RE: 25cc 2-stroke conversion
 

Foamcut,

The mufflers from Ken are 1” x 2” x 2 1/2” outside or .750 x 1.750 x 2.125 inside. The volume is 46cc. And yes the mufflers are not particularly quiet.

The original muffler without packing is slightly more than twice the volume of Ken’s muffler. A 16 to 1 ratio would be hard to achieve. I believe a ratio of 4 to 8 would be achievable and quieter but don’t have the material or equipment to experiment. 1 1/2” by 3” steel tubing would provide a starting point for experimentation.

Bill

RE: 25cc 2-stroke conversion
 

Bill,

I'm curious what the outlet tube diameter is on the Lambert mufflers. In a prior thread I believe you were getting an 800 RPM increase from the stock version.
I'm wondering if the larger volume design (gutted stock muffler) would benefit from larger outlets.

Thanks

RE: 25cc 2-stroke conversion
 

Bill,

I scanned a page from the August 2000 copy of the AMA magazine. In that article the author describes a 2 chamber design. he seems to indicate that the ratios between the chamber length as being critical to the design. I was unable to save a copy of the scan (pdf format) in my gallery. Let me know if you'd like a copy emailed to you. This might be information tha could be incorporated into a two-part Pitts design.....one more avenue to explore...

RE: 25cc 2-stroke conversion
 

Foamcut,

Ken’s muffler is good for a sizable RPM increase. There are two outlets and the ID is .580” each. I thought the stock muffler needed outlet pipes but don’t have brazing equipment.

Bill

RE: 25cc 2-stroke conversion
 

Foamcut,

I owned a Yamaha motorcycle dealership during the 70’s and watched many very skilled people experiment with 2 stroke exhaust systems.

The thermodynamics of muffler design for 2 strokes is very complicated and subject to much experimentation. The stock muffler appeared to have two chambers forcing the exhaust gas to travel further than it appeared.

Bill

RE: 25cc 2-stroke conversion
 

Interesting, I shall measure my Pitts style muffler and find out it's volume. It's your typical rectangular muffler with no baffles or chambering and a simple dual pipe outlet. I do know that it makes a significant difference to peak rpm but not the exact measure. This weekend should provide an opportunity for some measurements to quantify that.

RE: 25cc 2-stroke conversion
 

Check some of the other forum sites. Some fellas were using the new Bud Aluminum Beer Bottles for fabricatiing exhausts for gassers. Maybe Flying Giants site.

16 oz gets pretty close to your 16 to 1 on a 25cc!

RE: 25cc 2-stroke conversion
 

Mike,

That's really good feedback. If you find a particular thread on the topic please post a link.

Thanks

RE: 25cc 2-stroke conversion
 

Foamcut,

The interest in muffler design is high but I don’t have the necessary equipment to build various designs for experimentation. Measured the engine exhaust bolt pattern at 32mm. For all useful purposes 1.250”.

From the ‘What a Deal’ thread and the following mufflers are a close fit:
Poulan 25
G-23
Evolution 26

I would like to know the effect of larger volume mufflers is anyone experiments.

Bill


Seems that you sent the ice to the east coast.

RE: 25cc 2-stroke conversion
 

Maybe the exhaust used on a MVVS 26 would work good. Or one like it! Does that give you any ideas!!! Capt,n

RE: 25cc 2-stroke conversion
 

Capt’n,

I have the throttle shaft out of one of the carbs. The shaft is simply a .185” dia. shaft with a flat cut for the throttle plate, which could be easily made with nothing more than a file. Likely someone would have a similar shaft from a junk carb.

Did you make any mufflers?

Bill

RE: 25cc 2-stroke conversion
 

Bills: Not yet, I have installing all new fixtures in the bathroom...total make-over! Then the guy that does my welding is a poontoon boat builder. I have not seen him at his place of business in the last eek or so. I will keep checking. I was wondering why a person could not use a Map gas torch and silver solder a muffler (made of thin steel) using silver solder. I got to find out that also. Take Care, Capt,n

RE: 25cc 2-stroke conversion
 

Capt’n,

If I could find thin wall steel tubing anywhere near the correct size (2 3/4” x 1 1/4”) I would attempt to kluge something together. A large volume muffler could be made and volume could always be packed later for various experiments.

Bill

RE: 25cc 2-stroke conversion
 

Bills: Check this out! http://store.cyberweld.com/tukitsnseigp1.html Seems like this should work or something simular. Capt,n

RE: 25cc 2-stroke conversion
 

Hi, Please read my post under the thread "Dont forget Echo" if you make these mods to your engine I can see no reason it wouldnt perform the same.

Keith
BME

RE: 25cc 2-stroke conversion
 

Keith,

That's a great suggestion. Thanks for steering us to that thread.


Did you say that you still have some of the lightweight flywheels from your BME-24 design?
I'm wondering if they would fit this engine and make for an easy weight reduction on this engine.

Do you have prop hubs that would fit after removing the flywheel (for a glow conversion) on the BME 24?
I'm wondering if that type of hub would also fit these Mac25 engines. The major shaft diameter is .474" and the threads appear to be M8 X 1. I have no idea how to measure the taper.

Thanks again for your input.

RE: 25cc 2-stroke conversion
 

About the bud can mufflers, go to flying giants.com, forum, rednecks taking over the world, quietly.

RE: 25cc 2-stroke conversion
 

Bills: Check this out for welding a muffler. Capt,n http://store.cyberweld.com:80/tuextxprto.html EDIT...Will this silver solder hold up on exhaust temps. My guess is yes...not sure!

RE: 25cc 2-stroke conversion
 

Capt’n,

Soft silver solder melts about 1150 degrees F and hard silver solder melts about 1600 degrees F. Equipment is much easier to acquire than talent.

Bill

RE: 25cc 2-stroke conversion
 

Typical . . . . .

Aluminum Solder-It (ALP-21)

flows at 395F far below the melting point of aluminum. It is used to solder many types and gauges of aluminum as well as to solder aluminum to most other metals. It is especially useful for joining dissimilar gauges of aluminum. Use Aluminum Solder-It for aluminum tubing, aluminum antennas, aluminum radiators, air conditioning condensers, refrigeration, aluminum boats, doors, windows, anodized aluminum and aluminum foil. Great for constructing and repairing Remote Controlled Aircraft Parts and cosmetic repair of Homebuilt Aircraft/Parts.

Aluminum Paste $6.00

Aluminum itself melts around 1280F or so

RE: 25cc 2-stroke conversion
 

w8ye,

Don’t want to question your judgment but have you used low temperature solder on a muffler for a gas engine?

I think you will find that “Exhaust gases emerge from the cylinder at about 1200 degree F and the temperature in the exhaust pipe will often be 800 degrees F.” Gordon Jennings.

Bill

RE: 25cc 2-stroke conversion
 

I stopped into Low,s and Menards. Both places have 2 kins of brazing torches. One uses MAPP gas only and the other has 2 small tanks...one MAPP gas and Oxygen tank. The one with Oxygen claims to weld steel to over 3/16 inch thick. I think the temp can reach well over 3800 degrees on the 2 tank model. The price was $39 for MAPP only and $49 for MAPP & Oygen. I could not locate any silver solder yet to read about it. The tanks was about $8 for Oxygen and $6 for MAPP gas. The first set of tanks comes with torch. Also at Menards the had aluminum welding rod with flux on it ....to be use with a DC welder? The search goes on! Capt,n

RE: 25cc 2-stroke conversion
 

Here is sone words about brazing. Capt,n Advantages of brazing
Although there is a popular belief that brazing is an inferior substitute for welding, it has advantages over welding in many situations. For example, brazing brass has a strength and hardness near that of mild steel and is much more corrosion-resistant. In some applications, brazing is highly preferred. For example, silver brazing is the customary method of joining high-reliability, controlled-strength corrosion-resistant piping such as a nuclear submarine's seawater coolant pipes. Silver brazed parts can also be precisely machined after joining, to hide the presence of the joint to all but the most discerning observers, whereas it is nearly impossible to machine welds having any residual slag present and still hide joints.

The lower temperature of brazing and brass-welding is less likely to distort the work piece, significantly change the crystalline structure (create a heat affected zone) or induce thermal stresses. For example, when large iron castings crack, it is almost always impractical to repair them with welding. In order to weld cast-iron without recracking it from thermal stress, the work piece must be hot-soaked to 1600 °F. When a large (more than fifty kilograms (100 lb)) casting cracks in an industrial setting, heat-soaking it for welding is almost always impractical. Often the casting only needs to be watertight, or take mild mechanical stress. Brazing is the preferred repair method in these cases.
The lower temperature associated with brazing vs. welding can increase joining speed and reduce fuel gas consumption.
Brazing can be easier for beginners to learn than welding.
For thin workpieces (e.g., sheet metal or thin-walled pipe) brazing is less likely to result in burn-through.
Brazing can also be a cheap and effective technique for mass production. Components can be assembled with preformed plugs of filler material positioned at joints and then heated in a furnace or passed through heating stations on an assembly line. The heated filler then flows into the joints by capillary action.
Braze-welded joints generally have smooth attractive beads that do not require additional grinding or finishing. The most common filler materials are gold in colour, but fillers that more closely match the color of the base materials can be used if appearance is important.

RE: 25cc 2-stroke conversion
 

Tower part number LXASN4 looks a lot like the original muffler with an exhaust tube. Wow $40.

Bill

RE: 25cc 2-stroke conversion
 

Is "wow" mean too much$$ or good deal????:D:D Ca[t,n

RE: 25cc 2-stroke conversion
 

Appears it would leak around the tube joint to me?

It's just tacked in three places with the stinger?

If you could get the cad coating off, it would be a good candidate for brass brazing to prevent leaks?

RE: 25cc 2-stroke conversion
 

captinjohn
I would think a good deal, $40 muffler for an engine that costs marginally more.[sm=72_72.gif] Sure would not want it to leak anything an inch from the outlet.

RE: 25cc 2-stroke conversion
 

Well guys the weather was pretty bad today at our field, definitely not weather for flying my big plane. Still I test ran the engine with the pitts muffler fitted (after removing the gutted muffler) and came up with some interesting results. I'll start with my original 25cc conversion as a baseline and then list the 30cc figures. Both engines are similar with differences noted:

25cc with pitts muffler - Best rpm with 16x8 prop 8000rpm
30cc with gutted muffler - Best rpm with 16x8 prop 7300rpm (note the only difference from the 25cc was a different spark plug).
30cc with pitts muffler - Best rpm with 16x8 prop 7430rpm (same as previous except for pitts muffler with a 10mm spacer to fit the muffler clear of magneto).
30cc with ZERO muffler - Best rpm with 16x8 prop 7610rpm (near zero throttle response, either a high idle or near full revs)

Something doesn't seem right here, I would have expected the 30cc engine (particularly the third listing) to have turned at least 8000 with a potential for 8300rpm. This is now making me think what differences there could be between the piston and cylinder (only parts that vary). I suspect the port timing to be different or that the 2nd ring on the 30cc adds some massive drag (weird).

Future testing will involve the same engine refitted with the 25cc piston/rod and cylinder. If the rpm is 8000rpm then something is definitely different with the 30cc parts, which would most likely be the port timing or transfer ports.

RE: 25cc 2-stroke conversion
 

One of my 30's is completely stock except for the muffler and carb. It turns a DynaFlite 18-8 at 7500 rpm.

RE: 25cc 2-stroke conversion
 

Too much!

RE: 25cc 2-stroke conversion
 

Raven,

There does seem to be something wrong with the numbers between the 25 and 30. The numbers just for the 30 seem to be OK. I have seen a similar circumstance where the prop number seemed out of whack but it turned out that a different prop was being run. I get a 500-RPM difference on two props that are both 16x8 (MA classic vs. MA “K” series).

Data for same 25cc engine:
8000 RPM MA ‘K’ series
7500 RPM MA classic

Bill

RE: 25cc 2-stroke conversion
 

The Raven: Is the 30cc Homelite engine broken in good yet? Some engines take a long time to loosen up and get the higher RPM,s. Capt,n

RE: 25cc 2-stroke conversion
 

I've discovered what I believe to be the cause for the rpm discrepancies between the 25 and 30cc Homelites. I stripped down my 30cc and converted it back to 25cc last night and while doing so carefully examined the ports. While the intake ports look similar in size/shape/timing the 30cc has a very late exhaust port opening when compared to the 25cc. It looks like I could port that but as the exhaust port is angled rather steeply I need to research it some more.

Can anyone provide me with some good port timings that they have used for a 30cc Homelite? I'd prefer something proven that can have a prop size and rpm measurement provided.

Also, can anyone recommend a good site that explains how to measure the port timing? I understand the concept but as I've never done it before it would be best to review the correct technique.

RE: 25cc 2-stroke conversion
 

The angle of the port might not indicate anything about the exhaust duration. The only way to know is to measure both engines.

Bill

RE: 25cc 2-stroke conversion
 

Maybe this will help:
http://www.macdizzy.com/cylinder_map2.htm

RE: 25cc 2-stroke conversion
 

True. However, my main concern is when the port opens. It's relative duration may be the same but if the open/close events are much later on the 30cc.

Trying to describe the cylinder difference without a picture: While the muffler flange is in the same relative position (to the 25cc) the 30cc port descends to a much lower position at the clyinder bore wall. This would make the opening and closing events occur much later. As this is the only significant difference between the two engines it is the most likely cause of the performance differences.

I will degree the engines once I sort out a degree wheel and get an idea of what some of the more 'optimal' timings should be for aviation use.

Can anyone provide their port timing and performance figures, it would be greatly appreciated?

RE: 25cc 2-stroke conversion
 

Not quite what I was looking for but a wealth of information all the same. Thanks.

RE: 25cc 2-stroke conversion
 

Searching the RPM forum for Homelite gets interesting reading and probably a few lies.

Measuring exhaust timing.
http://www.rcfaq.com/ANSWERS/ENGINES/TIMING.HTM

Couldn’t find my degree wheel, which had not been used in twenty years and used the following kludge. One engine came out 155 and the other 157.

Bill

RE: 25cc 2-stroke conversion
 

Well check out issue #13 of ModelGasBoats.com Engine Porting
This should get you there.
http://modelgasboats.com/magazine/in...d=72&Itemid=47