custom pipe nightmare need help!!

[hawaiirider]

hi i own the banshee dirtbike on this page. i finally had the pipes fitted to the bike- not sure if you saw it before but they hung out the sides. now it fits and looks perfect but the bike will NOT hit the powerband anymore. lowend up to the point where it hits is ok then it just backfires and spits. we have done all jetting options that routine seems to be out ive gone down twenty increments with no change on top the pipes must just be too short? heres some pics of the pipes we made (middle) next to a dynoport left and rm125 pipe right for comparison. looks like my header is too short? im not sure what to do but im scared to even ride the thing as is. any pipe builders?

[tw05tr0k35]

If the Pipe on the left is the correct one for the motor package your running then the one in the middle will (most likely) be totally wrong. The length and diameter is different, the cone angles and lengths are different, and the stinger size looks diff. also. The way the pipe was sectioned is not good, the curve is not smooth and the abrupt change in diameter can 'clip' your pulse wave, turning it into a complex wave form that does not stuff the cylinder the way its supposed to (even if the pipe is the right shape), and can in fact draw most of the charge right into the pipe in a worst case scenario (best case scenario the complex wave dilutes the incoming charge with spent gasses). This is especially true near the exhaust port as in the first 9 or so inches of a pipe it is very important to have consistent bends and correct divergence angles. Dents and dings in your pipe after this point usually don't have an adverse effect on power, as long as they're not major, and sometimes enhance it slightly. The smaller stinger diameter will, with no other changes, likely increase top end power output at the cost of high engine tempertaures and lower end torque output. Shortening the pipe increases tuned RPM and reducing the diameter generally decreases wave strength. The combination of the two is at best disastrous. High RPM and weak pulse waves mean lots of fresh air/fuel in the pipe. Sorry if this isn't the answer you were looking for,

[hawaiirider]

sounds super complex!! the pipe on the left isnt for the bike just a donor i was thinking of using the headpipe off of. do i have to just go back and use the pipes forr the banshee or will the yz125 pipes work? i originally chopped 6 inches off the header to get it to fit and it ran like a bat outa hell still.

[tw05tr0k35]

Pipe design is a factor of the engine's exhaust port area and operating range and specific output (hp/cc) measured in BMEP (Brake Mean Effective Pressure). Piecing together pipes without doing the required math is dicey at best. I found this out the hard way with snowmobiles when I was younger (sometimes it worked, but more often than not the results were less than satifactory and one blew up as soon as it hit the power). Sounds like you got lucky with the other pipe. Maybe, that is, there could be things going on in the motor that your not aware of that are damaging things slowly. Improperly designed pipes can cause high engine operating temperatures hotspots on the crown and face of the piston on the exhaust side. If that pipe works then the best thing to do is section it CAREFULLY until you can rotate the sections to fit the way you want it to. When cutting you have to figure out where the angles need to change, by how much and cut the pipe at half that angle so that the section can be rotated on the cut 180 deg. and welded back together. If you do not rotate the sections 180 deg. every time they will not match up perfectly and your divergence/convergence angles will be changed or there will be abrupt changes in the diameter of the pipe that contribute to the 'clipping' effect and therefore the pipe will be changed, never turn more than 12 deg in one cut. Pipe designs are VERY complex, don't be fooled by the simple looking nature of it. When companies like FMF and Pro Circuit build pipes they use very complex math and may make 40 different variations of a single pipe and dyno test and track test each one before they ever offer it to the end user. Every inch of the pipe you buy off the shelf has been designed specifically by the manufacturer for a certain purpose. If you do decide to modify the pipe that works stay away from the first foot of the pipe if at all possible. This is the most important part of the pipe and making changes here will get you into trouble more often than not. I can not tell you what will work without knowing your effective exhaust port diameter and the state of tune of the engine and what you do with the bike. Newest genereation yz's have large exhaust ports and are generally tuned for midrange punch, you could get lucky and have them work with your motor or it could be another disaster. It is impossible to tell without careful measurement and inputting data into complex math equations. Prof. Gordon Blair's equations are available freely with complete explanations on how to use them (google gordon blair or prof. blair). If you need to custom build a pipe I recommend you do it this way, your approach is a crapshoot and can lead to engine damage. Coming up with the dimensions is relatively easy as long as you have the equations and an explanation of how to use them, translating it into steel is the hard part, there are several articles available on the internet about building pipes if you google it. Good luck man,

[hawaiirider]

thanks for all the advice!! are you on e2s as well? im suprised the original cut i did worked so well? i cut a good 4 inces right off the header maybe more. the bikes engine is beat its onlyy got 100psi warm and it never overheated even when pused past limits and still ran fast!! so that pipe design was a go. what if i make a pipe that fits the dimensions im looking for (fits the curves of the bike) and send it to dynoport as a mold. could they take it from there moving the expansion chamber and header lengts to proper design for the engine?

[Billy Golightly]

Yeah that pipe has WAY too much cut out of it to be working right. I can't really add much more to what tw05tr0k35's said (Nicely covered). The only way to really make the pipe work right is get one that BELONGS with that engine, and then section and twist it till it fits. Don't ever cut pieces out. You've got to use every single bit of what you started off with, and everything in the same order so that tapering diameters stay in the same places. If you don't it'll mess up the timing of the entire thing. Stay away from sharp bends and angles in your sectioning. Make everything smooth and gradual as it snakes around.


edit: Also @ that pipe mount on the back, what is that stuff 3/8thick? That is going to bust out of the side of the pipe the first 10 seconds there is any vibration. You need something thin and triangulated like the first pipe in your picture.

[tw05tr0k35]

Did you make it work?

[hawaiirider]

i found some 125 pipes that are near identical left and right forr older ktms by dynoport, picked both up for 40$. im in process of welding the up, its minimal work they almost bolt right on, i hope shee runs ok w em!!

[hawaiirider]

thanks for the help, the bikes done check bikersville banshee dirtbike.

[300rman]

hows it run now?