Gnarly Pipe on 99 KX hybrid?

Discussion for swapping a KDX motor in a MX frame...
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bryawn
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Gnarly Pipe on 99 KX hybrid?

Post by bryawn »

I have a 2000 KDX 200 motor in a '99 KX125 frame. I originally used a Pro Circuit pipe mated to a stock header pipe and it worked fine for many woods rides. Unfortunately, the pipe got damage beyond repair due to a untimely meeting with a big rock hiding behind a bush on a re-routed trail section. Hate it when that happens.

I've purchased a Gnarly woods pipe and it's a bit too short and contacts the frame. I can just get the pipe up to the first o-ring before the bend hits the frame rail. I'm thinking about welding on the end of the old pipe to the end of the gnarly, effectively lengthening the inlet tube about an inch. Pretty sure this would fit great, but what would it do to the powerband? I think this would probably shift the powerband down even further, and it's a woods pipe to start with. Probably not optimal. I also might be able to bend the tube and dent the pipe a bit and get it to just barely work.

Just curious if anyone has gotten a Gnarly to fit on a 99-02 KX hybrid and what option worked best.

Thanks.
sarrant
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Re: Gnarly Pipe on 99 KX hybrid?

Post by sarrant »

Word from the two-stroke tuning theorists that I've heard is stinger length is not really as critical as stinger diameter, but generally that type of pipe tuning is often as much trial-and-error as anything.

Try it.
Robkx250
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Re: Gnarly Pipe on 99 KX hybrid?

Post by Robkx250 »

Did it take manny modifications to fit the kdx motor in the kx frame? I would give it a go if it doesn't work you could just grind it back to the original place.
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Re: Gnarly Pipe on 99 KX hybrid?

Post by chkdx »

Word from the two-stroke tuning theorists that I've heard is stinger length is not really as critical as stinger diameter, but generally that type of pipe tuning is often as much trial-and-error as anything.
While that is true, what he's talking about extending is the headpipe, which will change the tuned length of the pipe, and shift the powerband down in the rpm range.
bryawn
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Re: Gnarly Pipe on 99 KX hybrid?

Post by bryawn »

chkdx wrote:
While that is true, what he's talking about extending is the headpipe, which will change the tuned length of the pipe, and shift the powerband down in the rpm range.
Correct. That's what I'm worried about. I have no idea how much impact that will have. Where we ride in Northwest Oregon, it's all tight single track and steep climbs, so a little extra torque may not be all bad. Maybe I'll just weld on the extension, go for a ride and as noted I can grind it off and go to plan B if it's terrible.
sarrant
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Re: Gnarly Pipe on 99 KX hybrid?

Post by sarrant »

Ah. Sorry, misread what he was intending to do.
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Re: Gnarly Pipe on 99 KX hybrid?

Post by chkdx »

Ok, I got curious and checked A. Graham Bell's book for some info. The KDX 200 service manual lists exhaust duration as 185.4 degrees. According to Bell, on a 200cc enduro bike, exhaust duration of 184 to 186 degrees corresponds to a power peak at about 9000 rpm. Sounds reasonable.

The formula for the tuned length of the pipe, in mm, is: L = (ED * 42545)/rpm

ED is exhaust duration in degrees, 42545 is a given constant representing sonic wave speed, and rpm is obvious. Using the formula:

L = (185.4 * 42545) / 9000, gives a tuned length, L = 876.43 mm, or 34.51 inches. The pipe you have may be slightly different, but this gives us and idea of the approximate tuned length you have now. Now let's say you extend the headpipe 1 inch, to 35.51 inches, or 901.95 mm. Rearranging the formula to find the new rpm peak: rpm = (ED * 42545) / L

Put in the new tuned length: rpm = (185.4 * 42545) / 901.95, gives an answer of rpm = 8745.

So the power peak is shifted down about 250 rpm. Is that enough of a change to matter? Dunno, that's up to you.

Hope that helps!
De250r
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Re: Gnarly Pipe on 99 KX hybrid?

Post by De250r »

Chkdx
Good bit of info there on how much an inch will change the peak hp


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bryawn
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Re: Gnarly Pipe on 99 KX hybrid?

Post by bryawn »

Thanks for the scientific explanation there Chkdx. Very interesting. So the question that Bell doesn't answer with this formula is whether shifting peak power down by a couple of hundred rpm will result in a similar reduction in peak power output. Since HP is a calculation of torque and rpm, it seems likely that there would be some power loss.

In the case of my KDX, it turns out that I will only need to add about 18 mm to the header pipe, not a full inch as I had thought, so the shift in the power peak will be even less. Since most of my riding involves small throttle openings dodging tree roots, rocks and tight turns, I rarely see peak power. Chances are, I won't even notice any difference 99% of the time.
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Re: Gnarly Pipe on 99 KX hybrid?

Post by chkdx »

I agree, there would probably be a small power loss, and I also agree, I don't think you'll really notice it. It's even possible that when the power peak rpm is shifted down a small amount, the torque increases a small amount at that lower peak, compared to what it was before, resulting in a smaller loss than might be expected.

Ok, that wasn't very clear. Let's put it this way: The current pipe produces say, 20 ft-lb of torque at 8750 rpm. I'm just picking the 20 as a plausible number. By lengthening the headpipe, the pipe might produce 20.25 ft-lb of torque at 8750 rpm, since the peak power rpm for the pipe has been shifted down. The engine will produce less absolute peak power, but the power at 8750 rpm may be up slightly from what it was before.

In any case, I don't think the addition of 18 mm to the headpipe is going to "ruin" the powerband.
bryawn
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Re: Gnarly Pipe on 99 KX hybrid?

Post by bryawn »

Good news. I'll let you know how it turns out.
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scheckaet
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Gnarly Pipe on 99 KX hybrid?

Post by scheckaet »

Adding length to the pipe will give you a bit more low end and less up top.
check my sig for my build thread, I have some pic of my modded gnarly / wood pipe.
bryawn
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Gnarly Pipe on 99 KX hybrid?

Post by bryawn »

Finally sucked it up and bought some more acetylene ($98 for a tank swap. Dang this stuff has gotten expensive). With the ability to weld restored, I hacked off the end of the old stock pipe - about 20 mm worth, just past the second groove for the o-rings. Then I welded it onto the end of the new Gnarly Woods pipe. The connection is very smooth and won't disrupt the flow. After a bit of wrangling and shoving I got the pipe past the frame rails and into the exhaust port. Also had to make up a temporary extension for the pipe retention springs. The lower pipe mount also was a bit short, so I had to come up with a solution for that, but the upper mount worked fine. The muffler also fit on fine.

In a test ride on my street, I could definitely tell that the revs cut out earlier than with the PC pipe. How much is due to new pipe vs. the extension, I have no idea. The bike would keep revving, but at a certain point it was just making noise. I have not had a chance to ride it, but my son took it out to the woods today and had a big grin on his face. He said there was noticeably more pull on the bottom end and he was able to tractor up hills easier. I'm looking forward to riding it myself, but so far seems like this was a good solution.

Thanks all for the info.
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Re: Gnarly Pipe on 99 KX hybrid?

Post by bryawn »

Rode the bike this weekend up in Hood River. Works great with the woods pipe and the extension. The top end tails off a little, but hardly a problem in tight woods. One of the guys in the group couldn't believe it was a 200 because I was able to short shift and grunt around pretty fast without even getting on the pipe. Third gear wheelies up hills? Yep. Jetting seems cleaner too (but maybe that's just because the pipe isn't full of carbon). Love it.
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Re: Gnarly Pipe on 99 KX hybrid?

Post by bufftester »

Real world you'll see a couple hundred RPM drop on the top end, but since the KDX already is a little soft up top you likely won't miss it. On my hybrid I'm running about 15mm of extension on the pipe and don't miss the top at all (although truth be told it's a 220 so some would argue there was no top to start with :lol: ), but like you my jetting seemed much more spot on afterwards. Since I spend most of my time in the woods in W. Washington it works for me, over in the east side of the state I just change gearing and live with it (I don't really need to do 85mph across the desert anymore anyway).
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