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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-38006 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration2020 ktm 5002002 FE 501 Previous bikes09 Ktm 30004 ktm 52502 ktm 520(540cc)00 FE 600 x298 ktm 62095 Ktm 62090 xr 63090 tt25091 kx 25083 yz 125 k

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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-38006 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration2020 ktm 5002002 FE 501 Previous bikes09 Ktm 30004 ktm 52502 ktm 520(540cc)00 FE 600 x298 ktm 62095 Ktm 62090 xr 63090 tt25091 kx 25083 yz 125 k

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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-38006 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration2020 ktm 5002002 FE 501 Previous bikes09 Ktm 30004 ktm 52502 ktm 520(540cc)00 FE 600 x298 ktm 62095 Ktm 62090 xr 63090 tt25091 kx 25083 yz 125 k

  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"!

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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-38006 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration2020 ktm 5002002 FE 501 Previous bikes09 Ktm 30004 ktm 52502 ktm 520(540cc)00 FE 600 x298 ktm 62095 Ktm 62090 xr 63090 tt25091 kx 25083 yz 125 k

  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

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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-38006 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration2020 ktm 5002002 FE 501 Previous bikes09 Ktm 30004 ktm 52502 ktm 520(540cc)00 FE 600 x298 ktm 62095 Ktm 62090 xr 63090 tt25091 kx 25083 yz 125 k

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    Default Re: Expansion chamber construction/modification

    I am interested in your project. My 380 has the gnarly but I have fitted the long 'hockey stick' silencer from an 02 300 to keep the noise down. I had to expand the socket on the silencer to fit the 30mm stinger but the long silencer is the smaller diameter. The smaller diameter may have the effect of increasing gas temps and therefore lowering density (so the sonic return is slower and top end friendly), but it may be offset by the greater gas flow of the 380 which would be restricted at the stinger and therefore increase density. So you may get a gas temp increase (as elemetal pointed out) without the top end benefit you are chasing. Many variables in the pipe design effecting volumetric efficiency that we are guessing about here, all interrelated to some extent.
    FWIW my 380 (with head mods and the 300 silencer) pulls really smoothly all the way through without having a far reaching top end. A fast motor that doesn't feel fast. A little more upper hit would be nice. The red PV spring doesn't seem to make much difference.
    By comparison my 07 300EXC with head mods, fatty and standard silencer pulls much better than stock right through, with a much better top end.
    I am keen to see your results once you try it.

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    Default Re: Expansion chamber construction/modification

    Quote Originally Posted by Miva View Post
    I am interested in your project. My 380 has the gnarly but I have fitted the long 'hockey stick' silencer from an 02 300 to keep the noise down. I had to expand the socket on the silencer to fit the 30mm stinger but the long silencer is the smaller diameter. The smaller diameter may have the effect of increasing gas temps and therefore lowering density (so the sonic return is slower and top end friendly), but it may be offset by the greater gas flow of the 380 which would be restricted at the stinger and therefore increase density. So you may get a gas temp increase (as elemetal pointed out) without the top end benefit you are chasing. Many variables in the pipe design effecting volumetric efficiency that we are guessing about here, all interrelated to some extent.
    FWIW my 380 (with head mods and the 300 silencer) pulls really smoothly all the way through without having a far reaching top end. A fast motor that doesn't feel fast. A little more upper hit would be nice. The red PV spring doesn't seem to make much difference.
    By comparison my 07 300EXC with head mods, fatty and standard silencer pulls much better than stock right through, with a much better top end.
    I am keen to see your results once you try it.
    Thanks for that info, I know its common now for people to put 300 silencers on 380's specifically the fmf power core which has a 28mm OD but what may not be common knowledge is the 380 fmf power core is still available with the 30mm OD and I have tried both with the 30mm 380 silencer defiantly delivering more top end and noise.

    I did some research and the only pipe of the shelf I can buy in Austrailia is the Gnarly so I did buy another one for a spare as it's only a matter of time until it's discontinued as well.

    While I was working on my 2016 380 I tried to fit the 250 fatty and it hits the radiators, the radiators being much lower on this chassis but I never intended it for this bike anyway just interesting to see it is a different shape or it's bent.

    I had a conversation with pipe masters about doing the welding and it all sounded good until he said pipes only comes in 28mm or 31.7mm diameters which killed it temporarily for me until a few days past and I realised I still have the piece we cut off the 300 silencer where we made a custom bend for the 2016 chassis, looks like there's just enough length to make the job work.

    Next step will be fitting the 250 fatty to the 99 380 and checking every thing lines up ok. This project may take some time as I have just picked up a 2020 ktm 500 and also a dead 2002 fe 501 so my apologies to keep you waiting, I'll post any updates but the garage is loaded with projects at the moment
    Current bikes 16 ktm 380 custom build http://ktmtalk.com/showthread.php?543034-2016-ktm-38006 ktm 380 custom build http://ktmtalk.com/showthread.php?54...ld-restoration2020 ktm 5002002 FE 501 Previous bikes09 Ktm 30004 ktm 52502 ktm 520(540cc)00 FE 600 x298 ktm 62095 Ktm 62090 xr 63090 tt25091 kx 25083 yz 125 k

  11. #10
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    Default Re: Expansion chamber construction/modification

    It's always nice to have a project or two on the go. We in Melbourne are staying sane with 'Lockdown' projects. Not permitted to travel to buy motorcycle parts (and retail is not open anyway). Postal delays are trying our patience nearly as much as our elected dictator.

    I will try the 380 FMF powercore silencer for a top end comparison. I removed it soon after I got the bike to keep the noise down and the neighbours happy. More important than ever at present while we are all stuck at home. I'm not a big fan of silencers that sound faster. I believe the stock EXC silencers do a good job of remaining 'civilised' without robbing performance.

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