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03-01-2012, 12:19 PM #1
precision expansion chamber design
Have you ever noticed how each pipe design program gives you greatly varied designs? And they dont tell you upon which formulas or ideas they base their calculations on. So I developed a way to design pipes based on Blairs research paper and present it to you for your use and understanding. It is at http://dragonfly75.com/motorbike/ECtheory.html
Since I believe that the perfect header length can only be found by trial and error I suggest pipe builders first measure the mid-header internal gas temperature at peak rpms (while riding), plug that into my Excel file, and use the suggested wave speed as a baseline to design the pipe. Then put the pipe on and test it with different header lengths, longer and shorter than what my Excel file recommends.
I have done extensive testing which led me to believe that the diffuser needs to start so that the beginning of its return wave comes back right around BDC at top rpm. Also the more focus you put on return wave strength from the diffuser coming from the beginning of the diffuser, the better it is for peak rpm power, whereas multi-cone diffusers are better for a broader range of power. And for the baffle a dual cone baffle is preferred for enduro riding whereas a triple cone baffle is for MX.
I also made a pipe with an adjustable stinger size and found out that smaller favors low rpm power, and larger favors high rpm power. Since there are different formulas available for stinger size and length I recommend a pipe builder experiment with different sizes to best fit the engine and the application.
All my testing so far has been with parallel-walled headers, not with expanding headers as MX bikes frequently have. I believe those headers increase baffle return wave strength but I need to start making and experimenting with them to find out for sure.
Last edited by jaguar; 03-01-2012 at 12:23 PM. Reason: addition
m y s i g n a t u r e:
My last two bikes: '89 Honda CR250R (stock), '89 KDX200SR with PC pipe. See http://www.dragonfly75.com/motorbike/ECtheory.html for full info on expansion chambers and how to design your own.
03-01-2012, 02:30 PM #2
Very impressive and I want to read through it when I get a chance...not because I am a pipe builder but this stuff is fascinating. That ahd to take some time! Thanks!
m y s i g n a t u r e:
If you see me coming up to pass you then you better move out of the way cuz I'm going the wrong way!!! :-O
03-03-2012, 12:39 PM #3
Nice read Jaguar, do you have a project bike you are working on?
m y s i g n a t u r e:
(¯`·.¸¸.·´¯`·.¸¸.-[ DRN Support Team Info ] -.¸¸.·´¯`·.¸¸.·´¯)
2012 KTM 300 XC-W, 2008 Christini KTM 530 EXC-R, 2001 CR500AF, 2001 XR650L, 2000 Gas Gas EC/XC300
03-03-2012, 09:46 PM #4
My 55cc bicycle engine (1 speed) has been my ongoing project, first to get it to climb hills without losing top speed and then to experiment with porting and pipes till I really understand 2 strokes. My next pipe will be to try to make it lightly "pipey" although that trashes its usefulness on the street unless I want it for popping wheelies. But where I live now I am practically downtown and I walk everywhere and only use the bike to test things on. Like tonight I tried to test what powerband changes happen when transfer plates are installed that direct the intake charge into the cylinder horizontally. But I ran out of gas before it warmed up and I have to wait till tomorrow to get more gas. I test late at night when theres no traffic and no cops. People think that a flat transfer roof accomplishes a horizontal flow but I totally disagree. The intake flow, at the beginning opening of the port, is determined by the transfer roof and the piston. After that the velocity and inertia of the fuel/air make somewhat more use of the transfer roof. Its all about physics which makes it more complicated especially since the scenario changes as the port opens more and the air velocity increases. fascinating stuff.
Been a long time since I have been on this forum. Things have died down a lot. This place used to be smoking! Damn four strokes. that and bad world wide economy. I grew up when half the high school kids had motorcycles. Me and my dad bought a 125cc Penton (forerunner to KTM) for $400. Now hardly anyone has dirt bikes. crying shame. Utopia, whenever and wherever that is, will have lots of dirt bikes.
03-03-2012, 09:59 PM #5
here is a pic of the transfer guide in one of my cylinders that has had the transfer walls removed so that the engine could rev to 9500 rpm. If I had the right tools I could of dremeled out the transfer channel but I dont. I had maxed out the port opening and then realized the bottom entrance of the port was the limiting factor to the flow. The colored pic showing flow angles was found by someone else on the net and put on my post about this topic of transfer guides at MacDizzy.
transfer guide picture
transfer flow picture
Last edited by jaguar; 03-03-2012 at 10:00 PM. Reason: addition
04-30-2012, 06:09 PM #6Registered
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I applaud you for taking on a project like a custom engineered expansion chamber!
I thought about replacing the one on my bike because it had a medium sized dent and I thought that was hurting it alot when I read how an expansion chamber works...figured the gases were not bouncing around right in the chamber.
Ended up seeing an article somewhere about doing testing on how dents effect a chamber on a 125cc bike. They tried all different sizes of dents on different parts of the chamber and did not see any adverse effect on performance and in some cases even saw a slight increase in HP on the dyno!
Point is I don't think the pipe is that big a deal anymore after reading that article, sure it makes a little difference in how the power band hits but doesn't seem like it'd be worth it to make a custom pipe if something like a dent could make it run better lol.
Maybe don't make it a giant project to try and craft some custom pipe that won't make too big a difference. But hey, if you do it then right on!
My 2 cents
05-01-2012, 05:18 AM #7
Sure dents don't affect the power a lot but they also don't change the location or angle of the cones. The cones are where the tuning comes in and can make a huge differences in power distribution.
05-15-2012, 01:15 AM #8
Pipes almost totally dictate what kind of powerband the bike will have. Seeing as how most 2 strokes are used for woods or play riding now I think knowing how to make a pipe to give a broader less peaky powerband would be very useful.