KDX versus KTM EXC? Experiences please.

Dbeast

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Aug 24, 2002
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Comically I've put a post on the KTM board but I've gotten much more help over here.

Can some explain or point me an explanation of how the powerband is built? I'm familiar with 4 stroke and cars, there's a camshaft deciding the power and RPM characteristics but what decides that on a 2-stroke? How can you have a KDX and an MX bike, what make the difference.
 

CaptainObvious

Formally known as RV6Junkie
Damn Yankees
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Jan 8, 2000
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Dbeast said:
Can some explain or point me an explanation of how the powerband is built? I'm familiar with 4 stroke and cars, there's a camshaft deciding the power and RPM characteristics but what decides that on a 2-stroke?

Dbeast, your question would be best answered in the Advanced Technology & Engineering forum. There are many things that affect the powerband, from pipe shock waves, bore and stroke to exhaust porting. Many variables. Do a search on this subject and you'll be reading for hours.


Dbeast said:
How can you have a KDX and an MX bike, what make the difference.


I don't understand the question. I have a KDX and an MX bike, and I can tell the difference. The bikes have totally difference personalities. The KDX has a soft compliant suspension and a ton of motor...power everywhere. Perfect wood woods riding.

My KX has a stiff, fast action suspension. I have to twist the throttle to extract power. That's not an indictment, rather, I enjoy keeping it on the pipe.

They are two dirrerent bikes built for two different purposes.
 

Dbeast

Member
Aug 24, 2002
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The KDX MX question was just exemplifying my powerband question, how they can be so different with almost the same parts. I understand the suspension differnce just not the power.
 

Dbeast

Member
Aug 24, 2002
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I dug out the Pipes I have, I have a gold series and the stock. The stock pipes has some dents and dings, would I be better off with the stock in that condition over the Fatty that's on it now?
 

cicone

Member
Sep 29, 2003
310
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Dbeast said:
Comically I've put a post on the KTM board but I've gotten much more help over here.

Can some explain or point me an explanation of how the powerband is built? I'm familiar with 4 stroke and cars, there's a camshaft deciding the power and RPM characteristics but what decides that on a 2-stroke? How can you have a KDX and an MX bike, what make the difference.
Dbeast---Obviously a two stroke doesn't have a cam or valves. The ignition timing is is set at by stator position and and then the timing curve is usually controlled electronically by the CDI(capacitor discharge ignition)module i.e. the black box. Typically a trail or enduro bike will be ported to provide a wide powerband and plenty of low end torque, partly enhanced by a much heavier flywheel. An MX bike is designed for maximun horsepower and consequently runs at very high RPM with a much lighter flywheel---to get there quicker--and has a narrower powerband, but considerable higher peak horsepower. This makes them difficult to control on in tight conditions where hooking up with the ground is most important. Also, MX bikes are prone to stall at low RPM's and develope little power unless on the pipe---which can produce excessive wheel spin--not good for tight spots. MX bikes are set up for all or nothing power and trail bikes are set up to put power to the ground over a wide RPM range and provide that extra throttle control without excessive wheelspin. Hope that helps. My $.02 worth. Jay :thumb:
 

Dbeast

Member
Aug 24, 2002
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But how does it do it? No cam, no valves, nothing. What makes the powerband so different? I understand the flywheel and recip mass, I 4-wheel trucks. Theres the timing, I see that but is that the only difference? Can you explain the "ported" part?

Thanks
 

Patrick

Member
Mar 6, 2002
37
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I have an '02 220 with the RB carb mod, pipe, reeds etc. and also a '04 KTM 300 EXC. The KDX is the smoothest of the two thanks to the carb mod but the characteristics of the power delivery are surprisingly similar at least to me. However the 300 pulls harder as one would expect from a larger engine. The KTM suspension is far superior and that is why I stopped modifying my KDX and bought the KTM. I was not quite sure that spending $600 + on the suspension would yield the same results. I also don't like the way the KDX gas tank keeps smashing my ..... (ouch!!). However, I have a lot of issues with my KTM - so many that I would not recommend an '04 KTM 300 at this time. Maybe next year. I had no issues with the Kawasaki other than the questionable piston skirt.
 

skipro3

Mod Ban
Dec 14, 2002
902
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Patrick, What are the issues you have with the '04 KTM 300?
Dbeast, I think you are asking how a 2 stroke works? There are channels (ports) within the cylinder. When the piston goes up, it draws air/fuel into the lower crank of the cylinder via a vacuum effect. When the piston goes down, that AF mix can't go back up the carb, because of the reeds, it goes up the channels in the cylinder to the upper part where it exits out ports and is now transfered to the top part of the piston. When the piston goes up, it compresses and it also draws a fresh mix of fuel into the crank again. The spark fires, the piston is driven down, and the crank AF mix goes up the channels out the port, etc. There is also and exhaust port that vents the burned mix out the pipe as well. You need to get a book with pictures to really understand it all. Google search may lead somewhere too. Good Luck.
 

cicone

Member
Sep 29, 2003
310
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Dbeast said:
But how does it do it? No cam, no valves, nothing. What makes the powerband so different? I understand the flywheel and recip mass, I 4-wheel trucks. Theres the timing, I see that but is that the only difference? Can you explain the "ported" part?

Thanks

dude, you have a lot of reading to do. I suggest Eric Gorr's site at a minimum to understand two strokes. The piston ports accomplish the same thing as valves on a four stroke---just in a different manner. This is too much for me to get into here, but searching for info should clue you in. Not only does the porting make a difference, but the geometry of the pipe is critical. Once again, a search will provide the answers. Good luck!
 
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