OzVMX Forum
Clubroom => Tech Talk => Topic started by: John Orchard on May 14, 2014, 05:14:57 pm
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Those that are knowledgeable of such things, what are the basic mods one would, or could, do to shift the power in the rpm scale?
Example: extending the header, say 10mm, this would shift the reflecting cone further away, with the pulse heading back to the exhaust port having to travel further, and hence arriving later, would this be conducive to better timing at a higher rpm?
Are there any clues we can use to shift power around?
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GMC. J
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easier to lift the exhaust port a tad! that will shift the power up the top (you should ride the 400 john)
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Making the tuned length of the pipe longer will drop the RPM for peak power. 10mm would be worth about 100 to 150 RPM.
The tuned length is from the piston face to half way along the rear cone projected to a point.
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Read the Bible by A Graham Bell,
Read it once, then get a pad of post it notes and highlighter pens,
Read it again writing notes on the postits
Read it again highlighting important things
Read it again
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The short brief version
Long chambers make low revving motors
Short chambers make high revving motors
Long headers make good low down power
Long mid sections broader mid power
Long baffle cones will let the motor over rev
Make all sections long and you will have a low revving motor and this is where the compromise begins, you will always have to compromise somewhere to gain power somewhere else.
There's more in it than that but as Son says, buy some books from the experts
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Those that are knowledgeable of such things, what are the basic mods one would, or could, do to shift the power in the rpm scale?
Example: extending the header, say 10mm, this would shift the reflecting cone further away, with the pulse heading back to the exhaust port having to travel further, and hence arriving later, would this be conducive to better timing at a higher rpm?
Are there any clues we can use to shift power around?
Short answers
This would bring the peak rpm down.
Yes quite a few but that's how I get the big bucks ;D ;D ;D and I would nearly bet you don't want a higher peak rpm but more over rev ;)easier to lift the exhaust port a tad! that will shift the power up the top (you should ride the 400 john)
These days we lower the exhaust port with much better results.
The tuned length is from the piston face to half way along the rear cone projected to a point.
That's a Jennings idea which is basicaly useless today we measure to the end of the baffle cone.Read the Bible by A Graham Bell,
Read it once, then get a pad of post it notes and highlighter pens,
Read it again writing notes on the postits
Read it again highlighting important things
Read it again
I put it under my pillow, while a good read Bell didn't get a few things exactly right (mainly about transfer port timing). Luckily today with social media/internet we learn from the master.
(http://i155.photobucket.com/albums/s297/Lozza85_2007/janthiel.jpg) (http://s155.photobucket.com/user/Lozza85_2007/media/janthiel.jpg.html)
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Yes Son, I have that book, very good reading, some places a bit flighty and some a bit deep. but i have read some sections 4 times to get the point. Maybe a sign of my Leichhardt high school education showing through.
Brett
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The tuned length is from the piston face to half way along the rear cone projected to a point.
That's a Jennings idea which is basicaly useless today we measure to the end of the baffle cone.
I'll stick with the Jennings theory, it's given me some very good results over the years. Bell also uses the Jennings theory.
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True story
Many years ago I was trapped on a houseboat for days
I had the Bell book with me
My wife could not believe I could read the same book over and over again
I explained that every time I thought I understood something I realized I didn't.
About a year later Mr Bell walked into my shop, he was quite humble when I recognized him and pulled a copy of his book from my office and had him autograph it.
Several days later we had a Technical Commitee BBQ and the Brains Trust got to ask him many questions, it was one of those great experiences,
For such a simple pump the 2 stroke is an amazing thing !!
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Another Great Experience
In the early 90's I was invited to the Lucky Strike Suzuki GP Teams testing at Eastern Creek
Schwantz was testing four or five different sets of chambers
A Japanese Mechanic crushed last years set, lucky me found them,
The next day they started testing the new ignitions, funny the chambers never changed, but the ignition did many times, by the end of the day I had a far greater appreciation of the ability of ignition to control the power of a two stroke.
Get the Exhaust Close
Get the Ignition Just Right !!!
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Never been more evident that the switchable maps on modern production bikes.
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I'll stick with the Jennings theory, it's given me some very good results over the years. Bell also uses the Jennings theory.
Jennings was plain wrong on quite a few things as well, Bell and Blair NEVER considered the most important phase in a 2T engine. Jan Thiel did the same things at Aprilia Corse as what he did at Jamathi in the 60's, ended up with 25 world constructor titles
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Another Great Experience
In the early 90's I was invited to the Lucky Strike Suzuki GP Teams testing at Eastern Creek
Schwantz was testing four or five different sets of chambers
A Japanese Mechanic crushed last years set, lucky me found them,
The next day they started testing the new ignitions, funny the chambers never changed, but the ignition did many times, by the end of the day I had a far greater appreciation of the ability of ignition to control the power of a two stroke.
Get the Exhaust Close
Get the Ignition Just Right !!!
.
Never been more evident that the switchable maps on modern production bikes.
Another story about timing. Did some porting and head work on a late model 80cc mx and the owner assembled it and raced off to his favorite dyno man. Came back the next day with all this expert advice on why the bike had no power. Things like CR to high, port timing all wrong etc. I asked did the dyno man play with the ign timing. Big no was the answer. Cutting the story short, back to the dyno played timing and hay presto the most power from an 80 he has seen. That did surprise me if he doesn't play ignition as part of a tune.
After Lozza's claims.
Jenning and Bell being so wrong!!! I think I'll have to go and commit suicide.
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I expected a few 'differences of opinion' on this matter but I love to hear everyones beliefs & opinions. I think we all learn from our mistakes, I've made my share.
Thanks for the suggestions so far, I will study some books, thanks Lewis for the offer of a loan of some books. I hope more people can contribute their experiences.
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The short brief version
Long chambers make low revving motors
Short chambers make high revving motors
Long headers make good low down power
Long mid sections broader mid power
Long baffle cones will let the motor over rev
Make all sections long and you will have a low revving motor and this is where the compromise begins, you will always have to compromise somewhere to gain power somewhere else.
There's more in it than that but as Son says, buy some books from the experts
yep what GMC just said, we have used these biblical instructions and got a T250 with 315cc pots, (despite it being a sidecar) that pulls like a bastard, we dont have to rev the crap out of it either ;) plus get the gearing right and suspension is another story
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Another story about timing. Did some porting and head work on a late model 80cc mx and the owner assembled it and raced off to his favorite dyno man. Came back the next day with all this expert advice on why the bike had no power. Things like CR to high, port timing all wrong etc. I asked did the dyno man play with the ign timing. Big no was the answer. Cutting the story short, back to the dyno played timing and hay presto the most power from an 80 he has seen. That did surprise me if he doesn't play ignition as part of a tune.
After Lozza's claims.
Jenning and Bell being so wrong!!! I think I'll have to go and commit suicide.
Must be like when you found out the truth about the tooth fairy,the easter bunny and santa claus :D
No claims just simple facts that speak for themselves,Jan Thiel has been head of 4 or 5 factory racing departments which won 25 world titles, in the 1960's challenged the Japanese factories with an engine he designed and built himself(including the casting and machining), went on to build the highest output per litre two stroke engine ever built (Aprilia RSA125 with 53hp at the sprocket). Jennings ,Bell and Blair never designed or tuned a world title winning engine.
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Another story about timing. Did some porting and head work on a late model 80cc mx and the owner assembled it and raced off to his favorite dyno man. Came back the next day with all this expert advice on why the bike had no power. Things like CR to high, port timing all wrong etc. I asked did the dyno man play with the ign timing. Big no was the answer. Cutting the story short, back to the dyno played timing and hay presto the most power from an 80 he has seen. That did surprise me if he doesn't play ignition as part of a tune.
After Lozza's claims.
Jenning and Bell being so wrong!!! I think I'll have to go and commit suicide.
Must be like when you found out the truth about the tooth fairy,the easter bunny and santa claus :D
No claims just simple facts that speak for themselves,Jan Thiel has been head of 4 or 5 factory racing departments which won 25 world titles, in the 1960's challenged the Japanese factories with an engine he designed and built himself(including the casting and machining), went on to build the highest output per litre two stroke engine ever built (Aprilia RSA125 with 53hp at the sprocket). Jennings ,Bell and Blair never designed or tuned a world title winning engine.
25 World titles. Can't compete against that I've only built pipes using Jennings formulars for multiple VMX Oz title winners.
Still not going to use Thiel formulars though. I am sick of building pipes, no money in it with all the cheap imports.
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there is a dutch guy in Bendigo who is a mr mechanic and talking to him it turns out he did pipes for the Suzuki road race team in the 70's... should see him do a muffler repack! amazing....
I got him to gut a dx 200 pipe for me years ago, beautiful job
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Walter Kaaden at MZ (who did a lot to bring about the modern 2T engine and exhaust) reportedly said something to the effect that you'll know you are getting the exhaust near right when it can't be mounted on the bike without requiring some major component (or the rider) to be relocated so you can keep it from dragging on the ground.
:)
cheers,
Michael