Improving low end torque

[willis617]

Not really, if your using a grinder make sure the disc is fresh otherwise you will have to cut the inner pipe separately. The only thing that was a bit of a pain is the end cap, I had to torch mine to burn the paint off after drilling out the rivets to remove it. That and the inner pipe simply slides onto the end cap manifold (engine end) but is freaking welded to the tail cap so it takes a bit of work to remove enough. My advice would be to leave the welded bit on and remove the nonwelded bit. Sounds kinda obvious but it wasn't for me until I was in too far to turn back.

To attach the endcap you can use self tapping screws but I was able to simply press fit mine, took some hammering but it aint coming off.

If I understand correctly, the inner tube is welded to the end cap end, and is a slip fit at the stinger end? So shorten from the stinger end?
I was thinking of shortening mine by about a third, from 11.5in to 8.5in, would this help power without much increase in noise, or do you cut it in half?

 

[extremeodd]

You could cut to any length you want, the inner tube being welded isn't a real issue in the slightest. Personally I went with roughly in half, but you could go as short or as long as you'd like. The shorter it is, the less of an impact on power and sound reduction it'll have. Worst case start with a short cut as you can always cut it further. If you overshoot and don't like it, another pipe is less than $20.

 

[Greg58]

I did a good bit of testing a few years ago to find more low end torque, I built several pipes. The best I managed to make has a 16” head pipe, on my bike it proved to be the best but not pretty. I bent the head pipe 180* to improve flow, the body is a conveyor roller body with 1/2 Id stinger. Like I said not a looker but it works.

 

[gary55]

If your using the MZ pipe even though it's more of a mid to upper range pipe you will still notice considerable more power at lower RPM as well. As for the muffler I used it full length and cut the stinger to just long enough for full insertion into the mounting bracket plus 1" for silicone tubing to get a good bite. The reason for the full lenght is it acts as a stinger, and since the stinger on the MZ pipe has a considerable larger ID than it should the addeed length of the muffler helps provide the proper back pressure that a correctly sized stinger would.
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[FNTPuck]

Great points like always from Gary.

Cutting off the stinger and replacing it with a smaller diameter can help too. The KX85 pipe is tuned for a damn near 30hp 85cc 2stroke and its stinger ID is only 20mm with a big 31mm header ID for a 55% ratio. With the tiny little barely ported ~1" exhaust port of my mild motor I replaced the 19mm end of the stinger with a ~16mm pipe instead(I actually left a few inches of the stock 19mm stinger since I only had a ~10" length of 16mm ID pipe) and gained more power everywhere in the RPM range. It would probably like an even smaller diameter, and I'd like top test a 15mm, but I don't have anything smaller to test. Honestly the bike is just a test bike and is already lacking the supporting mods I should have to keep it safe for how fast it goes so I don't mess with it much anymore. Keep in mind the stinger length makes relatively little performance difference when compared to the much more important stinger diameter.

Smaller stinger increases exhaust gas temp, which increases velocity. Following the VERY rough guidelines of 50-70% header ID diameter for stinger I would recommend a 14-15mm for a stockish motor as a starting point but every pipe setup is different.

 

[gary55]

for the stinger 58 to 62% of the head pipe is the suggested "close" correct size of the id with 12 x it's own ID in length. Still like Jennings says that is a "close" estimate, and with the wide variety of pipes out there I would be much more inclined to listen to FNT's outcome of hands on experiments considering a specific pipe for a true optimum size.

 

[JerboaJohn]

I love your guy's tuned pipes lol. if it works, it works,, we should have an ugly pipe contest hahaha

 

[Hello Moto!]

Well I got the pipe all finished and added the HC head. The head didn't improve torque but I noticed an improvement in the engine running smoother it seems like. It really smoothed out the throttle response..not really sure how to word it lol. It seems to accelerate smoother and a little quicker.

Thanks everyone for all the help and input. It was a fun project and I'm happy with the resulting improvements!