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  • over bore

    so my question is pretty simple:

    if I were to bore my engine lets just say .030 over and run stock compression, which i believe is 9.5 to 1, am I going to drop the compression at all from just boring it. Someone told me that i would, but i wanted to hear from someone with some real engine experience. mabye it'll go up a little? IDK?!? thanks much.
    98 SL2m - Intake, Raceland coilovers, daily driver
    94 SW2a>m - slammed and boosted 9 PSI - sloppymechanics.com inspired. Now owned by Importsdontlikeme.
    94 SC2m - Build in progress, Ford EDIS, 720cc's, stock engine, MS2v3.57 7 on the road again

  • #2
    it could go up a little.

    The piston will really determine that...if it's a 9.5:1 compression piston corrected for the overbore, then you would have 9.5:1 compression.
    If the piston maker uses a 9.5:1 piston height for stock size on the oversized piston, it will raise the compression slightly.

    either way, an overbore slightly increases volumetric efficiency.
    LSJ powered 1998 Chevy S-10 turbo
    visit my Albums - http://sixthsphere.com/album.php?u=7267

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    • #3
      Here's my question.

      Why?

      You're increasing the size of the combustion chamber without correcting the stroke...I would think it would go down...

      Dished pistons increase the size of the combustion chamber...not the same principle???

      BTW, your sig is freaking hilarious...
      -2002 Saturn SC2 - Clean, slow, low daily
      -2008 Saab 9-7x Aero
      -1996 Suzuki Bandit 750 swapped
      -1998 VW Golf GTI VR6 Mid Engine LeMons car
      -1986 RX7 - 275whp @ 10psi

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      • #4
        look at it this way:

        you're increasing the volume of the cylinder without changing the upper and lower extremes.

        now when you use the same stroke as before, you're compressing more volume with the same distance traveled.
        -Vinny

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        • #5
          +1 to the sig, how long did it take to make
          99 Saturn Blackberry SL2
          <br />
          <br />7/17/2006

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          • #6
            What Vinny and Jay are saying, if you didn't understand that, I'm sure you did... but for others who read this, I'm going to lower it down to PLAIN english. You're increasing the size, allowing more air into the cylinder, and compressing it to the stock piston height..... more air..... higher compression.

            Now, with a corrected stroke, you're retaining the stock compression ratio, but still allowing MORE air into the cylinder, thus, both ways increase volumetric efficiency. You'll get slightly more power either way...... but not anything drastically noticeable.

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            • #7
              Originally posted by project92SC2
              What Vinny and Jay are saying, if you didn't understand that, I'm sure you did... but for others who read this, I'm going to lower it down to PLAIN english. You're increasing the size, allowing more air into the cylinder, and compressing it to the stock piston height..... more air..... higher compression.

              Now, with a corrected stroke, you're retaining the stock compression ratio, but still allowing MORE air into the cylinder, thus, both ways increase volumetric efficiency. You'll get slightly more power either way...... but not anything drastically noticeable.
              no, that's not what I said.

              you're increasing diameter without increasing length is the basic idea. by doing that, and using the same stroke, you can be compressing it more unless the piston design is altered. if stock bore is a certain dish amount, and you keep that cc of dish for the larger bore, it can increase the static cr.

              if you altered the stroke that would directly effect compression (shorter = lower; longer = higher), but I never said anything about touching that, nor did Jay.

              your VE will go up though with the increased breathing ability
              -Vinny

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              • #8
                lol

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                • #9
                  I think i'm gunna go ahead and believe the admin of an automotive forum.

                  What you said makes a lot more sense now that i really think about it. I knew the very basics about running a larger bore, but not too much else.

                  easy enough, i'll look into it more when i have some money to put together a nice engine.
                  98 SL2m - Intake, Raceland coilovers, daily driver
                  94 SW2a>m - slammed and boosted 9 PSI - sloppymechanics.com inspired. Now owned by Importsdontlikeme.
                  94 SC2m - Build in progress, Ford EDIS, 720cc's, stock engine, MS2v3.57 7 on the road again

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