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Flat top Piston Compression Ratio

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  • Engine: Flat top Piston Compression Ratio

    I installed flat top pistons in my Sky Hawk 289 when I rebuilt it. What would the compression ratio be now...about?

  • #2
    With the Truck, large combustion chamber heads, it would be 9:1. With most other heads 10:1 or there about. 1/2 a point less with composition head gaskets. How well they will perform without detonation will depend on quench. If they sit "down in the hole" you will have a hard time without retarding the timing, and running premium fuel. If it were me I would use the shim gaskets just for this reason. The 1/2 point rise will result in LESS detonation because of the reduced quench area, all else being equal.
    Bez Auto Alchemy

    "Don't believe every internet quote" Abe Lincoln


    • #3
      Originally posted by 345 DeSoto View Post
      I installed flat top pistons in my Sky Hawk 289 when I rebuilt it. What would the compression ratio be now...about?
      Since scant detail is provided and no specs, "about" is the answers you'll get. Most Studebaker heads (other than truck/R2) vary only a few CCs in the combustion chamber, so as Bez says, about 10:1. But then some engines have been rebuilt more than once and the heads milled one or more times, so that would increase the CR. Was the block square decked? This increases compression. Most engines when rebuilt are overbored .030", some .060" and that definitely increases the CR. Were the connecting rods reconditioned? This shortens them a few thou, lowering the compression. Thin or thick gasket? Finally, cam choice is a major factor in actual dynamic compression. Stock cam? R1 cam? R2+cam? Longer duration lowers the dynamic compression ratio.

      Bottom line - compression ratio mostly matters uphill on a hot day. If it actually has 10:1 or more and a stock cam, it will ping like a poltergeist, guaranteed. If it doesn't, ping, either it's got more cam, or someone tuned it exceptionally well or the compression is less than 10:1 or like some of us older guys, the driver just isn't hearing it.

      jack vines


      • #4
        Head casting numbers?? If other than the truck low compression head, similar to R2 head, anywhere from 9.5 to 11:1. Does your starter spin it over?


        • #5
          Starter spins it over fine...