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  1. #1
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    AFTER MARKET S38B36 CONNECTING RODS

    What would be the pros and cons to use shorter connecting rods on a FI s38b36 .? Reason

    i ask is b/c my engine builder ask me i would be interested on custom made shorter rods for a better

    rod ratio on a FI s38b36. The oem rods are 144mm. why would i want to use shorter rods?

    ThanksClick here to enlarge

  2. #2
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    You wouldn't want to use shorter rods, ever.


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    Click here to enlarge Originally Posted by 5mall5nail5 Click here to enlarge
    You wouldn't want to use shorter rods, ever.
    I wouldn't say ever ever but with this motor yes, I'm not sure why the guy recommended shorter rods. He would need a new crank too...

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    No really there's really never any reason to make a rod ratio worse


  5. #5
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    Click here to enlarge Originally Posted by 5mall5nail5 Click here to enlarge
    No really there's really never any reason to make a rod ratio worse
    The times I have seen motors de-stroked was usually for much higher RPM so usually to lower the piston speeds at that RPM range.

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    The times I have seen motors de-stroked was usually for much higher RPM so usually to lower the piston speeds at that RPM range.
    You're confusing "de-stroking" with "shortening the rods".

    Example: M50B25 = 75mm stroke with 140mm rods. M52B28 = 84mm stroke with 135mm rods. Stroke != rod length.


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    Click here to enlarge Originally Posted by 5mall5nail5 Click here to enlarge
    You're confusing "de-stroking" with "shortening the rods".
    Probably but why would you just want to shorten them unless you wanted to shorten your stroke?

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    Again, ideally you would never want to shorten them, even when shortening the stroke. The shorter the rod, the worse the angle is as side load on the piston (pressing it against the wall of the block).

    You basically ALWAYS want as long a rod as possible so that it doesn't side load much. BMW made compromises - they were able to lengthen the stroke but needed to keep the rod down 5mm in the above example else they would have had to move the wrist pin up high which for whatever reason, they did not want to do (probably cost of re-manufacturing a piston).


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    1 out of 1 members liked this post. Reputation: Yes | No
    I suppose good questions to the OP would be:

    "Has your engine builder done his "what works and what has NOT worked for X whp to X rpms" homework pertaining to the S38?"
    "If he hasn't (which he most likely has not) then why is he still even LOOKING at your S38 engine, let alone building it?"

    A few personal thoughts.
    Click here to enlarge
    www
    milkt org

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    Keep the rod as long as possible, shortening a rod has ill effects in general.
    Some people live long, meaningful lives.

    Other people eat shit and die.

    I'm not racist, I hate everybody equally; especially fat people.


    Click here to enlarge

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    2 out of 2 members liked this post. Reputation: Yes | No
    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    Keep the rod as long as possible, shortening a rod has ill effects in general.
    Thanks guys for your input.

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