ttoks
Posts: 3665
Joined: 3/15/2007
From: leongatha,VIC, AUSTRALIA
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selecting the right tuned pipe is probably one of the biggest you can think of, there are so many pipe out there it's not funny, and choosing the right one can be a bit of a problem.
we here a lot about how pipe A give you more bottom end grunt, and how pipe B is a good mid range pipe for balancing the top and bottom end, and how pipe C will give you loads of top end power, but how to we choose the right pipe for a particular engine? well it is really a very complicated matter, with a lot of maths involved (for you people out there who hate maths, don't worry, i have done it all in advance so you don't have to).
(all measurements in the following section are metric)
so let's get the complicated part out of the way first, you need to know what RPM range you are targeting, that is simple enough no?
once you know what the RPM you are targeting is, you need to figure our exactly how long the tuned length of your new pipe will need to be, the reason for this is simple, when the exhaust port open on your engine, the high pressure in the cylinder creates a bang noise, as we all know all sounds are waves in the air, so this bang noise creates a wave and to begin with a sonic vacuum in the exhaust pipe, this wave is moving at the speed of sound, and the vacuum helps to scavenge all of the burnt fuel and air out of the cylinder.
the speed of sound at sea level on a mild day is 344 meters per second, but with the engine being so small, we don't deal in meters, so we have to times that by 100 to get the speed of sound in centimeters (CM) per second, so the speed of sound is 34'400 CM per second.
now with the way a two stroke engine operates, some fuel and air is wasted out of hte exhaust port every time it opens, the tuned pipe puts that wast to use.
now we know the speed of sound, and we know the wave in the exhaust pipe is moving at the speed of sound, and we have some fresh unburnt air and fuel in the exhaust after the sound wave, the wave travel through the tuned pipe, and when it hits the end of the pipe, it bounces back, or echoes back towards the engine, we know that there is some unburnt fuel in the exhaust pipe as well.
the wave is now moving back towards the engine, with the unburnt fuel and air in front of it, so the unburnt fuel and air get pushed back towards the engine along with the sound wave, now if the tuned pipe is the right length for the RPM that the engine is turning, just before the exhaust port closes again, that extra unburnt air and fuel get's forced back into the engine, in essence supercharging your nitro engine.
now all this is all well and fine, but how does this help in determining how long the tuned length has to be? for this to work, the sound wave traveling in the pipe has to take exactly the same amount of time to get back to then engine as it takes for the piston to cover the exhaust port, so the engine rpm you are targeting and the length of the tuned pipe have to match up perfectly.
(in the following section this key applies:
S means Speed of Sound, the speed of sound for with article is 34'400 centimeters per second
RPM mean Revolutions Per Minute
TL means Tuned Length
/ mean divide
x mean multiply)
(the tuned length of the exhaust pipe is the length of the pipe and header combined)
we now know how the tuned pipe works, lets get onto how we find out how long it has to be.
the basic formula you need to use to determain this is S/(RPM/60)/2=TL, but to the average person, that is just a jumble of number and letters that mean nothing, i will break this down into a more understandable layout
S/(RPM/60)/2=TL
Speed of sound divided by the number of revolutions per second, divided by 2 = the tuned length you need
so, as an example, i will find the tuned length i need if i want my engine to perform better at 25'000 RPM
firstly you need to find out how many revolutions the engine will do per second at the RPM, so you divide 25'000 by 60, and you end up with 416.667, you then divide the speed of sound by this number and you will get 82.6 centimeters (CM), this is the total distance that the sound wave in the exhaust pipe must travel to get back to the engine just as the exhaust port is closing, you then divide 82.6 CM by 2 to get 41.3 CM, which is how long your new tuned length must be.
Now, what happens if you have a pipe and header and want to to use, but you don't know what RPM it is tuned for.
the basic formula for this is S/(TLx2)x60=RPM, but like finding what the tuned length must be, this is just a jumble of numbers and letters to most of us.
so what you must do is measure the the length of the pipe and header together, then you must times this number by two, you then divide the speed of sound by the number you got from multiplying the tuned length by two to get how many revolutions per second the pipe is tuned to, you then times that number by 60 to get what RPM the pipe and header is tuned for.
in the below example i am using the tuned pipe from the RB TM523 with the header for the traxxas REVO
i measured the length of the pipe and header together, and that number is 26 CM, so i multiply that by 2 to get 52 cm, you then divide the speed of sound which is 34'400 centimeters per second by 52 to get 661.5, you then multiply 661.5 by 60 seconds to get 39'690 RPM, so the pipe and header for the RB TM523 is tuned to 39'690 RPM.
so we know how it works, and how to get the right length, but then how much more power will this give me? well there is no exact answer to this, there are a lot of variables to this, the volume of the crankcase, the piston stroke and the exhaust port size, so it will vary from one engine to the next, but it is fairly safe to say that by adding a good, efficient tuned pipe, you will gain roughly 30% more power at that given RPM.
if notes a fualt with the article, or would like to add a suggestion, please pm me.
< Message edited by ttoks -- 7/18/2007 1:37:44 PM >
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All Forums >> RC Cars, Buggies, Trucks, Tanks and more >> RC Car Engines >> Car Nitro & Gas Engines >> selecting the right tuned pipe. 
selecting the right tuned pipe. - 
Here's a link with a speed of sound formula/calculator.