how a tuned pipe works????

[dag]

can someone explain how a tuned pipe works in laymans terms, and why it is the best type of exaust?

thanks

[Hydro Junkie]

In one of the last two issues of RCBM, there was a whole article on that very subject. You might want to check and see if it's still on the website :)

[BoatDoc]

i think that there is a magic migdet inside the pipe! but...if you can't find the article, here's the layman's explanation:
basically, the length and the volume of the pipe has to match the displacement of the engine. but the main player in the equation (if i remember correctly...) is RPM. if you look at the pipe you will see that you pretty much have two cones that have been welded together. the theory of a pipe is based simply on the the sound waves of the exhaust. for optimum performance the sound wave and the RPM need to be timed. simpy put...when the sound wave generated by the exhaust hits the point where the two cones meet, it will reflect back towards the engine. if the length of the pipe is correct, the sound wave will reflect and hit the exhaust port just in time to shove the air fuel mixture back into the motor. why?? well...as the engine runs, some of the fresh air/fuel mixture will leave through the exhaust port (we must remember that the intake and exhaust are open at the same time). when the sound wave returns to the engine, it will "shove" that part of the mixture back into the engine, thus, supercharging it. i hope i helped, because the actual theory behind a pipe is a bit much to explain here on the forum.

[Hydro Junkie]

Nice job Doc :D I went and checked the site, only to find the article was gone :( It gave suggested pipe lengths and all kinds of info to "tune the pipe" :cool:

[BoatDoc]

hey HJ...the hard part is the fact that temp, fuel type, and prop also play in to the equation! that's what really make's it hard! until recently, i was only into outboards and my RTR miss bud. so i didn't worry about pipe's that much. but i've read up on pipe's a bit, and i'm a junky for info, so i try to remember what i can from everything i read!

[Hydro Junkie]

All of that was in the article. I'm just glad I bought a copy with all of that in it :D You didn't happen to get one, did you ?

[BoatDoc]

i buy the mag every time i see a fresh copy at the LHS. i don't know any other boaters down here, so that's the only way i can keep up. i wish they'd do an article on how to NOT break your boat. it seems that every time i run, i break something. and i'm fanatical about making sure i throw a good boat in the water!!! but for some reason, i can't stop from breakin' my babies.

[Hydro Junkie]

Must be the Water Moccassins :D

[BoatDoc]

i think it's the mysterious water gremlins of alabama! that, and the lake God's of alabama don't like me. i've crashed or sank a total of 3 boats in the last ten months!!! only one has been repairable! the tunnel hull i wrecked was a dumas...and i think everyone know how i feel about those! but at least i got the engine and radio back from that one! otherwise i'd be a little more upset.

[dag]

thanks guys--it all seems more clear!

[water dog]

I read a story on pipes called THE COLD WAR ON PIPES .It has good info on pipes.

[wakeracer]

A tuned piped really works because it allows the engine to take advantage of what they call scavenging. In a 2 stroke engine, such as a rc boat engine, overlap occurs, in which the intake and exhaust are open at the same time. The pipe simply creates a slight vacuum in the cylinder at this point, which is due to a change in temperature as the exhaust gasses exit the pipe becoming more dense. This vacuum allows the piston to draw more air/ fuel mixture into the cylinder, which in turn creates more power.

[Hydro Junkie]

Yes, but you're missing the other half of the story. The dual cone shape forces exhaust pressure back to the engine, pushing fuel that would have gone out the exhaust pipe BACK INTO THE ENGINE. That is the magic of the tuned pipe, and why they increase the engine power levels so much.

[hydroracer]

yep, they pull all the burnt gases and some fresh mixture out the exhaust port (scavenge) then push the fresh mix back in under pressure for a larger volume of fuel/air mix in the combustion chamber, thus more bang. that's why the pipe length you set is so important. you have to time the return pressure wave to push the overflowed fresh fuel/air mix back in to the cylider right before the exhaust port closes for maximum supercharge effect. if the pipe is to short or to long, the waves miss the "window" to gain the tuned benefit. the pressure waves or pulses travel at the speed of sound, hence the harmonics of "coming on the pipe". :cool:

[wakeracer]

Although a slight back pressure is created by the tuned pipe, it is hardly enough to create the huge horsepower gain you are speaking of. If in fact this were the case, the diameter on the pipe's end opening would be just as much a factor as the pipe's length, as both would help achieve the correct back pressure needed to fulfilll the demands of this precise window you speak of. On the contrary, the lenghth of a tuned pipe is so important because it times how long of a vaccum occurs through scavenging which fills the combustion camber as much as possible without drawing out too much of the new air/fuel mixture. Now, this is the main purpose of a tuned pipe as these small 2 stroke engines don't have a functioning valve train such as four strokes, to regulate overlap a save horsepower! ;)

[BoatDoc]

i'm not trying to sound like i know everything here, but what wakeracer has described is more like the idea of a "mini-pipe". i'm reaching back a few years here but a mini-pipe has more to do with vacuum and back pressure than a tuned pipe, as a tuned pipe uses sound waves. i used to run a mini on my k&b outboard, as i couldn't afford the tuned pipe. the mini is cut to such a length as to increase the scavenging, but it will not push anything back into the cylinder (all it does is create a vacuum). i learned this from some airplane guys back in my flying days. you can pull some serious rev's but you'll lose torque. it also causes you to waste a lot of fuel, by spitting a lot of raw mixture out the pipe. and since i haven't heard of anyone using mini's lately...i'm going to assume that they've gone out of style. they were really more of a way to increase performance a little bit without dropping a lot of green on a pipe. the trade off is the fact that they don't provide the performance of a good tuned pipe. if i'm wrong let me know...i can handle it! i think i've still got my mini set-up...maybe i'll slap it on one of these days and see how well it works (haven't used it in about 6 years).

[wakeracer]

Boat Doc, to be honest u probably know more about model engines than me but i'm just using my knowledge of racing engines to give some insight on this subject. Considering I have been a part of a number of drag racing teams and i'm currently going to school to build build race engines, i have to know a great deal about the engine and it's basics, and a two stroke engine (such as these) are as simple as it gets.

[hydroracer]

Although a slight back pressure is created by the tuned pipe, it is hardly enough to create the huge horsepower gain you are speaking of. If in fact this were the case, the diameter on the pipe's end opening would be just as much a factor as the pipe's length, as both would help achieve the correct back pressure needed to fulfilll the demands of this precise window you speak of. On the contrary, the lenghth of a tuned pipe is so important because it times how long of a vaccum occurs through scavenging which fills the combustion camber as much as possible without drawing out too much of the new air/fuel mixture. Now, this is the main purpose of a tuned pipe as these small 2 stroke engines don't have a functioning valve train such as four strokes, to regulate overlap a save horsepower! ;)

Well wakeracer, if a 50% of peak hp or more increase isn't huge, then I stand corrected. :) I've no doubt you know more than I probably ever will about the inherently more complicated high performance 4-stroke automobile engines, and this is just friendly info exchange, not a flame. :cool:

But the tuned exhaust theory of a 2 stroke is a little different than that of a 4 stroke. It does sound like you are referring to a straight pipe exhaust, but I may be wrong. :confused: You are correct about the vaccuum duration created by the expansion section of the pipe, and also port overlap. The end opening you referred to on a 2 stroke expansion chamber (tuned pipe) is a "bleed pipe", or more commonly known as the "stinger". And it does just that, it "bleeds" off exhaust gases at the desired optimal rate. The location of the stinger is not critical, but the diameter is. Too small of an opening will cause over-backpressure and overheating, and too large will reduce backpressure wave for the supercharge effect. The length of the stinger is important as well, but to a lesser degree. I'm talking about a modern 2 stroke expansion-type tuned pipe design with a convergent and divergent cone. If we're talking straight pipe or "megaphone" typ pipes, then the all important "return wave" is not created. The real power is made by the convergent cone at the end of the pipe. A straight pipe or megaphone send a much smaller return wave than a convergent-cone designed 2 stroke pipe. My description of how they work is much simplified, but accurate. I can write an entire page on how they function and their design theories if you'd like.

Good boating my friend :D

[BoatDoc]

i'm just going to go back to my "magic midget" theory. it's much simpler. i can explain a lot of stuff by saying it was the magic midget, or the lake gods of alabama. hopefully the lake gods of wisconsin will by kinder, as i'll be back in the cheese state in a few months. the lake gods of alabama have claimed 3 of my boats and caused numerous headaches, and i think the magic midget in my tuned pipe had been sleeping.

[wakeracer]

Hydroracer, i'll have to take your word on this one as i had no idea a tuned pipe had and extra set of cones inside as you explained. This completly changes the scopes of things... In other words I stand corrected. :( Any how, thanks for the insight! P.S: As u may already know we don't deal with tuned pipes often in drag racing.

[mjmsprt40]

Don't forget one other factor--- this is important to the way a tuned pipe works.

If you take a look at your cylinder liner, you'll find that the intake ports are somewhat lower than the exhaust ports. This means that the exhaust opens a bit earlier and closes a tad later than the intake. The tuned pipe uses this difference to make that power, by sucking out the exhaust gasses and some of the fuel charge. Then, on the piston's upstroke, it forces the fuel charge back into the cylinder AFTER the intake closes but BEFORE the exhaust closes. That gives us a bit of supercharging. Not much, maybe, but in these small engines it doesn't take much to make a big difference. This also explains why pipe length is so fussy--- it's all a matter of catching the intake and exhaust timing right to give that boost.

If this doesn't confuse you enough, I'll try a little harder later. ;)

[BoatDoc]

nope...it's the magic midget :D actually...i think we've finally got all of the finer points pinned down! now that we all have the theory down...here comes the hard part!!! you need to remember that the pipe may need "re-tuning" from time to time. things like air temp, air pressure, prop, and fuel type are all going to play in. from what i know, you can usually leave the pipe alone and just play with air/fuel mixture. from what i learned from flying planes is that a pipe that works well in one situation doesn't always work in other situations. sometimes, you'll need to shorten/lengthen the pipe just a tad to get it timed if your running condidtions change enough. he he he, i think i just complicated things. but like i said, it seems like everyone has the basic idea down pretty good. most of what i know about pipes comes from planes, and those engines are operated under very different conditions than boats. in a plane, a change in air pressure is enough to not only effect the engine, but it also changes how the prop will act and how fast the plane can actually go...thus changing EVERYTHING!!!! i knew there was a reason i loved this r/c stuff :D

[hydroracer]

Right on Doc, magic! :) Unlike the airplanes and air propellers, water density remains the same (discounting salt water or extreme suspension of sediment, etc.), so unless the air density is a whole lot different, a needle adjustment will usually suffice in boats, without a pipe change. One less thing to worry about, right? :) Once you get a particular tuned pipe setup to work well on a particular boat, it probably won't need adjustment unless there is a very drastic change in altitude or temperature, or you changed to a different propeller size, or changed nitro content. Something like a change from 40 degrees to 80 degrees outside may need a pipe length adjustment, but to be honest, I've not experienced that type of drastic change. I live in florida, so the altitude and temps are fairly constant. I love these little engines man!

BTW, nitromethane acts kinda like liquid oxygen, and has a similar effect as supercharging, without a supercharger. Kinda like nitrous oxide does. That's why you need a richer needle setting if you go to a higher percentage of nitro fuel mix. The nitro burns more fuel by releasing it's own oxygen during combustion, hence more power. :)

Hey Wakeracer, as you already know, the second cone would not have the same effect on a 4 stroke engine, so you won't see that type of pipe in your line of work. I don't know squat about 4 strokes. The power game is the same as with a supercharger/blower/turbo on a 4 stroke, to get more air/fuel mix into the combustion chamber for more power. 4 strokes do it from the intake side, where as 2 strokes do it from the exhaust side with a pipe. I know there are intake superchargers on some industrial 2 stroke engines, but I can't figure out how they can work, but apparently they do. The port designs of those cylinders must be quite different from our little nitro engines to keep the compressed air/fuel intake from blowing right through the boost ports and on out of the exhaust port during the 2 cycle's port overlap, and therefore defeating the purpose.

blah, blah, I'm just rambling :)

Later guys

[wakeracer]

I know this is somewhat off the subject but i just wanted to share a little 2 stroke trivia for those of u who don't know. A two stroke engine such as these, don't have a intake stroke like that of a four stroke. The only two strokes that are completed are a power stroke and an exhaust stroke, hinting the name "2 stroke."

[mjmsprt40]

Wakeracer, it gets a tad more complicated. The two strokes are actually compression/power. Then the fun begins. As the piston is going down, it's pressurizing the crankcase. When the cylinder intake port opens near bottom dead center, a pressurized air/fuel mix enters the cylinder and helps force the exhaust out of the exhaust port. Then the piston starts up, closing the ports and entering the compression stroke all over again.

[wakeracer]

I know all about it mjmsprt40. In all actuality a 2 stroke really goes throw four strokes, but at any given moment 2 strokes are occuring simultaniously. And the pressure on the crankshaft u speak of is known as crank case compression.