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MAXIMUM BORE FOR XJ750? aftermarket valve springs?

Discussion in 'XJ Modifications' started by skylrk62, Aug 31, 2012.

  1. skylrk62

    skylrk62 Active Member

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    I know there has been some talk on here about the 820 bore kit. My question is, how big can you bore before structural integrity becomes an issue? Is 900cc possible? If not, anyone gone bigger than 820? Also, I've been looking into a Megacycle cam #256-40. I need race pistons & springs. I've found a source to make pistons, but I don't know what size to go? The other question is, where do I find hi-performance valve springs? Anyone have any experience with this one?

    Project Shadow Warrior
     
  2. darkfibre

    darkfibre Member

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    820 would be it as there is not much metal left:

    [​IMG]

    Why would you change the valve springs? Heavier would just cause more valve recession for no gain.
     
  3. skylrk62

    skylrk62 Active Member

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    Thanks for the info on cc's. I'm not sure on the springs? All they say is the oem springs and pistons won't work?

    The cam specs are:

    Valve Lift - .410"
    Duration @ .040" - 250º
    Lobe centers - 105º
    INTAKE OPEN/CLOSE - 20 btc/50 abc running - .005"
    EXHAUST OPEN/CLOSE - 50 bbc/20 atc clearance - .007"

    Would I need a softer spring? I'm new to this and I'm trying to gather the right part combination.

    Darkfibre, I'm a big fan of your work with the efi mod. Did you have to modify the r6 throttle bodies at all? They look a little different on ebay. I'd like to replicate your design and use the intake runners you've designed. Would you be interested in making another set? I can wire you money through Pay Pal? I'd like to discuss this with you.
     
  4. mlew

    mlew Well-Known Member

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    When a cam manufacturer says their cam won't work with factory springs there are a several things to consider.
    1- the OEM spring will bottom out under full cam lift, dammaging cam and head.
    2- the OEM spring does not have enough tension to fully close the valve under high rpm.
    Most improntant is to ask them what springs they recommend to use.
     
  5. streetbrawler750

    streetbrawler750 Member

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    I talked with the megacycle cam man and the spring kit he sells is dicontinued, but said reason was to eliminate valve float. Wich is mlews number 2. Stiffer spring is necessary.
     
  6. darkfibre

    darkfibre Member

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    What sort of bike are you building? Those cam specs would give you more top end power, but you will lose out on the bottom end and mid range.
    I must admit most of my riding is not in the top end and I would probably only hit redline once or twice a week. If you ride in the non noisy part of the rev range most of the time, you will sacrifice some usability.

    Efi manifolds, wait until its proven (soonish, I promise). I would be more inclined to get one measured up for CNC replication. Those manifolds took a huge amount of time for me to do.
     

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