Home made YZ250 expansion chamber

[762SPR]

During my YZ250 rebuild project, I looked at the smashed up factory exhaust and got the crazy idea that I wanted to tackle a custom exhaust. How hard could it be, right?

 

After taking measurements, and crunching numbers, and comparing to the stock pipe, I came up with a model that I thought would work well. Aiming for a little more, little lower, and little broader power, but as long as it didn't suck I would be happy. I mostly wanted it to look cool!

 

This is the adjusted model I came up with in solidworks.

 

 

From there, I segmented it into 35mm sections. 32 of them in all not including the stinger and the header stub. From the solid model I measured the diameters at each 35mm section and built a spreadsheet to keep track of each cone section. Now the fun begins!

 

Piece by piece I would make an educated guess for the mieter angle, printed out each section, cut it out, taped it together, and tested it's fit, then repeat the process until it was right... Then start the next piece. I don't know how many trees had to die for this process, but their sacrifice was for a good cause.

 

 

 

\

 

 

 

 

The next step was optional, but cool and a good way to confirm the fitment. I recently got access to a 3D printer, so why not test it out?

 

 

 

 

 

[762SPR]

With a little super glue magic...

 

 

 

 

Now I was 99% sure it would fit. Back to your regularly scheduled fabrication!

 

I took each cone section and modeled it as a 20ga. sheet metal part in solidworks, and then flattened them out, and had the pieces laser cut out of stainless steel.

 

Looks like a fish... sorta.

 

 

Now on to the next hurdle. How the hell am I going to get these flat pieces into round pieces??? I could just hand shape each one, but the farther off round each one is, the harder fitment will be to the other pieces. So I made myself a little slip roller!

 

 

 

 

 

 

 

In the end, I still had to do quite a bit of hand tweaking for each section, but the roller helped a lot!

 

 

Tick tack weld!

 

 

 

After that, it was time to machine the header stub, also from 304L, of course! I made and broke two cutter tools for the oring grooves. Not fun! Finally I ground a custom cutter out of a solid piece of tool steel and that worked like a charm. Should have just started with that, but I'm lazy I guess...

 

 

[Turboloader]

During my YZ250 rebuild project, I looked at the smashed up factory exhaust and got the crazy idea that I wanted to tackle a custom exhaust. How hard could it be, right?

 

After taking measurements, and crunching numbers, and comparing to the stock pipe, I came up with a model that I thought would work well. Aiming for a little more, little lower, and little broader power, but as long as it didn't suck I would be happy. I mostly wanted it to look cool!

 

This is the adjusted model I came up with in solidworks.

 

 

From there, I segmented it into 35mm sections. 32 of them in all not including the stinger and the header stub. From the solid model I measured the diameters at each 35mm section and built a spreadsheet to keep track of each cone section. Now the fun begins!

 

Piece by piece I would make an educated guess for the mieter angle, printed out each section, cut it out, taped it together, and tested it's fit, then repeat the process until it was right... Then start the next piece. I don't know how many trees had to die for this process, but their sacrifice was for a good cause.

 

 

 

\

 

 

 

 

The next step was optional, but cool and a good way to confirm the fitment. I recently got access to a 3D printer, so why not test it out?

 

 

 

 

 

Impressive!

[762SPR]

Now it's finally starting to take shape!

 

You can see by my crossed out marking that some of the sections were very similar to each other. Make sure to double check your measurements kids!

 

 

... Except for some pieces that didn't get cut right... They must have shifted in the machine or something. Not exactly what I want to see after spending $170 to have these things cut out, but what can you do but work with it?

 

 

 

 

 

The magic of fast forward...

 

 

 

Presto! Pipe-oh!

 

 

 

 

Well, almost. It fits, not it's time for the final hurdle. Welding everything together! For anyone that cares, I'm no pro at welding to say the least. I've never taken a class, and aside from watching youtube videos, and asking pointers, I'm 100% self taught on various projects. Needless to say I was pretty intimidated by butt welding 20 ga. stainless, when the welds need to be cosmetic. I really, really, REALLY wanted it to look good, not have some nasty bird poop explosion welds!

 

For anyone that cares. Back purge with argon is a must, 1/8th tungsten, 1/8th 308L filler, around 35 amps (I was using pedal control) Gas lens with maybe 15-20 cfm, and a 5 cup. The machine was an old transformer miller synchrowave 300 that's probably twice as old as me, but it got the job done just fine!

 

Pause, dab, move, pause, dab, move...

 

I'll spare you the 7 straight hours of welding and skip to the results!

 

 

 

 

 

It did warp a tiny bit because I did most of the welds on the inside first for practice, then did the outside where you could see them.    Not the best idea, but it worked, and I just had to stick a pipe in the header and give it a few grunts to get it spot on!

 

 

 

The final step was to add all of the little tabs and weldments that would keep the dang thing on the bike.

 

 

 

 

Note that I ended up welding the gaps on the stinger... what is that ring spacer silencer sealy thing called anyway? Anyway I ended up welding them because it was leaking smoke when I test rode it.

 

 

 

 

[762SPR]

A couple shots of the welds. They aren't perfect, and they aren't even all this good, but I'm pretty tickled with how it turned out!

 

 

 

 

 

 

Evolution, or creation?

 

 

 

 

Turns out it's a hair lighter than the stock one, too. I'll be faster for sure!

 

 

 

 

 

 

Ride report. I only have maybe a half an hour seat time on it, but I swear it pulls harder than the stock one. Maybe I'm just dreaming, and like I said I honestly don't care as long as I can't notice that it's worse. I still need to tweak the jetting too. It seemed almost perfect when I rode it, but it was also only 50f so I'mm probably drop the main and pilot a size or maybe two when it warms up more... Still not an expert on two stroke jetting... I wish I could dyno it, but for now I'll just have to say by the seat of my pants that it was a success! Plus it looks amazing if I do say so myself, and who cares how fast you are as long as you look good... Right??

 

 

 

 

 

 

[2010crf]

Love the look of the welds! Great work

[rpt50]

Good God I hope you don't crash!  That is a work of art!

[GasMixer44]

Thats awesome. I know from experience that tig welding stainless is a bit tricky. I would be interested to see how your pipe stacks up against an aftermarket one (PC or FMF). When my KX needs a new pipe i might have to hit you up

[U1arunit]

Great job. Way more patience and time than I would ever have. Glad you're happy with it.

 

I agree.. I would cry if it got smashed into a huge rock!  I would be like--->      Haha!

[218]

Yeah, I'd have to hang that one on the wall. Too Cool. ?

[turbo dan]

Too cool. 

[cowboyona426]

Very impressive!  Now just keep yourself from smashing it ?

[DrScandinavia]

Holy shit! Dunno what you do for a living, but you have endurance and talent!

 

Funny you don't change the front tire which is toasted... Out of laziness?  But you go ALL IN on the exhaust? LOVE IT!!!

[motoxvet]

 OP, Where did you get the data for measurements on your project pipe?

[desrider891]

That is badass!!! Nice work. I love the way it wraps around on the left side, almost like a 500 pipe.

[762SPR]

 OP, Where did you get the data for measurements on your project pipe?

 

Its basically a mashup of measuring the existing one, using programs to calculate it, math from the two stroke tuner's handbook, and of course a healthy dose of pulling numbers out of my ass/going with my gut for what "looks about right". I ran quite a few calculations for different tuned results and compared the results. For example, a smaller, more gradual expansion chamber yields a smoother power curve. That made sense to me, especially comparing it to a trials bike which has pretty much a straight header and rides like a four stroke as far as power curve goes. Compare that to the stock pipe diameter X at length Y so I didn't end up with something that was WAY off, then blend it all together for a nice average. Smooth out the transitions some and that's about it.

 

Holy shit! Dunno what you do for a living, but you have endurance and talent!

 

Funny you don't change the front tire which is toasted... Out of laziness?  But you go ALL IN on the exhaust? LOVE IT!!!

 

 

Yeah, the original plan was to rebuild the motor, and suspension on that bike but keep the rest of it rough so I wouldn't have to worry about thrashing on it... Now I got this pipe... oops! ?  I'll change the tire eventually, but I pretty much burned up my budget for the rebuild because the motor had a lot of unforeseen problems that added up fast. There's a couple miles left on it anyway

[woods-rider]

That thing looks awesome! Now you just gotta make a clear pipe guard for it to protect your beauty and show it off at the same time.

[DrScandinavia]

Its basically a mashup of measuring the existing one, using programs to calculate it, math from the two stroke tuner's handbook, and of course a healthy dose of pulling numbers out of my ass/going with my gut for what "looks about right". I ran quite a few calculations for different tuned results and compared the results. For example, a smaller, more gradual expansion chamber yields a smoother power curve. That made sense to me, especially comparing it to a trials bike which has pretty much a straight header and rides like a four stroke as far as power curve goes. Compare that to the stock pipe diameter X at length Y so I didn't end up with something that was WAY off, then blend it all together for a nice average. Smooth out the transitions some and that's about it.

 

 

 

Yeah, the original plan was to rebuild the motor, and suspension on that bike but keep the rest of it rough so I wouldn't have to worry about thrashing on it... Now I got this pipe... oops! ?  I'll change the tire eventually, but I pretty much burned up my budget for the rebuild because the motor had a lot of unforeseen problems that added up fast. There's a couple miles left on it anyway

Ha! I hear ya. I wish I lived close to you, I would give you a tire for free. Your effort, commitment stands out for me.

[M A T R]

Dudes got skills

[d_man]

Holy crap man. NICE WORK! I have to admit, when I read the title I figured it was someone asking if it could be done, or some haphazard attempt. You on the other hand went balls to the wall and created a very well documented masterpiece. 

 

 

You might have a future in a side business building custom expansion chambers. The welds really aren't that bad either. If you did a few more of them you'll be surprised how fast you get good at it. 

 

Once again, impressive!