Mysteries of pipe design

[Chouca]

I have tried to find out the theoretical consequenses of what will happen if I change to a 125 pipe on my 200. The reason I would like to try it is that the left portion of the 200 pipe really easy gets dented when I crash - and that happens :)
So far I have not been "enlightened" and I am getting more and more curious/confused by the answers I have got.

The general opinion is that I will lose low end and eventually gain top end.This makes me puzzled because in my understanding;

The exhaust pulse has to travel a certain distance back and forth in the pipe.

This distance/volume is changed if I for an example put a 125 pipe on my 200.

In this case the distance/volume decreases and the pulse travels faster back to the exhaust port.

So far so good, I can understand that changing this distance/volume will change the timing of the pulse and have effects on how well the pulse will help to fill the cylinder with fresh air/gas.

But how is it that this is changing over the rpm register???
How can this change in distance/volume alter the power characteristics in a certain range in the rpm spectrum?
My guess would have been that it would remain constant over all rpm´s!

Yes, I will also try to find a 125 pipe also to do a practical test, it will be very interesting to see if I will feel a difference at all???

:)

 

[billyghog]

There will be a change at a certain RPM level because of the way the pulse travels. Each pipe is tuned so that the pulse makes the engine most efficient at a certain RPM level - that is why FMF can make 4 pipes for one bike and each one has different powerband effects. The ultimate pipe would be one that has a variable length and volume expansion chamber to match up with the RPM's of the engine as they vary, though this is just not practicle. Hope this helps, maybe someone else can explain it better than I did.

 

[billyghog]

Here's a good link that will explain it:

http://motorcycle.com/mo/mcnuts/em-pipes.html

 

[cujet]

A shorter pipe will in fact return the plug pulse faster, therefore the engine will be on the pipe at a higher RPM.

I took 25mm ( 1 inch) out of the headpipe one of my Husky's pipes in an attempt at raising the RPM at which the power signs on. Sure enough, the torque curve moved up quite a bit.

Chris

 

[elf]

I dont know to much about this but I know the length of the headpipe can have a big effect on where your power is located, a shorter headpipe will move your power higher in the revs and a long one moves your power lower.I had a ESR pipe on my yz250 that came with three inserts that fit between the pipe and the cylnder with each one being a different length,Well thats about all I can help you. Have you considered a [pipeguard?

 

[bclapham]

in running a 125 pipe on a 200, you should look at outlet diameter also. the 200 will flow a lot more air through the engine, but the smaller pipe wont let it leave......bad things will then happen

 

[Chouca]

Thanks to you all,

Billy G - "that is why FMF can make 4 pipes for one bike and each one has different powerband effects" - a very good point, it seems like it is a quite complex matter... :ohmy:

elf - what difference on the insert lenght are we talking about, a couple of inches or fractions of inches???

Bclapham - Regarding the flow out of the engine, I think the KTM 200 and 125 are having the same outlet diameter, as well as the stinger diameter, so physically it will be OK I guess.

 

[Detonator]

Low end on a 125

There was a recent article on the effect of dents and dings on pipe performance. The basic conclusion was that the first few inches of pipe closest to the engine were the most critical in terms of keeping "ding-free." Dents in the expansion chamber yielded no noticeable difference in performance; I'm assuming these are reasonably small dings.

I would be really interested to find a way to add an inch of pipe length right at the cylinder. I'm trying to shift the powerband of my 125 down a few thousand RPM...the power delivery is perfect, but in order to get the woods characterstics I want, I've had to drop a tooth on the front sprocket. It's a bit of a trade-off, because a geared-down 125 has a tendency to spin the rear wheel. The power comes on suddenly, hard, and not always in a way that creates more speed on the trail. Plus, lower gearing reduces the effective time you're in the "meat" of the powerband, necessitating more careful gear selection.

What would you recommend in my quest for the holy grail? Combination of pipe and reed change, porting job, or just leave the small sprocket and shift more often? (Please don't say "buy a KTM 200"; tinkering with bikes is part of the challenge!)

Elf...did you ever experiment with the pipe length on your bike? I think that'd be a really interesting report!

 

[Chouca]

Detonator, there are lots of posts dealing with fine-tuning of the KTM 125 (and 200). My experience from my 200´s, (2000 and 2002), shows dramatic results in a replacing the NOZ_ needle to a CEK AND verifying the Power Valve timing/adjustment. The 200 was good before and now it is much better :) .

Why not combine that with your idea of adding an inch or two on the pipe lenghth?

 

[Detonator]

Thanks...I hadn't thought about the needle. I am a total loser at jetting, so I'll have to do a search to figure out what NOZ and CEK are...but I've had other people more knowledgeably than I get my jetting set up, so I think my bike is in the zone for running properly with the stock setup.

The powervalve is an interesting idea, but from what I understand once it's timed properly, the most important thing is to make sure it's opening and closing completely and with as tight a seal as possible. I had a mid-80's CR with one of those ATAC bowls on the underside of the pipe/engine junction, and the butterfly valve needed to be de-carbonized fairly frequently to prevent partial opening. It's surprising how many guys have exhaust leaks between the pipe and the engine, or at the stinger/silencer junction. When they fire up a cold engine you can see exhaust (not just condensation) leaking from the cracks. Got to raise the Decible level, too.

As curious as I am to experiment with pipe length at the motor, I also don't know enough about it to risk creating a very hot environment for the engine, like increasing back-pressure too much. That's why I was asking Elf if he'd had any feedback from the various pipe lengths that came with his.

 

[elf]

To all,the difference in length between the spacers was about 3/4 inch. I did not measure just guessing and I no longer have the bike.I tried all three in the bike. The shortest one gave the bike a mid and up powerband, the mid length one gave it a beefy midrange and the long one gave it a low to mid power that flattened out early. With each spacer the motor made the same ammount of power but just moved it to a different rev range. IMO 1 inch of headpipe makes a bigger difference than changing silencer length. I would listen to what Bruce says about the stinger end of the pipe. If the stinger end is the same diameter on the two pipes it should be safe to try it. The fat part of the pipe can vary . My ESR pipe had a huge midsection much bigger than the stock pipe. Also I have noticed some of fmf rev pipes seem to have a smaller midsection. My uneducated guess is that the 125 pipe would lose some lowto mid power over the stocker but thats just a guess.