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Thread: Expansion chamber construction/modification

  1. #1
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    Default Expansion chamber construction/modification

    Hi guys

    I have purchased a used 99 250 fmf fatty with the intention of modifying it to be a 380 fatty as the fatty is NLA for the 380. I have a new 380 gnarly for comparison. The obvious difference between the 250 and 380 pipe is the stinger is 28mm and 30mm. By eye the only difference i can see between the fatty and gnarly is the final tapper into the stinger is kind of longer and narrower on the fatty but not sure if that's a 250 thing or a fatty thing.

    So does anyone know what's the physical difference between a fatty and gnarly?

    Other than the stinger diameter is there any difference between a 250 and a 380 Pipe?

    I have 30mm pipe and I can modify the 250 fatty but I'm not sure if I will have a genuine 380 fatty after doing this.

    I'm also curious as to why the 250 pipe has reduced in length so much over 20 years and length to the silencer as well.
    Current bikes
    16 ktm 380 custom build http://ktmtalk.com/showthread.php?543034-2016-ktm-380
    06 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration
    04 Ktm 525
    98 ktm 620

    Previous bikes
    09 Ktm 300
    2000 fe 600 x2
    02 ktm 520(540cc)
    95 Ktm 620
    90 xr 630
    90 tt250
    91 kx 250
    83 yz 125

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  3. #2
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    Default Re: Expansion chamber construction/modification

    20201011 161342
    Current bikes
    16 ktm 380 custom build http://ktmtalk.com/showthread.php?543034-2016-ktm-380
    06 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration
    04 Ktm 525
    98 ktm 620

    Previous bikes
    09 Ktm 300
    2000 fe 600 x2
    02 ktm 520(540cc)
    95 Ktm 620
    90 xr 630
    90 tt250
    91 kx 250
    83 yz 125

  4. #3
    KTMTalk Member
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    Default Re: Expansion chamber construction/modification

    20201011 161004
    Current bikes
    16 ktm 380 custom build http://ktmtalk.com/showthread.php?543034-2016-ktm-380
    06 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration
    04 Ktm 525
    98 ktm 620

    Previous bikes
    09 Ktm 300
    2000 fe 600 x2
    02 ktm 520(540cc)
    95 Ktm 620
    90 xr 630
    90 tt250
    91 kx 250
    83 yz 125

  5. #4
    KTMTalk Member elemetal's Avatar
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    Default Re: Expansion chamber construction/modification

    You have to do some very careful measure of the diameter and length of all the cone sections in the pipe as well as head pipe length; you will find they are indeed different. Look on the iterwebs at a pipe for a 150 and a pipe for a 550; gross comparison but it will show you where and how they play with the shape to get different characteristics. There is a basic set of sizing that is based on the amount of exhaust flow; then you want that rebound sound wave to bounce back to the exhaust port at a specific time ("on the pipe"); that time is dependent on what rpms the motor is most efficient at/where you want it to be most efficient. Sound rebound is dependent on medium density so as the pipe is more filled with fumes the slower it moves, same with gas temp.

    This is just a basic example of pipe tuning parameters, they get way more complicated than that. Note how the pipes on a 2t road race bike have really long chambers and dirt bikes have shorter ones...they want the power at higher rpms.

    There are some online programs to help figure out what shapes and lengths work but it is head scratching to figure and you still have to test it out. If you mod the stinger and just run the 250 pipe it will work on the bike (small stinger will hole the piston), but likely have power at a higher rpm than you want and be soft down low. Could be fun to try out but I wouldn't expect it to behave just like the real 380 pipe.
    '19 TX 300, '18 Scorp Factory 300, '15 500 EXC, '14 Gasgas XC300, '04 Gasgas EC300, '96 550, '93 550 MXC, '91 540 (CA plated and sold to Picklito), '72 H-1 500 Qwacker
    "A Gentleman can never have too many motorcycles"!

  6. #5
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    Default Re: Expansion chamber construction/modification

    So in short I would end up with a top end pipe, if I then put that pipe on my 380 with the widened exhaust port and reed block spacer for more crankcase volume which already has a top end charge that goes forever and pulls my arms out when it comes on, this pipe mod should complement and already crazy motor?

    Sounds like a plan, I'm using this bike for vintage natural terrain scrambles, just a bit of fun it's not racing so the wilder the better!

    Thanks for the informative run down
    Current bikes
    16 ktm 380 custom build http://ktmtalk.com/showthread.php?543034-2016-ktm-380
    06 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration
    04 Ktm 525
    98 ktm 620

    Previous bikes
    09 Ktm 300
    2000 fe 600 x2
    02 ktm 520(540cc)
    95 Ktm 620
    90 xr 630
    90 tt250
    91 kx 250
    83 yz 125

  7. #6
    KTMTalk Member Rebuild's Avatar
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    Default Re: Expansion chamber construction/modification

    you might want to read "The Two Stroke Tuners Handbook" by Gordon Jennings, if you haven't already. It is old, mid 1970's.
    It has formulas for pipe design, etc.
    It can be downloaded.

    Torque pipes have shallow cone angles, topend pipes have steep cone angles, is about all i remember.
    Steve
    Ariel MCC
    09 300 EXC-E
    89 125 EXC

  8. #7
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    Default Re: Expansion chamber construction/modification

    Thanks ill have look it up
    Current bikes
    16 ktm 380 custom build http://ktmtalk.com/showthread.php?543034-2016-ktm-380
    06 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration
    04 Ktm 525
    98 ktm 620

    Previous bikes
    09 Ktm 300
    2000 fe 600 x2
    02 ktm 520(540cc)
    95 Ktm 620
    90 xr 630
    90 tt250
    91 kx 250
    83 yz 125

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