Expasion chamber (pipe) issues 250cc

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#1
I am looking for a liitle technical help with an exhaust issue.

I have a 2005 KX 250 powered SKM shifter kart that I race at a family track in the off season.

I am currently running a tunable LRD pipe made for a 250R honda quad. (tunable by lengthening or shortening the largest section of the pipe just before it tapers back down)

In order to make the pipe fit and also to tone down the power I had to remove a six inch section from the pipe. I took the six inches out of the very begining of the pipe right after the flange.

Can someone explain to me what effect the now six inch shorter exhaust is having on the engine?

Do you see any negative effects on the longevity of my engine?
 

Rich Rohrich

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#2
By shortening the length of the straight section that leads into the diffuser section you'll narrow the effective range of the pipe tuning effects. You will potentially have a greater maximum wave tuning effect, but the trade-off is it will be over a much narrower rpm range. This could potentially put you in a situation where peak pipe pressures will raise piston crown and combustion chamber temperatures to a point where detonation could occur. You may need to play with the ignition advance curve and the fuel curve to keep things safe.

You also changed the total tuned length of the pipe which will tend to raise the peak wave effects to a higher rpm range. You may be able to counter that by taking advantage of the tunable center (dwell) section of the pipe.


Significant pipe changes tend to require significant change to fuel and ignition settings to keep everything in balance.
 
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#3
Rich

Thanks for the reply. The "modification" of the pipe definately lowered the bottom end power of the engine. I have not noticed any detonation, but I do not know if I am able to.

I will be runnning the kart tomorrow. I am happy with the performance, though even a little less low end power might be better.

Thanks again

Chris
 
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#4
When you fit the pipe, instead of removing sections, cut it at all of the bends. Then, rotate the bends as necessary to fit the pipe and weld it back together. This method will have minimal effect on the characteristics of the pipe as you maintain the lengths and volumes of all of the chambers.
 
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#5
Well who is going to give a lesson on exhaust harmonic waves and pressure waves?Its a lot more complicated than hacking and welding!Isn't the idea to put the pressure wave and sound wave out the pipe at the same time?Doesn't the sound and pressure travel different routes?I would bet there is a software program to simulate what is going on in the pipe and for design,I would be real surprised if pipe companies actually used them!
 

Rich Rohrich

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#6
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#7
Now that is better than coffee for a wake up call!Being as it is already designed,why does an exhaust come out the front and loop around,why not the back and straight out!
 

Rich Rohrich

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#8
whenfoxforks-ruled said:
why does an exhaust come out the front and loop around,why not the back and straight out!
Fitting an airbox and having enough space for the required exhaust length would be pretty tricky on a dirtbike, but the reverse cylinder setup has been used successfully in a number of road race designs like the TZ500 OW48.
 

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SpDyKen

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#9
whenfoxforks-ruled said:
Now that is better than coffee for a wake up call!Being as it is already designed,why does an exhaust come out the front and loop around,why not the back and straight out!
Do you mean like the way that Yamaha did on their last few years of their built-for-sale- to-customers TZ 250 road racers, which I believe ended their 'production' phase 4,5, or 6 years ago.

It really is a shame that the manufacturers quit, out of fiscal necessity, research & development on 2 strokes. I believe that another 10 years of work on 2t's would have yielded us some great bikes!

Of course, what comes around, goes around! :nod: :ride:
 

SpDyKen

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#10
I forgot to mention that, at first, Yamaha simply reversed the cylinders on their parallel twin TZ250's, similar to the works bike that Rich showed, but then in the "V-twin" revolution era of the early 2000's, went to a V-twin with the carbs. in the middle, bottom cyl. laying down, so that it's pipe had only a slight bend, with the top, (or rear) cyl. exhaust pointing almost straight back.

These were all VERY COOL production bikes! My good friend Charlie F. has one on a display stand in his living room. :cool:
 
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#11
Well I talked to Dave yesterday(2 1/2 hours!)then I seen this thread about this guy's shifter cart that will probably frag a piston due to changing exhaust pressure and he brought up his idea about putting barriers inside the pipe to force the sound waves back in line with the pressure wave!I was like my jaw hit the floor,of course this is beautiful and why has this not been done before!Or has it,once installed who would know its there?
 
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#12
If I am destined to "frag" my piston what can I do to prevent it?

The kart would be undrivable on my dirt "road course" track. Is there a better way to detune the engine?

I wasnt able to cut the pipe in the bends because it only had one bend. It was designed for a 250R fourwheeler and is not shaped like a pipe for a bike.

Thanks for the info

Chris
 
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#13
You honestly need to deal with the software program yourself or find someone who actually does!The hit or miss bs has gone on for too long as it is,changing pressures and harmonics in your pipe directly effects pressure on your piston.More pressure on your piston or less and boom!Out at the track,what is the frequency of piston failure on this application?
 
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#14
There have been no piston failures or problems of any sort,up to now.

The engine was run three out of four Sundays during the summer of 2006. However it is never run very hard or very long. The races last about fifteen minutes say three or four races a weekend. I doubt if the engine has ever ran more than 30 minutes at a time. Also it has very rarely been out of third gear at say 3/4 throttle.

I am going to try my hand with the program and maybe contact an exhaust manufacturer about a custom pipe for my application.

Thanks again for the help and please feel free to offer anymore advice.

Chris
 
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#15
A lot of the aftermarket pipes are dead knock offs of the oem,for mx and sx,the big changes are in the silencers!The guys running the short silencers are changing their powerband to hit harder on bottom,bye bye topend power!35 years ago,by mistake,I found out about putting a beer can on my stinger to improve bottom end.Now I have a better idea why that was!No horsepower gains,just put the power where it can BEST be applied!