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Since there are very few options for the exhaust for my 86' CR5, I thought I'd look into building a pipe for it. I plan on using 18 or 16 ga. sheet steel (undecided if I want to use stainless or not), with A. Graham Bells "Two-Stroke Performance Tuning" book as my guide.

I'm currently running a bridgeless cylinder with stock porting, but I plan on bumping it up to 189 exh and 125 transf. My goal is to make max power around 6,500-7,000 RPM.

According to the book, the overall "tuned" length of the pipe = Exhaust duration multiplied by 42545 divided by the goal RPM. So, it would be 189x42545/7,000 which equals 1148.715mm (I'll round it up to 1149mm). That length is measured from the piston to the reflection point of the reverse cone (roughly the middle). 

I plan on using a divergent "single stage" header. To find the length of that, I'll need to multiply the exhaust port diameter by a factor between 8.5 and 9.5. I don't have the diameter measurement, so let's assume that it is 48mm. 48x9-70=362mm. So the overall length to the reflection point is 1149mm, 362mm of which is the header/diffuser, less the distance between the piston and exhaust flange. (still need to find that measurement).

To find the physical size of the header, I'll need to subtract the length of the exhaust port and flange (let's assume it's 90mm). 362-90=272mm. The taper of the header will be 1.25 degrees, which has a co-tangent of 45.83. So I will multiply 272x2/45.83+48 which equals 59.86mm (I'll call it 59.9mm)

59.9mm is the major inside diameter, 48mm is the minor inside diameter, 272mm is the overall length of the header, less the exhaust port/flange length.

Header Measurements: 
59.9mm M.I.D
48mm m.I.D
272mm O.A.L (362mm including port)
1.25 Degree Taper

Because this is a bigger motor (>350cc), the diffuser outlet should be roughly 2-2.2 times the size of the exhaust port. The diffuser angle will be 9 Degrees divergent ( 4.5 degree taper). The length will be based on the taper, and 2.1 times the port diameter (so 48x2.1=100.8mm.) So the equation will be (100.8-59.9)/2x12.71, which equals 259.9195mm. I'll round it to 260mm.
Diffuser Measurements:
100.8mm M.I.D
59.9mm m.I.D
260mm O.A.L
4.5 Degree taper

The baffle taper will be 10 degrees (20 degrees convergent). To find the length, I will need to divide the baffles major inside diameter (100.8mm) by 2, multiplied by the cotangent of the baffles taper (5.6713). So the equation will be 100.8/2x5.6713 which equals 285.83mm. We can half the overall length to find the reflection point (R.P). So 285.83/2 equals 142.9mm

Baffle measurements:
100.8mm M.I.D
28mm m.I.D (stinger ID)
285.83mm O.A.L
142.9 R.P 
10 Degree Taper  

To find the length of the belly section, I will need to take the total tuned length of the pipe (1149mm), and subtract the length of the header, including port, plus the length of the diffuser, plus the overall length of the baffle divided by 2. So the equation will be 1149-(362+260+142.9)=384.1

Belly length: 384.1mm

I plan on using a stinger that has a 28mm I.D and is 165mm long (right in the middle). The stinger reduces the O.A.L of the baffle cone. To find the length of the baffle, I will need to take the Diffuser M.I.D (100.8mm) and subtract the stinger I.D (28mm), divide by 2, and multiply the cotangent of the baffles taper (5.6713). So (100.8-28)/2x5.6713, which equals 206.435mm. (I'll round it to 206.4mm)

All measurements so far:
Header: 
59.9mm M.I.D
48mm m.I.D
272mm O.A.L (362mm including port)
1.25 Degree Taper

Diffuser:
100.8mm M.I.D
59.9mm m.I.D
260mm O.A.L
4.5 Degree taper

Baffle:
100.8mm M.I.D
28mm m.I.D (stinger ID)
285.83mm O.A.L
142.9 R.P 
10 Degree Taper  
206.4mm "True Length)

Stinger:
28mm I.D
165mm O.A.L

Belly:
384.1mm O.A.L
100.8mm I.D


Not sure if my math is right or not, but that's the general plan. And yes, I did copy/paste this from my post on the CR500 forum  :devil: 

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What the heck did I just read?? Lol. Jk. As ROLLTIDE2014 said. You do seem to have it figured out. I looked into this when I wanted to take the engine out of my 93 CR500 and put in a street bike. When i found the bike, it had been leaning against a barn for 10 years in 100 degree 95% humidity summers and 10 below winters. If you refer to the guide, I believe you'll be alright. It might take a couple tries to get it right. I'm not sure what all can be used when hydroforming exhaust, but I've only seen it done with stainless steel. Looks pretty good stainless. Some guys have make their own exhaust when they build custom motorcycles like naked bikes. I imagine getting the bends just right is going to be difficult. There's some YouTube videos of the process if you haven't seen it already. Not sure if any of that helped. Not really knowledgable in this. Have a good understanding though. Excited to see how you do. Interested in what challenges you'll encounter. Post pics!! Good luck!!

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I've made a couple a LONG time ago but I remember I really didn't bend anything as much as welding the cones together at slight angles.  I figured if there was a detriment to performance from that, then the tight bends that the OEM's do are in the same category.  I always did try to find a head pipe because IF you can find the correct dimensions, it saves a LOT of time and money to buy those automotive pipes and angles and tack them together just right.  BTW - I used the same book.  I still have it.

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i would defiantly make at least a 2 stage pipe probably 3 or 4 actually

Take the diameter every inch down the length of a cone and give it the ole pi R sq to find the area.  Small end to big end when you compare the % area gain you will see that the gain from the first inch to the second inch is much greater than the gain from the second to last to the last inch,

 

to create a cone where the gain in area is equal for every inch you end up with something with a little bit of a trumpet profile A LITTLE BIT NOT ALOTA BIT

its sort of a section of the shape called a tractrix

 

so if you look at a two or 3 stage pipe this is what youll see

a long shallow taper a shorter steeper taper and a real short real steep taper.

 

i hydroform 4 stroke pipes with 27 angles and 2 bends,

IMAG1381_zpscf6cf79c.jpg

IMAG1379_zpsb265779f.jpg

the hydroforming is really usless on anything tighter than a 10" bend radius and wrinkles to HELL

 

so if i was doing a two stroke id probably depending on the desired powerband

do a linear area gain diffuser starting the header with rolled cones using this cone layout software (ITS FREE)

https://www.conelayout.com/

then a hydroformed section also linear area gain.

 

also check out the free pipe design software here

http://www.iwt.com.au/mota.htm

5 or 6 years ago i made a few two stroke pipes with this,

they were great, but stay away from the road race setting >.>

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