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Thread: Increasing 3.9 Compression Ratio

  1. #1
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    Increasing 3.9 Compression Ratio

    My 1988 RRC has a 3.9 litre 8.13:1 compression ratio with the composite type head gaskets. As I run on LPG I would like to increase this compression ratio to around 10:1. If I put some of the steel shim type gasket I think that would give me around 8.7:1, how much would I then need to remove from the heads to get to 10:1? The engine has the code 38D, I purchased the engine to replace the original 3.5 litre that had had it, not sure what vehicle the 3.9 came from.

  2. #2
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    You would have to calculate the combustion chamber volume required.

    composite gasket heads have 28Cc combustion chambers and shim gasket heads 35CC?

    40 thou shave takes a shim head to composite head.

    So I reckon about 20 thou.
    You will have 3 potential problems
    1 You will need pedestal shims to get your lifter preload to between 20 thou and 60 thou.
    2 The heads will be soft if you believe the LR stuff.
    3 The inlet manifold may need machining.

    The correct way is different pistons.
    Regards PhilipA
    PS 8.7 :1 is just about the limit for even 95octane as the head design is old.

  3. #3
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    makes sense if you're running on gas. Its probably easier to rebuild another motor with new pistons .... that way you can drive the car while the work is being done and just swap the motor one weekend
    Proper cars--
    '92 Range Rover 3.9V8 ... slugomatic
    '92 Range Rover 3.8V8 ... 5spd manual
    '85 Series II CX2500 GTi Turbo I :burnrubber:
    '63 ID19 x 2 :wheelchair:
    '72 DS21 ie 5spd pallas
    Modern Junk:
    '07 Poogoe 407 HDi 6spd manual :zzz:

  4. #4
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    What is the largest capacity Rover motor I could put into a 1988 model? I would have to swap the front cover to retain the oil pump and distributor. The camshaft would also need the provision for the distributor, and the bellhousing bolt up to my manual gearbox.

  5. #5
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    The largest LR engine would be a 4.6 - yes if you want to keep a dizzy you would need to change the timing cover etc over, spacer for the crankshaft and change the camshaft. It will bolt up to your manual box.

    My low comp 3.5 runs LPG OK, so having a low comp engine is not such an issue for LPG.
    REMLR 243

    2007 Range Rover Sport TDV6
    1977 FC 101
    1973 Haflinger AP700
    1971 Jaguar V12 E-Type Series 3 Roadster
    1957 Series 1 88"
    1957 Series 1 88" Station Wagon

  6. #6
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    I have done some calculations to see what ratio I would get if I took 0.5mm (20 thou) off the heads and installed the steel gasket.

    The swept volume of one cylinder is 492cc, so to get a compression ratio of 8.13:1, as standard, the total volume of the heads, piston dish and head gasket must be 69cc as 492cc+69cc/69cc = 8.13:1.

    The difference in the composite (1.2mm or 48 thou) and steel gasket (0.45mm or 18 thou) is 0.75mm so that volume is 5.20cc. If I take 0.5mm from the heads that volume is 3.47cc.

    Now the original volume above the piston is reduced to 69cc-5.20cc-3.47cc = 60.33cc.

    Now the new compression ratio is 492cc+60.33cc/60.33cc = 9.15:1

    I would be happy with 9.15:1 but then would probably need to machine the manifold as I am taking a total of 1.25mm away.

  7. #7
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    If you do go down the route of new pistons, my engine machinist recommended Hypatec pistons (decent quality and good value for money). From memory a set of eight 20 thou oversize 9.35:1 pistons for my 3.5L were $550-600ish dollars a few years back. (They sell good quality Hastings piston rings too). The company were great to deal with too.

    Precision International

    Edward

  8. #8
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    Repco used to be able to give you all the information you are after for almost any brand of car.

    You could actually buy a catalogue from them....it's probably on internet now.

    Repco would also do your engine machining for you.

    I have no idea what services they offer now , they went very quiet for a while but appear to be going through a rebirthing.

  9. #9
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    If you want to "build" a motor, there is a 5.2 stroker kit for the rover v8.

    My own honest opinion is to not bother, but rather to examine other offerings if you want reliable power

    When I say power I mean obviously 250kW and over, with the torque to match.
    Roads?.. Where we're going, we don't need roads...

  10. #10
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    The 38D is from circa 1992 non cat RRC, the attached doc relates to older head type with 3 row bolts fitted to post 1995 2 row block production, milling specs are given taking into account the step from shim to composite gasket. This would maintain the ratio of the late 2 row block which also had the higher compression ratio pistons.

    You don't want to mill any more than this, your best option is to try and find a set of the 9.35:1 pistons that were available on the engines fitted with a cat converter.
    Attached Files Attached Files

    Bonnie - MY03 D2a TD5 Auto (One Bourbon)
    Clyde - MY93 RRC 3.9 Classic (One Scotch)
    Arfa - S2a 109" custom trailer (One Beer)
    Earl - The "Duc" 2009 Ducati Sport Classic (chuck in a Lambrusco)

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