Compression Ratio Calculations and Head Gaskets - NCRS Discussion Boards

Compression Ratio Calculations and Head Gaskets

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  • Duke W.
    Beyond Control Poster
    • January 1, 1993
    • 15667

    #16
    Re: Compression Ratio Calculations and Head Gaskets

    Originally posted by Mark Hedberg (30810)
    Is there a maximum variability in CR across cylinders?
    No, but I like to see maximum variation of no more than 0.1. John McRae achieved that on his Special 300 HP engine. It's mentioned in the Tale of Two Camshafts and Compression Ratio Explained articles.

    Duke

    Comment

    • Duke W.
      Beyond Control Poster
      • January 1, 1993
      • 15667

      #17
      Re: Compression Ratio Calculations and Head Gaskets

      Originally posted by Mark Hedberg (30810)
      Well, after much consideration I think the following is the new plan:

      Take the block and heads back to the machine shop and have the decks straightened out (hopefully without harming the stamp pad - it is not an original motor but it is period correct).

      Ditch the KB 156 pistons for KB 157.

      The heads had already been ported, polished and port matched. Get the head volume after whatever machine work is done.

      Get the rotating mass balanced. Again.

      If my calculations are correct, doing the above should put me in the 10.0 to 10.2 range using a Fel-Pro PermaTorque head gasket.
      With the CR in the low nines, you're giving away 4-5 percent torque and power, but you can probably use regular unleaded or midgrade and still have a reasonably aggressive spark advance map. It's a lot of work and expense to get that torque/power back. I recommend you have the machine shop check for head and deck level and get their opinion on whether a shim gasket will seal.

      Duke

      Comment

      • Mark H.
        Very Frequent User
        • July 31, 1998
        • 384

        #18
        Re: Compression Ratio Calculations and Head Gaskets

        Weeks turn into months, but I now have the engine back from the shop and am taking measurements. Head of the shop said it was at 10.2 compression and spec'd FelPro 1003 head gaskets. We switched out the KB 156 pistons for Speed Pro L2166-AF domed pistons (5.3 dome). When I measure the deck clearance (range from .0.026 to 0.029 on the odd bank and 0.024 to .029 on the even), using my head chamber volumes above, I get the following:

        Piston CR
        1 10.13
        2 10.24
        3 10.34
        4 10.34
        5 10.28
        6 10.31
        7 10.16
        8 10.21
        I need to call the machinist to see how he measured 10.2:1; He had said he would take a bit of the dome of pistons as necessary to dial in the compression ratio, but I see no signs of work on the pistons. 10.34:1 is a little higher than I would have liked.

        Duke, to avoid further delays, I may call this good.

        Almost ready to install the cam. It came with two sleeves of camshaft assembly lube 10 years ago and they haven't been opened. I'll plan to kneed the heck out of them and use them unless that's a bad idea?

        Nice to be back to making progress again.

        Comment

        • Duke W.
          Beyond Control Poster
          • January 1, 1993
          • 15667

          #19
          Re: Compression Ratio Calculations and Head Gaskets

          Call me, Mark,

          310
          372
          5527

          Duke

          Comment

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