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Thread: LQ4 Compression Ratio

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  1. #1
    Join Date
    Sep 2007
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    LQ4 Compression Ratio

    Im running TFS 225s with 65 cc chambers on a stock internal LQ4 and want to mill them down to get between 10.8 or 10.9 to 1 with a stock head gasket. Im getting a lot of conflicting information on the internet which is preventing me from feeling real confident with the web compression ratio calculators.

    I know the bore and stroke but I've seen wide ranging stats for chamber cc on stock 317s (70-74cc) and piston dish volume. Anybody a real authority on the subject? I'd like to be able to fill in these blanks:

    62 cc = ? cr
    61 cc = ? cr
    60 cc = ? cr
    59 cc = ? cr

    Im thinking I need to go down to 59cc which would require milling .036 off (assuming .006 = 1 cc) to get close to 10.8:1 but other research indicates that 61cc (.025) will get me there. So I'm really confused at this point. Thanks for any input.
    ________________________
    2006 2500HD, RCLB, 4X4
    6.0, 4L80e, 14 bolt

  2. #2
    Join Date
    Sep 2007
    Location
    H-Town
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    212
    This is what I get.

    62 cc = 10.35:1 (would need to mill .018 off current 65 cc chamber)
    61 cc = 10.47:1 (mill .024)
    60 cc = 10.59:1 (mill .030)
    59 cc = 10.70:1 (mill .036)

    I'm using the following inputs into a compression ratio calculator:

    bore 4.0
    stroke 3.622
    chamber size 71cc (stock)
    piston dish -7.5 (plugged this value to get to stock cr of 9.4:1)
    gasket thickness .05 (stock)
    piston deck clearance 0

    These inputs get me to 9.4:1 which is the stock cr on a lq4. From there I just played with different chamber ccs. Anyone see any flaws in this?
    Last edited by Sizzle Chest; 04-05-2009 at 10:23 AM.
    ________________________
    2006 2500HD, RCLB, 4X4
    6.0, 4L80e, 14 bolt

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