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  • Engine: 289 Compression Ratio

    I am having the carbs for my Weber motor done soon. I need to know what my compression ratio will be. The motor is a 0.060" over 289 with "570" heads. It will be assembled with modern composite head gaskets and half dish pistons. Any help would be appreciated.

    JK

  • #2
    Originally posted by 3x2stude View Post
    I am having the carbs for my Weber motor done soon. I need to know what my compression ratio will be. The motor is a 0.060" over 289 with "570" heads. It will be assembled with modern composite head gaskets and half dish pistons. Any help would be appreciated. JK
    First, measure from deck to rocker cover rail.

    Jack Vines
    PackardV8

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    • #3
      Originally posted by PackardV8 View Post
      First, measure from deck to rocker cover rail.

      Jack Vines
      Is that to ensure the heads have not previously been milled? Cuz my 570 heads have already been taken down .125 and that would not lead to an accurate accounting of compression ratio if you were going by the tech page.
      sals54

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      • #4
        Outa site would be my guess. 1/2 dish avanti pistons yield about 9.5 to 1 per Jon Meyer, 1/8th cut maybe a full point.so 10.5 to one would be suspect with todays fuel? it will have to be noodled out by the Pro's like Jack V. sound's dicey but cool, what are cam specs? "Big Cams" will bleed off some pressure and you will have a Jack Hammer exhaust note, you will get noticed!!! Luck Doofus

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        • #5
          Yeah, I was using flat top pistons and shim gaskets. It had a pretty decent exhaust note with the 2-1/2" pipes and flow masters. I liked it.
          sals54

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          • #6
            "1/2 dish" pistons sounds a lot like a "3/4" race cam. Doesn't really say much or mean much.
            Need to know the actual volume of the dish. Unless you have all of the data below, you are just guessing.

            Bore (that could be figured by the .060 overbore... so guessing 3.6225)
            Stroke (guessing stock at 3.25)
            Volume of piston dish (not just "half" or "whole" dish)
            Thickness of head gasket compressed
            Distance from piston to deck (not from the dish, but from the part that quenches)
            Volume of combustion chamber.

            As for combustion chamber volume, that is why Jack wanted you to measure from deck to rocker cover rail.. so see if it has been milled.
            Even if it HAS been milled, there is always the possibility that someone worked over the combustion chamber, further changing the stock cc.
            You can get a close guess ASSUMING no one worked on the chamber, but to know your exact static compression ratio, you need to find the cc of the chambers.

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            • #7
              If the head has been milled the CC of the combustion chamber in the head will be telling and measuring the head thickness will tell if it's been milled but CC the head as some folks like to grind and hog stuff out usually not knowing why. Lots of measuring to be done, there are some cool books about building SBC engines that go into it at length. The camshaft is yet another black art so to speak. Cam timing,lift and duration lobe centers along with rocker ratios can change things a bunch as well. Another important thing is when you start changing the innards you need to know why or what you are building it for, racing with high RPM's or just wanting a little extra from the light because other factors like rod stretch and clearance between the piston and the head/ valves can be critical on the outcome. Just a bunch of other things to consider is all. Have fun

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              • #8
                Jack,

                What should an unmolested head measure?

                JK

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                • #9
                  JK,

                  Here is a calculator to help you figure out what your compression ratio is;

                  https://www.uempistons.com/index.php...5efb3a1aa56304

                  If you are using dished pistons, you need to measure the volume of the dish to get total combustion chamber volume. Also, the head gasket thickness is important as well as how far down in the bore your piston crown is, this too adds to chamber volume. Every cc added to chamber volume will reduce compression ratio.

                  Lynn's memory was faulty on stroke length, the 289 has 3.625" stroke. That or his strike on the "6" key didn't register, I'd bet the latter.

                  For instance, the engine I am (rather slowly) building has the following done;
                  Bore .060" with "full dish" pistons on 18-20 cc volume.
                  Block decked .010-.012"
                  Piston crowns .015-.018" below deck
                  Cylinder heads milled .040" and the valves unshrouded, chamber volume at 57-58 cc (stock-54cc).
                  Steel shim gasket .017" thick when compressed (estimated)

                  Result, about 8.2 cr

                  Ken

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                  • #10
                    Originally posted by 3x2stude View Post
                    Jack, What should an unmolested head measure? JK
                    3.5625"

                    Jack Vines
                    PackardV8

                    Comment


                    • #11
                      Originally posted by spokejr View Post
                      JK,

                      Here is a calculator to help you figure out what your compression ratio is;

                      https://www.uempistons.com/index.php...5efb3a1aa56304

                      If you are using dished pistons, you need to measure the volume of the dish to get total combustion chamber volume. Also, the head gasket thickness is important as well as how far down in the bore your piston crown is, this too adds to chamber volume. Every cc added to chamber volume will reduce compression ratio.

                      Lynn's memory was faulty on stroke length, the 289 has 3.625" stroke. That or his strike on the "6" key didn't register, I'd bet the latter.

                      For instance, the engine I am (rather slowly) building has the following done;
                      Bore .060" with "full dish" pistons on 18-20 cc volume.
                      Block decked .010-.012"
                      Piston crowns .015-.018" below deck
                      Cylinder heads milled .040" and the valves unshrouded, chamber volume at 57-58 cc (stock-54cc).
                      Steel shim gasket .017" thick when compressed (estimated)

                      Result, about 8.2 cr

                      Ken
                      Not going by memory. Got my info from the web (so it must be right) but a quick review (with the GLASSES ON) indicates I gave the stroke for a 259. Of course, you are correct on the 289 stroke.

                      Comment


                      • #12
                        Originally posted by Lynn View Post
                        (with the GLASSES ON)
                        But the world looks so much better without them...

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