Tinkering with stock exhaust



Vikingimike01

Active Member
Joined
Jul 1, 2018
Messages
879
Hey guys! Sorry for bringing back such an old post, but I have further questions, about the banana pipe. I actually get more top end from it.

Now I read somewhere a few days ago, that the ones with the stinger comming out at an angle, are actually GOOD, and off of a pocket bike, but not for these engines. I've read, that the power band increase is somewhere around 14k RPM, wich my 66cc 2 stroke will probably never reach, if it would, well I don't want it to. I'm not sure how to calculate this, I'm probably saying something VERY stupid, but here's my try:

The end of the stinger (visible) that comes out, is 2,5cm. I'll assume it goes in about 0,5cm too. That would make it 3cm long. Now if the increase is at about 14k RPM, and the stinger is ~3cm long, that would mean, that 1cm of stinger would be good for 42k RPM (???) I belive these engines shouldn't really go over 9k RPM. I want a mid-high RPM torque boost. If I did everything correctly (according to my logic) I should have a 6cm long stinger, for a 7k RPM boost. If its squared, then I'd probably need just a centimeter and half to add on. I was also thinking about drilling it out a millimeter, so the inside of the stinger is a bit wider.
 

Frogslayer

Well-Known Member
Joined
Jan 20, 2019
Messages
638
I'd like to see a picture of a pocket bike turning 14k. If that's where the power band was how would a person ride the damn thing lol.
 

Street Ryderz

Well-Known Member
Joined
Oct 14, 2016
Messages
3,119
Hey guys! Sorry for bringing back such an old post, but I have further questions, about the banana pipe. I actually get more top end from it.

Now I read somewhere a few days ago, that the ones with the stinger comming out at an angle, are actually GOOD, and off of a pocket bike, but not for these engines. I've read, that the power band increase is somewhere around 14k RPM, wich my 66cc 2 stroke will probably never reach, if it would, well I don't want it to. I'm not sure how to calculate this, I'm probably saying something VERY stupid, but here's my try:

The end of the stinger (visible) that comes out, is 2,5cm. I'll assume it goes in about 0,5cm too. That would make it 3cm long. Now if the increase is at about 14k RPM, and the stinger is ~3cm long, that would mean, that 1cm of stinger would be good for 42k RPM (???) I belive these engines shouldn't really go over 9k RPM. I want a mid-high RPM torque boost. If I did everything correctly (according to my logic) I should have a 6cm long stinger, for a 7k RPM boost. If its squared, then I'd probably need just a centimeter and half to add on. I was also thinking about drilling it out a millimeter, so the inside of the stinger is a bit wider.
Ok so the banana pipe is not from a pocket bike but rather a revised version of the one made and marketed for bicycle engines as a tunned pipe/expansion chamber.Not even pocket bikes stage their pipe at 14k more like 9-10k,and it's the pipe's overall length from piston face to 3/4 of the baffle cone's length that tunes where it stages.The stinger or the out let pipe on our engines should be around 50% of the headers dia and 12 x the dia in length,for race engines it's .58 - .62% of header dia and the 12 x dia for length.The biggest issue for the banana pipe is it's to small and to short,it's over all length would have you assume that it stages high in the rpm range but the reality is that due to it's chamber size with no header length to speak of the engine see's it as being longer but with a very narrow band width.In my experiance with these pipes they usually stage at around 8-9k but provide little actual power boost but instead when jetted properly just give a nice clean burn and are able to rev past the peak of pipe without going too lean.
 

Vikingimike01

Active Member
Joined
Jul 1, 2018
Messages
879
Hey, thanks for showing me. Is this book copyrighted? I mean, can I print it, so I can read it when I go to places? I would love to read it, as I am really into these kinds of stuff.
 
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