Exhaust pipe tuning

[hangtime]

Howdy again,
I have attatched photos of a stock KX500 pipe and a PC CR500 pipe. The length of the first section of the two pipes is quite different. I will weld the KX flange to the CR pipe. Could anyone tell me how changing the length of this first section of the pipe will affect performance? Will a longer pipe = better bottom end or better top end??
Thanks

[YUNGGUNNAZ]

my little knowledge of pipes tells me the longer the head pipe the power curve is shifted towards more bottom end power and just the opposite for mid to top end power.pipe science is tricky and i believe is still not a known.remember the cr starts with a 89mm bore and 79mm stroke compared to k5 86x86.maybe mun will chime in on this.

[KXcam22]

Hangtime,
  I'm assuming  you got the cr500 pipe for free, but it might be less work to start with an aftermarket KX500 pipe, specially if you can find one that is built out of cones and not a few stamped shapes like the gnarly. With the different stroke, the CR500 pipe will automatically have the wrong tuned length, but if the rest of the pipe is close, it can be changed by adding a section in the middle.  For the headpipe, the critical dimensions are the taper of the headpipe and the total length from the inlet to the midpoint where it meets the fattest part of the pipe.  As long as the tapers are close the pipe will work well.  I built pipes for my roadracer out of parts of different pipes that were close. I left the headpipe and end cone tapers alone (they were close but not exact), just changed their angles, and tuned the length. It worked quite well.  Somewhere on the site, Demographic ( I think) posted a .pdf of the "David Jenning, 2-stroke tuners handbook".  It is a bit dated but the formulas for pipe work are very good.  Should give you enough info for modifying the pipe.  It might be useful to measure the entire stock pipe (or a gnarly) and map it out to scale on paper, ie measure the entire pipe OD at  2" intervals using the centerline of the pipe for the length. Then at least you will have some dimentions to refer to. It's doubtful but if you end up with a pipe that is going to be too long, a cool trick is to cut the stinger where it attaches to the pipe and then slide the stinger inside the pipe and weld it up. Makes no difference to the performance, and makes it quieter to boot.  I did that on my roadracer with excellent results.  If you can't find the .pdf then PM me and I can email it to you (it's on my work computer).  Also consider looking at the zip ty pipe as it has a unique shape to clear the tank that could prove helpful.  Here is a pic of the zip ty.  For pipe work a power hacksaw or bandsaw is very handy. Makes it easy to slice a small 1/2 pie section out (to weld on the other side) to change pipe directions. Have fun. Cam.

[hangtime]

It is hard to compare the two because of the multiple turns in the KX pipe, (I don't see why engineering that pipe was easier than a lowboy pipe). I will try to match the distance from the headpipe inlet to the fattest part of the expansion chamber. Does a PC or FMF pipe differ from the factory KX pipe in this area (headpipe inlet to fattest expansion chamber) or is this a solved equation?? I think I'll pull a string inside the pipe tomorrow and start mapping the pipe to see just how different it is. I like the zipty pipe but I think it is a little pricey to hack up (and I am a Hack!!) Now for the stinger question, I assumed the stinger was only a pipe to connect the end of the expansion chamber to the silencer. How much affect will this have?? The factory KX pipe is MUCH longer than the CR (see photo)

[kx5tsi]

my experience with tuned pipes is limited to rc cars, but at least with the small 2 cycles, the longer the head pipe the more top end you get. But the actual pipe design is more important. The performance of a pipe is more depending on how quickly it goes from the head pipe diameter to the largest diameter of the chamber. The quicker it opens up to the chamber, the more low end, and vice versa. Lets say it opens up at a 45 degree angle, you are going to have a much more low end punch than one that opens up at a 30 degree angle, but the 30 will have more top end. Not saying its fact, but its just my two cents.

kyle 

[KXcam22]

Hangtime,
  I just put a gnarly on my bike so I recall the differences (somewhat). The gnarly gets fatter sooner and is a lot fatter than the stock pipe (that's not very scientific is it).  The overall length is about the same and going from memory the stinger on the gnarly might be longer.  I'm going riding this morning so I will take a few comparison pics of the gnarly (unfortunately it's on the bike) and post in my gallery.  The lengths and angles have a lot to do with things like overrev and where the focus of power is.  I don't recall exactly which does what. I'll have to dig out my Jennings book and refresh my memory.  Also keep in mind that the occaisional custom "dent" added to clear frame rails etc doesn't rob too much power and can sometimes simplify things lots. Cool project. Cam.

[Polar-Bus]

my experience with tuned pipes is limited to rc cars, but at least with the small 2 cycles, the longer the head pipe the more top end you get. But the actual pipe design is more important. The performance of a pipe is more depending on how quickly it goes from the head pipe diameter to the largest diameter of the chamber. The quicker it opens up to the chamber, the more low end, and vice versa. Lets say it opens up at a 45 degree angle, you are going to have a much more low end punch than one that opens up at a 30 degree angle, but the 30 will have more top end. Not saying its fact, but its just my two cents.

kyle 

The behavior of a tuned exhaust is no different in a nitro engine, KX500, snowmobile, etc. Generally speaking the more "gradual" the taper in the divigerant cone, the more top end, and higher RPM's. The more abrupt, and larger in diameter the cone, the more bottom end.

[bellracing2]

Thats why all the old school guys always said "fat is where it's at " when talking about pipes .

[doordie]

And then we have to calculate:

1.Exhaust portimes.
2.Extra pipe volume/length,when KIPS is closed.
3.Temp(thinner or thicker material)
4.Application(MX/Motard/Enduro/Desert)

A pipe isn?t just a pipe,Lol

[hangtime]

Thanks for all of the input... The cone on the CR pipe definitely gets bigger quicker, so I'm assuming the low end (which I prefer) would benefit. If when this is up and running, and the pipe is just not usable. I'll have to find someone to make a custom cone pipe.

[Polar-Bus]

I remember reading an article in MXA about the effects of adding .250" thick spacer rings inside a CR250 cylinder headpipe to alter the torque output. On the dyno there actually were very different hp and torque #'s. Adding spacers shifted the torque curve higher in the rpm range. So for a bike that feels flat on over-rev, those spacers were the hot ticket.

[BigGreenMachine]

How about reshaping a pipe to fit a different chassis? I need to turn the pipe around to exit on the opposite side as the KX500.
So my question is;

How does the flow/bends of the pipe affect performance?

[demographic]

By the looks of it I reckon Mun knows his stuff about spannies

You can get computer programmes to work out the cones and so on and it sure as hell isn't as simple as it looks.
Mota is one of the programmes I have heard of and you can buy it here..http://www.iwt.com.au/MOTA.HTM

Theres a free programme on that page but I don't have the foggiest notion how good it is, it's here..
http://www.users.bigpond.net.au/arrowkarts/private/Pipe_dsn.zip
After you realise thats it's too much like hard work these lot might be worth contacting ...
http://www.dmax.ca/

[John]

Alternativly, get Mun on the case...

Mun has hand crafted pipes and published dyno charts in this forum.

He's the Man.

//John

[demographic]

Judging by the dyno sheets Muns tuning does seem to be rather effective also