RE: selecting the right tuned pipe.

RE: selecting the right tuned pipe.
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RE: selecting the right tuned pipe. - 7/19/2007 9:43:49 AM

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I'm just saying what works, that was a formula I got from a man that used to make tuned pipes for a living.

The angles of the divergent and convergent cones will affect how broad the rev range is for a tuned pipe and as a rule of thumb a longer exhaust timing is needed to get the best out of a pipe,the transfer timing also affects the length,it isn't all about the speed of sound.

No disrespect intended,but maybe your calculations are wrong.I never said 1650 was the speed of sound,thats the figure to use in that calculation.

Allan

< Message edited by bikeracer -- 7/19/2007 9:48:46 AM >

(in reply to ttoks)

RE: selecting the right tuned pipe. - 7/19/2007 11:05:19 AM

ttoks

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the speed of sound in the exhaust of what i am assuming is a gasoline powered engine is 1675 feet per second (thanks to SManMTB for that fact)

you can cross check my calculations if you like, my calculations arnt wrong.

the formula is S/(TNx2)x60

that is divide the speed of sound (in your suggestion is it 1650 feet per second (19'800 inches per second) which is 48'906 CM per second) by your tuned length multiplied by 2 (in the case of the RB exhaust system that is 26 cm (10 1/2 inches) so the number you need to use is 52 cm (21 inches), that gets you your revolutions per second, then you times that by 60 to get RPM. you will see a number around 56'500, which simply cannot be true, thus the speed of sound in the exhaust for a nitro engine is not 1650 feet per second, and is affected by other factors that i am unable to calculate.

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RE: selecting the right tuned pipe. - 7/19/2007 11:38:45 AM

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For the second time.....I'm only stating what is a tried and tested formula.You don't have to use it.
For the second time.....I never said 1650 was the speed of sound.
What applies to gasoline engines does not necessarily follow for methanol engines,neither does full size practice convert well to the motors we use.

On your own admissions your calculations aren't giving you the correct answers.You seem to be quite intelligent,but that does not mean you will always be right.

I've got nothing more to say,you obviously don't want to doubt your own calculations.

I might add that I only race 1/5 bikes,one of which has been clocked at 68mph in a recent race.It's fitted with an OS 18 CV-R and a pipe I made.

Allan

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(in reply to ttoks)

RE: selecting the right tuned pipe. - 7/19/2007 11:48:33 AM

ttoks

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the only variable there is, is also the only one that i cannot confirm, that is why i am trieing to find out as much information as i can to try an identify this hidden variables and figure out how they affect the calculations, at i guess i can see the engines maximum power being made at around 32'000 rpms which would suggest a speed of sound of about 300 meters per second, which is lower then the speed of sound at sea level, so the glow fuel left over from the combustion process is slowing the pressure wave dramatically, at the temperature of the exhaust gases (which is about 400 F depending on the nitro content of the fuel and other factors) the speed o sound in air is 430 meters per second.

sorry if a seem a bit testy, but the formula (sorry i used the wrong term above, as the calculation is obviously off) is correct, the only problem is figuring out the last variable which is the speed of the pressure wave that i am having trouble with.

that is a nice bike by the way.

< Message edited by ttoks -- 7/19/2007 11:51:33 AM >

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RE: selecting the right tuned pipe. - 7/19/2007 1:44:54 PM

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quote:

ORIGINAL: ttoks
try an identify this hidden variables

Your "hidden variable" is likely what I pointed out way back in post #17. Either they're not a pipe at all or are a half wave pipe at best. Work out your figures as for a half wave pipe and see what the revs indicate it's tuned for. I'll make it easy for you, take your final revs and halve them.

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RE: selecting the right tuned pipe. - 7/19/2007 2:14:01 PM

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Before dutting the wavelength in half, first try to use 700degree F as the EGT (exhaust gas temp). 700degrees might seem too high, but keep in mind that not all of that heat radiates into the surrounding metal.

If factoring the speed of sound for 700degrees F still does not work, then calculate for half wavelengths, at 400degrees F and 700degrees F.

Hopefully one or both of these solutions will generate the correct factor. I will try to calculate these myself tonight once I come home from work. I too am very curious.

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RE: selecting the right tuned pipe. - 7/19/2007 2:50:56 PM

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quote:

ORIGINAL: SAVAGEJIM

Before dutting the wavelength in half, first try to use 700degree F as the EGT (exhaust gas temp). 700degrees might seem too high, but keep in mind that not all of that heat radiates into the surrounding metal.

I wasn't going to add any more,but.....

The average can be as high as 750 F, also the hotter the gasses the quicker the the reflected wave moves.If you cool a pipe too much the engine revs will drop because of this,this has been proven in practice.

A pipe made from steel also resonates better than an aluminium one.

Of course the pipes used in model cars may not conform to normal tuned pipe principles. However,the length from the face of the piston to the stinger will be very close whichever pipe you fit to the same engine to get the same revs.
What I've learned over the years applies to proper tuned pipes,so might be best to take anything I say with a pinch of salt,but a web search should find you all the answers and formulae for tuned pipes. There was no such thing as the internet when I first started playing with tuned pipes and knowledge was hard to come by.

Allan

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RE: selecting the right tuned pipe. - 7/19/2007 3:53:22 PM

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quote:

Of course the pipes used in model cars may not conform to normal tuned pipe principles.

Oh yes they do.

The best book about two stroke engines and tuned pipes on the market is a book called:
"Design and Simulation of Two-Stroke Engines" by Prof. Gordon P Blair.

It can be purchased from here: Link to book.
or maybe you can find it at your local University library.

I've read this book twice and there is enough info in there to keep anyone busy for years on end. I'm planning on buying it.
It explains everything in detail. It's +500 pages.
I don't claim to understand everything in that book because I never studies thermodynamics myself but there is still a lot that can be understood from the book.

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RE: selecting the right tuned pipe. - 7/19/2007 4:08:40 PM

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quote:

ORIGINAL: ttoks

thanks for the explanation there, i did overlook exhaust timing in m/ calculations, although i cant see this affecting the tuned length but rather the overall power band with a given pipe, i'll have to do some research to get a better understanding of it.

the only problem i see is that it is simply impossible for the speed of the pressure wave to be 501 meters per second, this would mean a 26 cm pipe (the RB pipe and header i use for my examples) would be tuned to 58k RPM, a good 18-20k over what the engine is capable of, i would be thinking that is the speed of sound in exhaust gases of a petrol engine no?, the speed of sound still has me stumped and i don't have enough time to sit down and think about it for now so for a while it will go unresolved.

Not really...
Assuming the exhaust timing is 175 deg (which actually is on the high end when it comes to off road). Insert this in the formula after rearranging it:

rpm = (V x E) / (L x 12)

Use the speed in [cm/s] and the result is:

rpm = 28636

A perfectly reasonable answer.

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RE: selecting the right tuned pipe. - 7/19/2007 4:26:04 PM

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To explain the exhaust timing.

Follow your initial thought about the wave traveling back and forth in the pipe. You assumed (IIRC) that it had one whole revolution to do this in. This is not true. The wave has to exit the exhaust and come back just at the time it closes again.

A longer duration exhaust is open a longer part of one revolution so for a fixed rpm the pipe has to be longer for an engine with longer duration exhaust port. Thinking this way when it comes to long exhaust timing is still kind of wrong.
The point of a longer duration is to make the engine 'breathe' and perform better at a higher rpm, and it will.

Keep the length the same and play with exhaust timing instead and you'll see that a longer duratrion will yield peak power at higher rpms.

Example:

Exhaust: 165 deg (typical OS RG or other 'sport' engine)
Pipe: 26cm

rpm = (V x E) / (L x 12), gives us rpm = 27000

Exhaust: 175 deg (RB WS7 for example)
Pipe: 26cm

rpm = (V x E) / (L x 12), gives us rpm = 28636

(in reply to SManMTB)

RE: selecting the right tuned pipe. - 7/19/2007 4:43:49 PM

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The speed of sound is not really hard to get either. It varies with the medium (air/exhaust fumes/water... and so on).

I'm at work and can't spend too much time looking up all the data but in general you can say that sounds travels faster when the temperature goes up.

< Message edited by SManMTB -- 7/19/2007 4:50:39 PM >

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RE: selecting the right tuned pipe. - 7/20/2007 2:45:30 PM

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I killed the thread.

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RE: selecting the right tuned pipe. - 7/21/2007 1:58:45 AM

ttoks

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no i had to work yesterday then i stayed at my brothers house last night.

28k for this engine seems low to me, having used the engine it will not make most of it's power until it hits second in the revo because i have the shift point set to early to allow the engine to rev out in first, i plan on doing some more research but i can see me getting a headache, i might see if i can get my hands on that book, but once im done i doubt i will have an idea on how ot calculate the speed, i think the best way would be to test the engine on a dyno power without a pipe and with a pipe and see where the the power increase is to find out the rpm, and then it might be possible to make a rough calculation from that, i don't know i'm thinking about just giving up on the tuned pipe issue and just focusing on my work instead.

< Message edited by ttoks -- 7/21/2007 1:59:46 AM >

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RE: selecting the right tuned pipe. - 7/21/2007 3:45:30 AM

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I don't know which engine you've got but that 28K sounds about right to me for peak HP. Maybe it'll spin to 35K easily enough but the HP developed then will be quite low. What a tuned pipe does when it comes on song is to give a sudden rise in torque which then translates into more HP but as the revs rise further and the pipe goes off tune then the torque (and HP) drops rapidly. I've just been looking at a few of the dyno graphs at http://www.nitrodynesystems.com/Engine%20Database.htm and concentrating on the torque curves and none of them show any sudden rise as they should if a car pipe actually worked.

When a true full length high performance pipe comes on song the effect is remarkable. I remember watching a control line speed model begin a flight and it was just pottering around at maybe 100mph when the pipe suddenly cut in. Everyone within about 100 yards fell flat on the ground in case the lines broke

. If you're interested in finding out how a pipe really sounds doing this then go to http://www.flyrc.org.uk/record/control_line.htm and watch the video of a world speed record being set with a .15 engine. Make sure you have the sound turned up

.

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RE: selecting the right tuned pipe. - 7/21/2007 4:40:56 PM

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Another thing I forgot to mention is the time the exhaust port is open. It is not a linear function sinc the linear piston speed is speeding up as the port is openeing and the linear piston speed is slowing down as the piston is closing. So to accurately calculate the time the enhaust post is open, you must factor the conrod length and its geometry to the crank angle from the instant the exhaust port opens to the instant it closes.

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(in reply to ttoks)

RE: selecting the right tuned pipe. - 7/22/2007 2:56:44 AM

Grits