True tunned pipe length

Street Ryderz

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#1
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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#5

Steve Best

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#6
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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#7
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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#8
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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#9
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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#10
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!
 




Street Ryderz

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#14
The earlier version is available free!!!:rolleyes::eek::D
Well thats good to know,There are many free software programs available now that I've not used just because of the time envolved in checking their validity I'm a little ocd that way and have to know that the end result is correct so I stick with the IWT Racing one that has proven to concure with manualy doing the math.
 

machanic

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#15
Well thats good to know,There are many free software programs available now that I've not used just because of the time envolved in checking their validity I'm a little ocd that way and have to know that the end result is correct so I stick with the IWT Racing one that has proven to concure with manualy doing the math.
I read some reviews and it appears good but the early version is not simpatico with my windows program.
 

Street Ryderz

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#16
These values are a starting point for proper pipe length (tunned) to the engines actual exhaust port timming and rpm.
First we need to know the exhaust ports open duration in degrees.Messured from deck to top of window.We'll start with the stock generic or older 38mm stroke and 85mm (short) rod @ .7 crown to deck.
height of window druration wave speed rpm length
28mm = 140.66 x 1,700 divided by 8,000 = 29.89"
27mm = 147 .48 x 1,700 8,000 = 31.33"
26mm = 154.10 x 1,700 8,000 = 32.74"
Now since I've yet to see a stock exhaust port higher than 26mm that should cover it,So we will move on to the stock 40mm stroke 89mm rod (long) also @ .7 crown to deck height.
28mm = 149.99 x 1,700 8,000 = 31.87"
27mm = 156.21 x 1,700 8,000 = 33.19"
26mm = 162.29 x 1,700 8,000 = 34.48"
Now we can look at a modded generic or older 38mm stroke and 85mm (short) rod @.7 crown to deck.
25mm = 160.54 x 1,700 10,000 = 27.29"
24mm = 166.84 x 1,700 10,000 = 28.36"
This is as far as I will go here since I can't recomend going above 24mm due to loss of torque.
We will now look at a modded 40mm stoke and 89mm (long) rod @ .7 crown to deck height.
25mm = 168.25 x 1,700 10,000 = 28.60"
24mm = 174.10 x 1,700 10,000 = 29.59"
The rpm used here is not the final rpm rather the peak rpm before power starts to fall off the rpm may still climb above this point depending on the type of pipe used.Next we can look at the chamber area's and cone angles to help determine the best style or type suited to the aplication. I will try to get to that soon as this takes me awhile to put up here LOL.
 

machanic

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#17
At the moment I have a Honda cr80 pipe which needs to be recurved at the cylinder end to fit so length is the most crucial numbers I need, so thanks, but I am looking forward to the pipe shape numbers.
 

HeadSmess

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#18
It looks ok but I'm not going to pay 35 bucks U.S. to only be able to use it twice in a 365 day period as per the piracy statment attached states.


i use two stroke wizard. always given me working results. i did prefer version 3 to version 5, and tried the "duel power" on one pipe with fairly inconclusive results. maybe on my NSR150 when i rebuild it (again...) plus finding which percentage of open space mesh is ideal... what hole size/shape, etc. plenty of variables to stuff around with.

as for the use two times a year? no. read it properly. its INSTALL on a PC system twice in a year.

once installed you can use it as much as you like...

anyway, it often comes up on special every now and then. i think i paid $10.

you also need to download "cone" or something similar. i think my version of cone now takes fifteen minutes or so to open. an extra second every day or something like that...

enough time to go make a coffee and have a smoke :)

be prepared for a heap of work laying out the pipe and welding it up. 180gsm paper is ideal for testing the final shapes before cutting any steel.

and my pipes were always a slight compromise because i would fiddle around with the peak rpm and power just so the lengths/diameters would result in nicely rounded figures. hard to cut things to 0.1mm accuracy. the quote i got for a local laser cutter was absolutely ridiculous.
 

gary55

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#19
These values are a starting point for proper pipe length (tunned) to the engines actual exhaust port timming and rpm.
First we need to know the exhaust ports open duration in degrees.Messured from deck to top of window.We'll start with the stock generic or older 38mm stroke and 85mm (short) rod @ .7 crown to deck.
height of window druration wave speed rpm length
28mm = 140.66 x 1,700 divided by 8,000 = 29.89"
27mm = 147 .48 x 1,700 8,000 = 31.33"
26mm = 154.10 x 1,700 8,000 = 32.74"
Now since I've yet to see a stock exhaust port higher than 26mm that should cover it,So we will move on to the stock 40mm stroke 89mm rod (long) also @ .7 crown to deck height.
28mm = 149.99 x 1,700 8,000 = 31.87"
27mm = 156.21 x 1,700 8,000 = 33.19"
26mm = 162.29 x 1,700 8,000 = 34.48"
Now we can look at a modded generic or older 38mm stroke and 85mm (short) rod @.7 crown to deck.
25mm = 160.54 x 1,700 10,000 = 27.29"
24mm = 166.84 x 1,700 10,000 = 28.36"
This is as far as I will go here since I can't recomend going above 24mm due to loss of torque.
We will now look at a modded 40mm stoke and 89mm (long) rod @ .7 crown to deck height.
25mm = 168.25 x 1,700 10,000 = 28.60"
24mm = 174.10 x 1,700 10,000 = 29.59"
The rpm used here is not the final rpm rather the peak rpm before power starts to fall off the rpm may still climb above this point depending on the type of pipe used.Next we can look at the chamber area's and cone angles to help determine the best style or type suited to the aplication. I will try to get to that soon as this takes me awhile to put up here LOL.
Nice info. I'm going to have to remember this thread so I can resource it later. Are the lengths your giving from the piston face to the mean reflection point in the diffuser?
 



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