Help designing custom expansion chamber for the cafe- YZ250/WR motor-Ninja 250R frame content.

The goal is to put a YZ250WR engine in the Ninja 250R frame.  The expansion chamber will by far be the most work. I've been doing lots of reading & watching YouTube videos about them.  I need to know the peak power rpm target which should be @ 8,500-9,000 & the exhaust degrees. I want to send the cylinder to Eric Gorr for porting & boring to 295cc's. So that will definitely affect it also. I'm sure he could tell me the degrees also.  If anyone has a grogeam or has done this before I could just use the size/design. I'll build it but I will just need the measurements. Can't wait for a 9,000rpm cafe/scrambler.

I can't help with specific numbers, but on a 2 stroke like that if you've already got the engine, the fastest way to get a number with any confidence is probably to pull the stator cover and the cylinder head and measure the crank rotation from when the exhaust ports are fully covered to when they are fully open. 

Have you looked at go karts?   I bet some shifter carts use that motor and might be a good starting place 

Highly recommend picking up a copy of Two Stroke Performance Tuning by A Graham Bell. Been a long time since I've leafed through it, but it's super comprehensive not just on expansion chamber design, but the interplay of all the myriad factors that go into how and where a 2 stroke makes power. Should have all the equations you need along with some guidelines as to what you want to shoot for in various applications. 

Super rad project, by the way! Can't wait to see how it turns out!

Buy this book. 

It taught me more about two strokes and tuned pipes than anything else when I was racing RC Boats and is well worth the read. 

Both the A. Graham Bell and Gordon Jennings books are pretty outdated as far as expansion chamber design goes, unfortunately.  I think the Jennings book pre-dates powervalves, so not exactly cutting edge tech in that one, and neither one has any information on tapered headpipe expansion chambers IIRC.

You're going to need a whole lot more information than expected peak power rpm and exhaust port duration to make a pipe that works well.   To do it right, you're going to need to know almost everything about the engine.  On something as ubiquitous as a YZ250, you might get lucky and find someone else's port map and combustion chamber volume, but a lot of it will be left up to you.   It sucks, but you're fooling yourself if you think you skimp on the numbers.  Trust me, I've tried because math is hard.

Michael Forrest's site has some good primers on what you're going to need.  His excel program, ECCalc29 produces pretty good results in my experience, but its an absolute bear to use (dude is also a nutjob conspiracy theorist, so watch out for that).  I think its $30ish bucks, and using it will make getting a root canal seem like a pretty ok time.

One of my fellow pipe building friends moved from ECCalc to JanBros FOSpipe or something like that.  I'll dig up the bookmark.  Its free, and he seemed pleased with the pipe he just built for his KDX.  This one

As far as layout goes, I highly, highly recommend the Pulserate Cone Layout software.  Intuitive bit of software, great results, and the free trial technically never expires.

This is going to sound silly but..............wouldn't it be easier to just measure the YZ pipe dimensions and create a downpipe based.

We did this with a YZ 125 engined dirt track bike and the motor worked every bit as well as it did with the up pipe.

We also did a similar thing with chamber on my my Formula 500  (it needed a different routing).