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Thread: High compression vs low compression

  1. #1
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    High compression vs low compression

    Hi,

    Rather than hijack a thread, I'll ask in a new one.

    This will sound possibly like a dumb question, but, what is the difference between a high compression engine and a low compression engine?

    Is my 2001 4.6 I believe it is a Thor engine high or low compression? Can it be either? What is the difference? If it is low compression how can I make it high compression? What are the advantages?


    Sorry abut all the questions, I just feel a bit confused?


    Thanks

    Steve

  2. #2
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    The compression ratio will be written above the engine number on the tab on the side of the top of engine block on the passenger side between the two centre exhaust pipes.

    It should have a 8.3 or a 9.3 (please correct me if the numbers out not quite correct) above the engine number indicating the compression ratio.

    The two engines are basically the same except for the pistons - so to upgrade from low compression to high compression you would need to change the pistons.

    As a general rule HC will produce more power and more torque.

    Garry
    REMLR 243

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  3. #3
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    [ame="http://en.wikipedia.org/wiki/Compression_ratio"]Compression ratio - Wikipedia, the free encyclopedia[/ame]

  4. #4
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    Hi,

    Also remember that a higher compression engine requires higher octane fuel to run it.

    As with everything, there are tradeoffs

    The other thing you may care to read about is power vs torque, motors can be tuned in various ways to provide more of one or the other.

    Have fun!

    Steve

  5. #5
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    Quote Originally Posted by intheozone View Post
    Hi,

    Rather than hijack a thread, I'll ask in a new one.

    This will sound possibly like a dumb question, but, what is the difference between a high compression engine and a low compression engine?

    Is my 2001 4.6 I believe it is a Thor engine high or low compression? Can it be either? What is the difference? If it is low compression how can I make it high compression? What are the advantages?


    Sorry abut all the questions, I just feel a bit confused?


    Thanks

    Steve
    Hey Steve

    There's a good chance that yours will be a Low comp motor as that was standard that the P38 was set up for the Oz market, however some High comp motor did slip through.

  6. #6
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    Thanks guys,

    I am just looking to squeeze a few more horses under the bonnet, looking for inexpensive ways do do this.

    I think a twin pipe exhaust system may help and make her sound a bit more V8ish

  7. #7
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    Staying on topic, I am hijacking! Sorry intheozone. My 3.5 has a psi reading of 120 -125 on all cylinders. I'm told this is low? I went looking for the number on the block but couldn't find it. I will look again.

  8. #8
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    Here is a pic of the engine number and compression ratio on the side of a 3.9 block. If only the engine number is there, the block has been leveled at some stage and the numbers removed - the engine number would have been stamped back on.

    You can see the compression is CR 8.13:1



    Here is a pic that shows where the numbers can be found - left side between the two middle exhaust ports.



    Garry
    REMLR 243

    2007 Range Rover Sport TDV6
    1977 FC 101
    1976 Jaguar XJ12C
    1973 Haflinger AP700
    1971 Jaguar V12 E-Type Series 3 Roadster
    1957 Series 1 88"
    1957 Series 1 88" Station Wagon

  9. #9
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    Quote Originally Posted by garrycol View Post
    Here is a pic of the engine number and compression ratio on the side of a 3.9 block. If only the engine number is there, the block has been leveled at some stage and the numbers removed - the engine number would have been stamped back on.

    You can see the compression is CR 8.13:1



    Here is a pic that shows where the numbers can be found - left side between the two middle exhaust ports.



    Garry
    Thanks Gary, what does that mean? I was expecting a PSI reading, so not sure how to interpret or convert it.

    Just checked, and interestingly I have the engine number but not the compression numbers. Why would the block be leveled?

    Regards

    Jez

  10. #10
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    Quote Originally Posted by snowbound View Post
    Thanks Gary, what does that mean? I was expecting a PSI reading, so not sure how to interpret or convert it.

    Regards

    Jez
    Hi Jez

    It is a ratio so it is expressed as 8:1 or 7:1 etc, this is different from compression pressure, which is measured in PSI.

    You can do a rough conversion, as detailed here
    How do you convert compression ratio and psi

    But as it says there are other factors involved in the actual pressure you finish up with

    Steve

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