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Thread: True 3.0 compression ratio (its low)

  1. #1
    Supporting Member Turbo Mopar Contributor Ondonti's Avatar
    Join Date
    Dec 2005

    True 3.0 compression ratio (its low)

    I measured 3 different styles of 3.0 pistons.

    1997 3.0 (supposed 9.3:1)
    1994 3.0 (supposed 8.9:1)
    198x 3.0 (supposed to be 8.85, its a full skirt piston with thicker old fashion 1.5, 1.5, 4mm ring pack)

    Every single pistons measured 12.5cc dish, even though they are completely different castings.

    I have measured a stock head in 2006 and I believe it was 47cc. Heads I ported for myself back then were around 51+cc (BIG...) Ed Kelly measured and found 46-47cc for all chambers, and I am assuming the motor he measured was around a 1990 model.

    Measuring OEM used headgasket, this leaves 3.0L SOHC 12 valve compression ratio

    8.19:1 for 46cc head
    8.3:1 for 47cc head.

    Super low! Its too bad the top ring land and intake valve relief are so close together or we would be able to make a lot more stock piston power safely.

    If you look at JE brand shelf pistons sold for vR4, which has a 43cc chamber (much smaller),
    Part # Bore Oversize Stroke (mm) Rod (mm) C/D (mm) Head CC's C/R With .040 (squish) Dome CC's

    321330 91.1 STD 76 141 31.5 43 8.0:1 -15.5
    321333 91.1 STD 76 141 31.5 43 9.0:1 -6.5

    Shows how deep our 12.5cc dish is for a much larger combustion chamber!!!!!
    1992 Duster 3.0 The Junkyard- Now - Megasquirt II/Extra. OEM 10:1 3.0 + funny business
    - Old - 11.5@125 22psi $90 Stock 3.0 Junk Motor - 516whp 519wtq 20psi 91 oct built 3.0 stock n/a computer
    1994 Spirit 3.0 - 12.3@113 15 psi - Daily Holset He341 280whp 304ft/lb @10psi
    1994 Spirit 3.0 a670 - Wife's - Disassembled
    1990 Spirit 3.0 E.S. 41TE
    1993 Spirit 3.0 E.S. 41TE
    1994 Duster 3.0 A543
    1990 Lebaron Coupe 2.2 TII non IC, T1 31TH
    1981 Starlet KP61 Potential driver, 1981 Starlet KP61 Parts, 1983 Starlet KP61 Drag
    1998 Dodge 12v Cummins 47re 4x4 4dr longbed. No fuel plate. Mack plug, AFC & Spring mods. Boost elbow.

  2. #2
    Garrett booster
    Join Date
    Jun 2016
    Janesville, WI

    Re: True 3.0 compression ratio (its low)

    That is very interesting. Thanks for posting those numbers! I always suspected that the combustion chamber volume was actually the same on all of the motors. I've owned all three "different" generations of 3.0l and they all look the same as far as the combustion chambers and piston dish. I measured from the wrist pins to the crowns and that's the same too. IIRC The top ring on the 87' full skirt piston is slightly lower making the distance from the ringland to the valve relief larger but I still broke 2 of those with the blower. Good find!

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