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Compression ratio on 250 CI Chevy IL6

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  • Compression ratio on 250 CI Chevy IL6

    I’m in the middle of rebuilding my old 1973 motor and see that I have high compression pistons since they have a dome on them. I have no idea what the compression ratio is, I asked that question before on here and no one seemed to know. I guess I could do some CCing on the head and see what dome is on the piston to figure it out now that I have it apart... I’m keeping the cam stock, and have a bit of a decision to make if I run high compression pistons so I can run the timing retarded or advanced to match the fuel I’m using while running it retarded is it possible to run hot like a car would do with the timing retarded or since I will be using a raw water cooling system will it run cooler? Or should I go with the low compression pistons and take a chance on losing power, it’s rated at 165 HP now and I have a 2500# dry weight. Does anyone have one of these motors stock with low compression pistons, if so what’s your horse power rating? Your opinions would be appreciated.

  • #2
    Re: Compression ratio on 250 CI Chevy IL6

    Until you find out what the combustion chamber size is, I think you're guessing. They maybe stock pistons.
    From Mercruiser:
    Anyone who uses a service procedure and/or tool, which is not recommended by the manufacturer, first must completely satisfy himself that neither his nor the products safety will be endangered by the service procedure selected.


    • #3
      Re: Compression ratio on 250 CI Chevy IL6

      I believe that engine was rated at 165 with the stock pistons.
      1980 Sylvan Aluminum 18' Sportster I/O - Mercruiser 140, Honda 7.5 Kicker, Bennett M120 Tabs
      1974 Glastron V-179, Mercruiser 140
      1984 16.5ft. Alumarine Sealark side console with 82 Merc 50
      Manitoba, Canada


      • #4
        Re: Compression ratio on 250 CI Chevy IL6

        compression ration on oem is 8.5:1 for what it's worth. i don't know if them dome pistons are oem. to me it don't seem likely. the pic in the oem book has a pic of the piston and it has a recessed D cut into the top of the piston. as marc c says, i think it's rated at 165 stock.... how you'd tune yers and what it's hp rateing is not for me to even guess at. as for running it hot. i don't know on that either, oem has a 140 tstat i think, that'd run ya up to 160. why do ya want to run it hot. does that help? sounds like you've been running these dome pistons already. what have been the ramifications?. i don't know building engines up well enough to comment. seemed to me the 165 IL6 was pretty torquey already and dandy in it's oem state....i'd put it back oem if it were me. i didn't even know there was anything that ya could do to a 165 IL6 to get more out of it(ie dome pistons to increase compression ratio).....what kinda fuel are ya useing too. i don't understamd that statement. i use premium in my 72, says thats what it wants on the valve cover. aslo says it wants lead. so i add lead aditive... if you use oem parts you will know where to set the timeing too. seems like running it retarded or advavced is not the way to go unless ya can figure out what ya've got and know where to set the timeing at. oem it's a givein. at 6* @ 500-600 rpm......you have an interesting problem, good luck with it...
        'Lime Time'
        1975 AristoCraft Nineteen
        Mercruiser 165hp (chevy 250 L6), MC-1 drive


        • #5
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          Re: Compression ratio on 250 CI Chevy IL6

          I just bought this boat a few months or so ago as a restoration project. I have no idea of the motors history. I can tell from the smog holes in the head that it has been changed and doing a little research I’ve learned the marine version of this engine had 64CC chamber heads. I also learned the automotive version was rated at 100 ponies they have to be getting that extra 65 HP from somewhere Ziggy, It sounds like you have the same set up as I do so in your Trisonic, I'm guessing in stock form mine is supposed to be a low compression engine too. I don’t want to run it hot, I was afraid that if I kept it a high compression engine and used the ignition timing to make it run on pump gas it would run hot. I never had the boat in the water. I’m an old time mechanic from before the computer age really took off in cars and boats. When you had a high compression engine and they did away with good gas, that’s what I call it, the solution was to retard the timing to keep it from pinging. Besides losing performance by doing this you stood a chance that the engine would run hot, but that was in an automotive engine where you kept reusing the same water, not pulling in cool water constantly like in a raw water cooling system. I was hoping someone had experienced this. I guess I’m going into unexplored territory, maybe I just worry to much? Thanks for the help folks.