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  • I-6 Bored .40 over!

    Wondering if I should bore the I-6 .40 over (258CI) ?

    Those of you who had it done was it worth it ?

    Did you notice a big difference?

    How much H.P. did you gain?

    Did the fuel mileage drop alot?

    Would you put a street cam back in or a moderate cam or a high lobe race cam in?

    Have A Great Day!


  • #2
    I went .30 over with my rebuild. I also used a Comp Cams Extreme Energy cam. Part # X4250H. I chose the small cam because I didn’t want to lose low-end torque. I noticed a little power increase, but not much. Finally put a 4.0 head on and that really woke the engine up. Now I wish I had used a larger cam as the 4.0 head increases low-end torque also. If you’re going to rebuild, find a 4.0 head to put on your engine. You won’t be sorry.


    • #3
      Most of my rebuilding exp is on a 350 chev, but as a general rule, 30 over is the most you really want to bore an engine. Every time you bore an engine you weaken it by reducing the thickness of the cylinder walls. Now I'm not saying you can't bore the engine 40 over, but personally I wouldn't. The difference in power between an engine bored 30 over and one bored 40 over would most likely not be noticeable.

      While there is no substitute for in^3 there are some things that help. A mild diy port job will benifit any engine, even a low reving I-6. The stock compression ratio on a 258 is about 8:1. The 4.0l has a higher rato, and that is why the 4.0l head will "wake up" a 258. I'm new to the jeep engine realm, so I can't tell you what the ratio will be after installing a 4.0l head.
      The ideal ratio (arguably) for a low reving high torque engine is about 8.5 to 8.75 to 1 and the upper limit is about 9:1. A higher ratio will also help improve fuel economy and top end HP. (You may also experience better emmisions, but don't quote me on that)

      If anyone has the exact ratio after a 4.0l head is installed, please let me know so I can put it in my notes.

      This one word of advice applies to everyone and every situation: Before you commit to any major upgrade to your engine, sit down with your favorite machinist or engine builder and have a nice long talk over lunch. Tell him what you want, and what you can spend. Don't be afraid to get a second poinion.

      Good luck


      • #4
        I’m not sure what the compression ratio is on this engine now. The 4.0 head does have a smaller combustion chamber. I remember that with the engine bone stock, I had to run 93-octane gas with a lot of pinging. I did the Nutter bypass and dropped the octane down to 89 with a little pinging. I put an HEI distributor on and went to 87-octane with no pinging. I was a little worried when I put the new head on that I would get into pinging problems again. But this did not happen. I’ve got the timing set at 6 degrees BTDC with no hint of pinging. I thought with this head change that I would help the engine have more power in the top end, which it did. I was surprised at how it also improved low-end torque. Over all this was a really good mod. If you’re going to rebuild the 258, it is well worth the trouble to put a 4.0 head on.


        • #5
          From what I was explained, boreing your engine over doesn't add power, just decreeses the cylinder walls. The less you have to bore it the better off you are, cause you can go back later and rebuild it again.