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Thread: V8 comp ratio

  1. 3rd July 2011, 08:10 PM #1
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    V8 comp ratio

    Hey guys I have my heads of my 99 3.9 D1 v8 currently getting new valves how much should i shve my heads to bump comp ratio up a bit this is a low comp motor with a wade 1166a cam going in now as well cheers for any advice
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  2. 4th July 2011, 08:33 PM #2
    PLR
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    G`day ,

    if you intend running unleaded petrol i`d suggest leave it as it is .

    The higher the usable ratio the better but difference between Rover 9.35 high and 8.13 low ratios is pretty much nothing .

    Peter
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  3. 4th July 2011, 09:30 PM #3
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    I always found a big difference between 8.13:1 and 9.35:1 in the 3.5 anyway.
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  4. 4th July 2011, 09:32 PM #4
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    from memory around 8hp between low and high comp 4L,,,
    "How long since you've visited The Good Oil?"

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  5. 4th July 2011, 11:37 PM #5
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    spoke to the guy doing the heads and were leaving it stock and just milling all the working surfaces re facing all the valves new stem seals and pressure testing cheers hopefully back together soon
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  6. 5th July 2011, 09:28 AM #6
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    Djam1 is right , adding more static static and dynamic comp is like adding cubes , as long as its matched with the right cam and the heads have been flowed correctly.You can run unleaded up to 10.5:1 easily with the right ignition map / curve . Its midrange that you need , so a cam like a TIGHE CSRV8 310 with some healthy bowl and seat work in the heads , it will outpull any standard 4.6 !! Deck the block to Zero ,install some holden 186 flat tops , shave .020 at least off the heads , shim the pedestals lookout 200HP easy !!
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  7. 5th July 2011, 10:28 AM #7
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    When I went from 4.0 to 4.6 and had to do the heads it's ended up at around 10.4 and I can tell you anything but BP ultimate and it pings it's haed off. Even then under loads it's still noticable. Elf 114 cures it but at $8- litre I'm not filling 90+ litres often.
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  8. 5th July 2011, 01:34 PM #8
    PLR
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    Quote Originally Posted by djam1 View Post
    I always found a big difference between 8.13:1 and 9.35:1 in the 3.5 anyway.
    G`day ,

    i guess it depends how you rationalize the difference you found .

    Since we didn`t get any 9.35 EFI your talking Carb .

    The 9.35:1 ratio 3.5ltr figure is 125HP .
    The 8.13:1 ratio 3.5ltr figure is 132HP .

    The figures for the engine in question are 8.13:1=179HP and the 9.35:1=181HP ( earlier 3.9 had higher 188HP )

    I don`t dispute a worn low comp 3.5 and a not so worn high comp may be different to use and visa-versa but i`d think it would be a struggle to notice a difference using the HP figs above .

    Peter
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  9. 5th July 2011, 08:50 PM #9
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    It was a high mileage 8.13:1 but it had a new cam in it
    The 9.35:1 was a new engine with the same cam to be honest there was a big difference.
    None the less the 8.13:1 was being replaced so it obviously was worn.
    They were both carb in the same vehicle the driveability was greatly increased. To be realistic the 8.13:1 also had composite gaskets thus reducing the compression ratio by another .7:1 from the original tin gaskets.
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