 bore, 56 stroke, exhaust port roof to deck height 29.5 mill .
              I need cc's for a 6.6.1 ratio. Start with 22cc' head dome and perhaps come down. That's all I got. Worst case stroke will have to increase to 58.

              Comment


              • #8
                To solve for a ratio of 6.6:1

                Your bore x stroke x pi won't change = 108.6cc


                a is the trapped area at tdc.

                108.6 +a/a = 6.6/1

                108.6 + a = 6.6a

                108.6 = 6.6a - a

                108.6 = 5.6a
                108.6/5.6 = a
                19.39= a

                So you need to reduce your trapped volume at tdc to 19.39cc to achieve a 6.6:1 comp ratio.

                That is a lot to try to get from decking the head, need a domed piston or something like that, you're getting rid of about 1/3 the volume.

                Comment


                • #9
                  i know calculating compression pressures for 2t is very unlike 4t but isn't a ratio of ~ 6:1 very low?
                  faster ya go closer to nirvana

                  Comment


                  • #10
                    Elsie - tuning

                    Then you need to work out if you want to use uncorrected CR or corrected CR which measures from when your exhaust port closes.

                    This link will help
                    Last edited by D.J; 12-10-2013, 06:51 AM.
                    There are two questions in life.
                    1:Which way do I go
                    2:Whats the lap record.

                    Comment


                    • #11
                      TKS to everyone who replied.
                      I applied the following calculation from ben and got this, maybe someone could check against their formula

                      Bore 6.45cm x PI 3.1416 x stroke 5.6cm= 1134.7459
                      +21cc
                      = 1155.7459
                      divide 21cc
                      = 6.4.1
                      Alternately 20cc = 6.6.1

                      DJ, Hi mate, Id really like it if you could, try 20cc in your formula, after all, you race the basdards. And I have seen how fast your bike is.

                      Big hug and thanks to those that have chimed in.

                      Ps Biffer no idea if 6.1 is to low, will know once a compression test is done, I believe it is possible to calculate, but I don even want to go there.
                      PS superoo, I have a draw full of nude barbies.

                      Comment


                      • #12
                        I guess it would also fantastic to find out what Comp ratio's the few LC 350 boys here are running. Come on DJ, coff it up mate.

                        Comment


                        • #13
                          Originally posted by seahorse View Post
                          TKS to everyone who replied.
                          I applied the following calculation from ben and got this, maybe someone could check against their formula

                          Bore 6.45cm x PI 3.1416 x stroke 5.6cm= 1134.7459
                          +21cc
                          = 1155.7459
                          divide 21cc
                          = 6.4.1
                          Alternately 20cc = 6.6.1

                          DJ, Hi mate, Id really like it if you could, try 20cc in your formula, after all, you race the basdards. And I have seen how fast your bike is.

                          Big hug and thanks to those that have chimed in.

                          Ps Biffer no idea if 6.1 is to low, will know once a compression test is done, I believe it is possible to calculate, but I don even want to go there.
                          PS superoo, I have a draw full of nude barbies.
                          Hi mate, you screwed your calc, your 1134 is cubic mm not cubic cm so when you add your 21cc to it its wrong. Otherwise you are saying have a ~1 litre per cylinder bike

                          Make sure you keep all units the same, which is why is changed your bore to 6.45cm from 64.5mm, same deal for stroke.

                          I had a quick look and apparently 2t typically run about 6:1 - 8:1, you need to factor in the supercharging effect of the expansion chamber means that you are still pushing the limits of the fuel/ignition curve when on pipe in terms of pinging.

                          Comment


                          • #14
                            Just read the link DJ, very good mate.
                            Made things less muddy. 6.1 ratio is OK then and over 7 sailing close to the wind, and raising the exhaust port will reduce CR. (are the old Crown Radious) wont forget that for a while.
                            When I put this lump together, I might have to catch up and fill your pippet with beer.

                            Comment


                            • #15
                              Originally posted by benlikesbikes View Post
                              Hi mate, you screwed your calc, your 1134 is cubic mm not cubic cm so when you add your 21cc to it its wrong. Otherwise you are saying have a ~1 litre per cylinder bike

                              Make sure you keep all units the same, which is why is changed your bore to 6.45cm from 64.5mm, same deal for stroke.

                              I had a quick look and apparently 2t typically run about 6:1 - 8:1, you need to factor in the supercharging effect of the expansion chamber means that you are still pushing the limits of the fuel/ignition curve when on pipe in terms of pinging.
                              Sorry mate it does work, should be 113.4.

                              Mind you I did like the 1.1 litr yammys, had a mate Clugg many years ago, who used to ride one, on dirt roads he was pretty handy, used to treat it like a motard.
                              Last edited by seahorse; 12-10-2013, 07:57 AM. Reason: calc

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