Mousse can logic?
 

I recently bought a tower hobbies .75 engine that needed agood cleaning but was still relatively tight at the top end. It did not have a muffler and mufflers forthat engine are hard to come by now unless you buy a whole 61 – 75 tower or gmsengine with muffler.I decided to make aheader pipe and mousse can muffler.Theheader pipe was easy but took a bit of time.It turned out good.I found twodifferent sizes of aluminum cans that I thought would be the right size.I modified them slightly and the differencesare shown in the picture attached.Theyare both the same volume but give very different rpm numbers.I was told that the size of the can was notthat important but perhaps shape makes all the difference.
The rpm numbers are listed below. I added a short piece of aluminum tubing on the short fat pipe tomake the header length the same on both setups which actually lowered the rpmsslightly more.What do you think is the main reason for the800 rpm difference in these two cans are.I am leaning towards the taper ofthe tubing coming into the can and the taper of the tubing going out of thecan.I guess I can add a tapered tubingonto the short fat can and see if I see any improvement and then put a taper onthe rear to see if that makes any difference also.Maybe it is the short fat compared to theskinny long can that makes the difference. I also did try to slide the short fat can up the header with no real difference.
Tower 75 engine 15% nitro 18% oil (50/50 castor syn blend)48 degrees F 3900 ft elev.
Apc 12x8 sport prop
11,400 peak - open face exhaust
11,500 peak-short fat can
12,300 -12,450 peak – long skinny can



I have seen that the bud light aluminum bottles that are 16fl oz work well with the 91 size engines. they have a tapered neck on the bottles.

I would guess that the length is the most important issue. How long are they?

Mousse can muffler are usually not made like that, i.e. just empty inside. Instead most people use them to mimic the "tuned mufflers" of MVVS and Jett etc.

the long skinny can is about 10 inches long and the short fat is about 7.5 inches long, the inside capacity of each can is almost the same measured after they were built.

I know what you mean by the pipe running inside the cans but doing a search for MCM or mousse can muffler builds, brings up about 95% empty can designs and to tell you the truth, I haven't actually seen a picture with the pipe inside the can like the Jett and similar tuned muffler except for the stock muffler modification mousse can shown on spad to the bone website. Just very few posts that describe the pipe any more than 1/2 - 3/4 inch inside the can.

I am going on the logic that bigger volume containers can use smaller exhaust stinger and smaller containers need larger diameter stingers. I did read on some sites that if the can is too large of a volume for the engine then it will not "get on pipe" this was from some very knowledgeable profilers judging by the amount of engines they have experimented with.

I did play with the idea of making a similar setup to the Jett mufflers or ultra thrust mufflers but it just seemed like the straight header pipe length with the included radius on my header would be too long overall with the size of can they have on their mufflers. I could probably do it with a sharp 90 bend which is what they kind of have on theirs.

The volume plays a role in the effective "tuning" of the pipe, but the length is going to have more influence. The exhaust pulse is a pressure wave traveling along the length of the exhaust system. When the length of the system coincides with the timing of the engine, you get the tuned pipe effect. Lower timing for bigger props requires a longer system whereas higher timing for smaller props requires a shorter system. If your header is long enough, you could move the can in and out (on/off the header) to find the optimum length for a given prop. Similar prop loads won't require changing the length of the system generally.

I did some experiments with an Enya SS30bb using .46 sized mufflers and got a very nice boost in power ~2000rpm in one case. In the end, the .46 muffler gave a slight increase in power over the open faced exhaust reading without being obnoxiously loud. The caveat is extra weight for the larger muffler and adapter.

Does the length you are referring to mean the length from the end of the pipe to the back of the can or the length from the exhaust port to the back of the can?

When I speak of the length of the system, I mean the overall length from the exhaust port to the end of the convergent cone of the pipe (in regard to a regular tuned pipe). For a mousse can pipe, your effective length is from the exhaust port (more technically the glow plug since that's where combustion starts) to the back of the can. The further the header protrudes into the can the shorter the effective length becomes, so moving the can in and out from the header will change the length of the system this changing the tuning of the system. If your pressure wave comes back too soon (pipe too short), you can end up with the engine running hotter and losing power because it's pushing exhaust gasses back into the combustion chamber. If the pressure wave comes back late, it will still be down on power but it just won't be pushing the full fresh charge back into the combustion chamber. In essence it's better to have the system a little too long than a little too short. Of course if the system is too short, running a little less prop load (more rpm) will get the engine on the pipe.

So would it be a good assumption then if the a stub of pipe is already welded to the inlet of the can, that I would start cutting the header shorter, little by little while using one load prop higher than I plan to fly with until the rpms stop increasing, if any increase at all. Then when I put on my flying prop, it would be just about right? Is that correct?

The general practice for tuning a pipe is to start with a long header and cut it down 1/4-1/2" at a time with peaking the engine each increment until you see no gain in rpm between two increments. But it's been said that this sometimes causes the pipe to run a little short in the air. So I think it would be good practice to run a prop with maybe 1" more diameter and same pitch as the intended flight prop and tune the pipe to the larger prop and then for flight swap to your 1" shorter prop. This way your pipe ends up being a little longer.

This might be of some help - Dub Jett has some how-to videos on YouTube now with tuning a pipe being one of them. Also - in one of his other videos, he shows a different way of finding your peak rpm that's different than just peaking the needle. It might be worth watching that video also. Here's the pipe tuning video link - https://youtu.be/KCIlP6_Z0pw

The idea of cutting the header, little by little will only work if the header is too long to start with!
If you have around 10" from the end of you muffler to the glow plug then I guess the optimum rpm would be above 20000rpm (depending on engine timing etc). So I would say in your case you need to increase the header length and then see what happens.

The MMVS "tuned mufflers" have a tube inside such that you get a "folded pipe". The pressure wave will travel on the outside of the inner tube to the front of the muffler and the back again. The stinger needs to be smaller than the inside tube in order for the pressure wave to travel back into the tube. Here is a picture of a cut-up MVVS "tuned muffler";

http://www.rcuniverse.com/forum/atta...mentid=2201607

At the "SPAD-to-the-bone" site they mimic the MVVS construction, as I have understood it;

http://www.rcuniverse.com/forum/atta...mentid=2201608

Your numbers indicate that with the open pipe, part of the intake charge is following the exhaust out into the pipe. The muffler that is getting the higher rpm is slowing the exhaust just enough to keep more of the intake charge in the cylinder. Exactly what you want.

You can play with a longer silicone tube for a longer length before resorting to cutting up what you already have. With a real pipe, the diameter of the outlet makes a difference too, and the length or the stinger a bit too, if it makes it restrictive. You can see if you are in the ballpark, by blowing into the stinger like a flute, and your finger over the entry point of the header. (not trying to be kinky here) Compare the musical note to the rpm coming from the motor. 13,000 rpm is equal to about an "A" You are not too far off with either pipe, and the stubby one may unload a couple thousand rpm in the air and be "on" the pipe better than the longer skinnier one. A smaller prop load could bring it up to be on the pipe too. You have already surpassed the open faced rpm so are much farther ahead than a stock muffler with either pipe.

Clarence Lee showed us that stinger length or diameter matters with his mufflers.

I made up a new can with a little longer pipe attached. I used a 12/9 apc prop and cut 1/2 inch at a time( should have stuck with 1/4"). 1st cut got + 200 next cut another 200 next cut lost 150. Then put on the 12x8 and was up by 600 over the 12x9.

I will I'll try to attach a picture of my setup with the three cans I used. I slid the short can all the way up to halfway around the header radius on the way up it peaked and then went back down a little. I still was a good bit lower than with the other two cans which are pretty close to each other.

75 tower , 15% nitro, 18% oil, 50 deg f

11,650 - 12x9 apc
12,250 - 12x8 apc
12,700 - 12x7apc
13,200 - 11x8 apc
13,290 - 12x6 apc
13,700 - 12x6 MA SIMITAR
13,500 - 11x8 Revup wood
13,900 - 12.25x3.75 apc

http://www.rcuniverse.com/forum/atta...mentid=2201739http://www.rcuniverse.com/forum/atta...mentid=2201741
I am thinking about using this engine combination on a Ultra sport 60. I kind of want as fast as I can go with the ability to still have enough thrust to go straight up until I let off the gas. The plane will weigh no more than 8.2 lbs ready to fly. Any sugestions for props based on the rpms given above.

I use the same fuel mixture and run my OS75 ax on an APC12/6
The engine is a true powerhouse in a 60 size airframe
Vertical performance is the true test of a props performance
You prop an airframe not an engine
Find the prop that pulls the plane straight up best
It is the prop that will pull you thru your manuvers best

Enjoy

One thing worth noting is that the reverse cone on the muffler/tuned pipe works best if it is a straight tapered cone. A curved cone doesn't work as well. Less of the shock wave gets reflected back towards the engine when the cone is curved. Thus the short fat muffler chamber using a curved cone is not as good as the long skinny muffler with the straight tapered cone. When you look at the tuned pipes and expansion chambers they all use a straight tapered cone shape.

But anytime a muffler lets the engine develop more power than just using a open exhaust makes that design successful. So both of your designs are good.

This is what gives the most power, it would sacrifice throttle response. The gradual tapers like the Macs pipes are designed for ease of use on throttled motors.http://www.technohobby.com.ua/online...5cc-f2-pioneer . For the OP's purpose, then yes a more straight taper is great.There was a concern of cutting the pipe too short. When the prop unloads in the air, it may be about what you would want in the end. A bit of trimming of the prop could bring it into the pipe's range too.

On a separate thought, just to confuse you, or give another idea at least, here is a pipe idea that was manufactured in Australia for a short time. I was going to make up something like this to try out sometime. I could not find any very light tubing. http://www.rcuniverse.com/forum/atta...mentid=2201954 The inside plug is adjustable in and out. The last plug is screwed in. I don't think I would use the back one myself, but it supposedly would help resonation at different speeds. It gave about 1,000 rpm more than open exh. which you seem to have got already.

It's too bad there aren't tuned mufflers for aircraft engines similar to what was/is made for rc car engines. Buku and CVEC used to make mufflers with baffles that moved back and forth with the exhaust pressure waves giving better resonance at higher rpm than at low rpm.

One classic technique used by us with pattern planes years ago was to run a little lower pitch prop on the ground and tune the engine and pipe for that. Then you put the higher pitch prop on to go fly it. The prop then unloads in the air and comes up on the pipe. That gets you really close to optimum on the ground then. In my example I used a 11x6 prop on the ground tuning and switched to a 11x7 prop for flying. The engines when they come up on the pipe get that power boost quite quickly then you need to richen them up quite a bit as if you don't the engines wind up being too lean then. That can lead to the problem where the engine is too rich on the ground to get airborne and unload the prop enough to come up on the pipe.

Ok, after looking at the ultra thrust mufflers and MVVS tuned mufflers and Jett Stream mufflers, I think that the plane will look a bit cleaner with just a tuned muffler instead of the long header pipe and long can. I think I am going to try and make a tuned muffler. I just don't think that with the "folded system" would be as efficient as a straight shot like what I am making now. I hear that the tuned mufflers give you quite a boost in power but is it as much as you would get with what I have now?

Thanks for the suggestion. I was trying to duplicate the jett mufflers or ultrathrust mufflers but the metal was just too thin to get a deep enough bell.

I am going to try to make a tuned muffler taking the measurements off of the 90 size ultra thrust muffler. I figure I will make it just a little long to begin and just keep cutting it shorter and shorter until I get it right. Then I will weld everything in place. Do you think that instead of the curved cone on those mufflers, I would do just as good with a straight taper cone?

Interesting, earlwb.

double post

I scaled a picture off a ultrathrust and jett sport muffler and they both are almost identical in size for the 61- 91 size muffler. They are both very close to the same shape at the rear end, and although I have not actually seen the inside of the jett, I would bet they are almost the same inside. I also noticed that the top end of a mousse can is almost the same shape as the tuned mufflers of these two brands. I just have to make and weld on the tip section I plan to use a 1.75" mousse can for the outer can and machine a top housing that the can will mount to and that a pipe will slide through. I then will make a rectangle tubing with a flange to bolt to the engine and weld this to the front end of the pipe. I will start with 7/8" od 3/4" id tubing and machine the tubing that will be inside the can down to 1/32" wall thickness. The 1/16" wall thickness will be stronger to weld the top housing that will hold the can and the exhaust flange rectangle tubing. I will make a temporary joint on the can to the housing until I get the right length and then permanently attach it. I can then take 1/8" at a time off of both the can, inner tube, and change the stinger outlet diameter (by adding bushings) each step of the way until I get the best rpm numbers for the props I plan to use.

I noticed that the inner pipe ends right where the taper of the outer can starts. I don't know how critcical this is but I might have to put in an extra step of cutting the inner pipe first check rpm with different stinger diameters, cut outer can, check rpm with different stinger diameters ....... and keep going until I find the right combination. http://www.rcuniverse.com/forum/atta...mentid=2202344http://www.rcuniverse.com/forum/atta...mentid=2202345

double post again, not sure whats wrong.

Doing a little research I found this claim on the Jett site. It is kind of a little disappointing that they are only getting 12500 peak with an os 91 and 12x8 prop when my .75 is at 12250 with the same prop. Maybe I should just stay with the header pipe and mousse can I have.


Bolt one on to your favorite engine, and enjoy the instant 25-50% increase in power
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[TD]http://www.dubjett.com/images/f-14-jettstream-os91.jpg
OS .91 with Jett-Stream Muffler
Great Planes F-14 - 12x8 prop
pulls hard at 12,500 ground peak RPM[/TD]
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Are you using an MAS 12x8 or APC 12x8? You will see more rpm with the MAS than you will with the APC generally.

FWIW, my ST S90K pulls around 14,800rpm on a 12x6 APC on 5% nitro fuel and red Jettstream muffler (.61-.91 size). 12,500rpm on a 12x8 seems like a misprint. It's probably supposed to be 13,500rpm. That seems much more realistic.

My numbers for the 12x8 are with the APC prop on a .75 engine using 15% nitro. I would imagine that if this is advertised on the Jett Web Page that they would make sure they corrected a misprint of 1000 rpm if it was a low side error. If I remember correctly, I think I get about 10,500 with the stock muffler and a 13x10 APC on my Magnum .91 XLS.

You can definently see the difference in the rpms from the APC to MA props with my numbers from the first page. With a 12x6 prop there was over a 400 rpm difference.

There has been a lot of misprints on the new Jett website. They state 17,000rpm for the Jett .35 on a 10x6 and in fact it's less than 16,000. I'm not sure who does the website, but I'll guarantee the 12,500rpm figure for the 12x8 is low. That or they were running a worn out engine. My ST .90 would easily top that on just 5% nitro.

I suppose I am willing to agree with you on that seeing that they advertise a 15-30% increase in power by bolting on a jet stream muffler on one page and 25-50% on another page.

I suppose I have nothing to lose except a little time to try to build a Jet stream type muffler. The least that could happen is I could gain more experience and maybe even get something that works great.

That's how I would look at it. If you want/need any measurements from an actual Jett muffler, let me know.

Thanks 1Qwksport2.5r, In looking at the design of the jett stream mufflers, I noticed that the sport mufflers are a bit longer than the racing mufflers. So I figure if I start a bit longer than the sport size and slowly cut shorter, I should come up with a design that will give me the best RPM for the prop I plan to use. My can diameter is a bit larger than the jett mufflers so the dimensions might be just a bit different. I am going to make this first muffler for my ASP 91 and if it works well might make one for the 75. My biggest concern is if the welds and thin aluminum mousse can will hold up to the vibrations of the engine.

The red mufflers are the largest with the gold ones shorter/smaller, and the black ones smaller and shorter yet. The red ones have the widest prop range, gold is narrower, and the black mufflers are pylon mufflers pretty much tuned for just a couple of props. For the red .91 Jett muffler, the tuned range is 12,000-15,000rpm which I believe is the ground peak rpm.