True tunned pipe length

Street Ryderz

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This topic has been covered before I'm sure but there is some points of intrest or curiousity I guess as more and more folks are using small displacement dirt bike pipes or simular designs.I'm suprized that many of these pipes really dont hit hard or at all in many cases and yet the user's will swear that they are working and giving a proper boost effect.When going through Jennings and Blair's works and formula's I found it interesting that both are within 10" of each other for our engines displacement and @ 8300 for peak power,but that still is alot Blair say's that 46" from piston face to the end of baffle cone (convergent section)is the starting point for overall length,Jennings formula works out to 34" to start and then states less an inch,So if we split the difference we are looking at 38.5" Jennings says that measurement is from window to just past half the baffle cones length.Now my pipe at this length stages at around 7 grand and falls off around 9300 but it hits hard with both an audible change as well as power boost,How long is the MZ pipe from flange to end of baffle cone?Many of the kx,yz,rm,cr pipes look much shorter than that and makes me wonder if they even hit at all being designed to hit at higher rpm and work with exhaust durations above 170 degrees.Any input here is welcomed.
 


Street Ryderz

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Steve Best

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Gordon Jennings was not a scientist or engine builder, he was a journalist and editor. He didn't make up these formulas, he simply reported on them. The beauty was, he put together all the black magic alchemy that was making 2 strokes fast in the late 60s and early 70s. He cut through the BS and fed us something we could get our teeth into. And for that, God bless him. We lost him far too soon.

https://www.motorcyclistonline.com/gordon-jennings-and-bike-changed-my-life-perspectives
 

Street Ryderz

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Oct 14, 2016
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Gordon Jennings was not a scientist or engine builder, he was a journalist and editor. He didn't make up these formulas, he simply reported on them. The beauty was, he put together all the black magic alchemy that was making 2 strokes fast in the late 60s and early 70s. He cut through the BS and fed us something we could get our teeth into. And for that, God bless him. We lost him far too soon.

https://www.motorcyclistonline.com/gordon-jennings-and-bike-changed-my-life-perspectives
I've been using both Jennings and Blairs findings and Jennings makes comments of dyno testing for his results in the handbook.
 

Steve Best

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Jennings had a wide swath of friends in the business, and his writing was very easy to read. A lot of people gave him access, and just like us he had projects and curiosity. He made CYCLE the mag to read. Generally when he said it, you could believe it.
 

Street Ryderz

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Jennings had a wide swath of friends in the business, and his writing was very easy to read. A lot of people gave him access, and just like us he had projects and curiosity. He made CYCLE the mag to read. Generally when he said it, you could believe it.
Interesting I've been hearing those names and others of course being refferenced for decades now and it's very useful information and refference materail that I still go back to when second guessing my self LOL.The pipe designer I've been using for awhile now is from IWT Racing and in the acknowledgment it states that the program is a numerical imbodiment of G.P.Blairs books and weather I do the math or use the designer (calc) it comes out pretty much the same using Jenning's math or the Blair based program.What I would really like to know now is the length of the MZ 65 pipe from flange to just past center of the baffle cone as I still dont have one to messure,I messured my son's rm 65 pipe and it's length from piston face to just past center of baffle is 26" and again goes right in line with the afore mentioned methods especaily since the style of pipe with steep baffle angles well above 8-9 degrees witch both concure stops rpm climb after the peak.I think in the case of the rm 65 the pipe design was purposely ment to stop young ridders over reving and doing damage it's also said that that design has lower return wave velocity witch means less power over a shorter span,So basicly I'm trying to come up with a pipe better suited to our engines in the proper rpm range because I'm not convinced that the dirt bike pipes are really working properly on our engines.
 

Street Ryderz

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Omg I just spent 2 hours listing port timmings and pipe lengths for the two main varients of our engines only to have lost it all due to a connection error.so now it has to all be redone again smh!
 
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