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Cox engine experience: hopping down for a muffler, compression, nitro

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Cox engine experience: hopping down for a muffler, compression, nitro

Old 01-26-2008, 08:51 PM
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David Ingham
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Default Cox engine experience: hopping down for a muffler, compression, nitro

I think I almost finished off a Pee Wee. I put a muffler on it. The top plate was missing and I fabricated a nicely fitting brass one, without a lath. I put it on my Pee Wee with the least sub-induction, perhaps older than my others but started normally. At some point or other it seemed to get quite lean and hot, and maybe that is when it quit starting. I took off the head and put in a few drops of 3 in 1 oil then a prime of fuel, and it started right up each time. So I plan to put a can of 3 in 1 in my flight kit from now on for such cases. It really started nicely with the oil to plug the loose bore. I thought of this because I had gotten a totally loose larger engine (that I had bought from a fiend) to pop with 90 weight gear oil, a long time ago.
It then ran fairly well but only turned the 4 1/2D 2P propeller at 12–13,000 RPM. So I added two ounces of nitro to an 8 oz. can of 20% "sport" fuel. It then started but would not lean out, so I added a second head gasket. This improved things but it still quit when I tried to lean it out, so I cut the above 40 or 50 % mixture half and half with 20%, but I have not tried it again yet.
Next I got out the reset tool and tapped until the rod upper end got a bit tight in the piston wrist ball and worked it loose again. This improved the compression, but it still has less than another good cylinder and piston I have. Maybe I'll try tapping it more.
Next I started trying to eliminate the sub-induction of the good engine. I could not get solder to stick, even though I sanded off the black surface from the bottom of one exhaust/sub port and used plenty of flux; probably the soldering iron I filed to fit in the port is too small to get the cylinder hot. As second choice, I applied layers of medium viscosity CA to the bottom of the ports. That looks as though it might work, but it might come off in all that fuel and oil and heat and vibration. Maybe I will have go back to soldering, this time with a torch or larger soldering iron. Anyway, narrowing the port to eliminate sub-induction does not obstruct the exhaust area at all. I have 049s made without sub-induction but no 020s. I don't know whether they exist.
[Trying to get my picture to show.]
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Old 01-26-2008, 11:34 PM
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injunnut1
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

.....As second choice, I applied layers of medium viscosity CA to the bottom of the ports. That looks as though it might work, but it might come off in all that fuel and oil and heat and vibration. .....

Try JB Weld slow cure epoxy.
Old 01-26-2008, 11:41 PM
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David Ingham
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

Thanks. Did you use it for a similar purpose?
Old 01-26-2008, 11:43 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

That's an old head with "knurls"... Kool... I wonder if that was part of the insert clamping procedure?
The hardware store in the plumbing section has fibre washers that may fit. I found a washer that was just right for the TD .09 muffler in that style at an auto farts store in the fuel system / carb repair section
Old 01-27-2008, 12:04 AM
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David Ingham
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

Fiber might do and would be easy to trim. I am surprised you found one that really fit; the top plate "washer" ID and OD appear to have no reason to be the dimensions that are simple fractions of inches. It might work in a wide variety of sizes, but the corner of my top plate fits in the notch in the top rim of the muffler body. Maybe I am only thinking in these terms because I happen to be taking a jewelery class at the moment.
Old 01-27-2008, 02:50 AM
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David Ingham
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro


[quote]ORIGINAL: skaliwag

"That's an old head with "knurls"... Kool... I wonder if that was part of the insert clamping procedure?"

What do you mean by the "insert clamping procedure", Steve? By "knurls" do you mean those marks on top like on high compression 049s?
I know you understand how these things were maid better than most of us do.
Old 01-27-2008, 03:29 AM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

You are in a bit of a no win situation here-putting a muffler on any engine that has sub piston induction will markedly reduce power-by as much as 30% in some cases. This is a double whammy-the engine now draws in exhaust gas not fresh air via the sub piston induction-so this reduces the oxygen content of the fuel mix PLUS it is also drawing in HOT exhaust gas rather than air at ambient-so the fuel charge each cycle is less dense-again reducing power. There is no escape from this particular issue-if you reduce or remove sub piston induction, again you reduce power!
Pee Wees peak around 17,500-18000 on 25-30% nitro fuel-and normally tirn the 4.5x2 Cox at around 17,500 so yours is well down-but how much of this is due to a worn motor, how much due to the silencer, and how much to the fuel [not to mention an old glowhead] is anyone's guess.

Forget cyano-nitro dissolves cyano! All you are setting yourself up for is a sticky mess.the JB Weld suggested is a good option-but the best way is to get someone with the appropriate gear to measure how much sub piston induction you need to eliminate, and make up a shim washer of that thickness and fit it under the cylinder base. Forget trying to raise the lower edge of the exhaust ports by adding material! Raising the cylinder this way (by a shim) will lower the compression slightly, so you may be able to take out the second head gasket you put in.

There WERE SPI-less 049 cylinders made (for the QZ 049 for example) but AFAIK this was never done for any of the Pee Wees or TDs-the only variation was some were ground around the exhaust belt OD for an exhaust throttle sleeve.

'ffkiwi'
Old 01-27-2008, 03:37 PM
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David Ingham
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

Thanks ffkiwi. Yes, I usually see around 17,000 RPM with Pee Wees, so the power is way down. I like the shim idea, especially as I lost an airplane with two head gaskets on it and so am short of them. I probably have brass or silver the right thickness in my jewelery tool box. It seems that even though the sub-induction opening is small, it looks much wider than a head gasket thickness, but I can "mill" the head with good old wet and dry SiC. I am not sure how to measure things without a micrometer. My brother has beautiful old Swiss micrometers, so I can go to him as a last resort. Does anyone know how thick the head gaskets are?
Yes, of course the sub-induction was put there, in the first place, to increase power by adding charge and leaning the mixture at high RPM, so I will need both getting rid of the s-i and high nitro, if I want close to normal power. This was my cylinder with the least s-i, so a newer one is unlikely to do better.
Probably there aren't any 020s without s-i because people used not to complain about their sound. The muffler does kill most of the high pitched part of the sound, anyway.
Old 01-27-2008, 04:01 PM
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David Ingham
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro


ORIGINAL: skaliwag

That's an old head with "knurls"... Kool... I wonder if that was part of the insert clamping procedure?
The hardware store in the plumbing section has fibre washers that may fit. I found a washer that was just right for the TD .09 muffler in that style at an auto farts store in the fuel system / carb repair section
Here is a muffler plate I make with a lathe but in less time. The cylinder has no s-i but is not a QZ, so the top plate that came with the muffler does not fit it. This plate fits over the top instead of in the groove. It works, but it slips around, and I am always having to push it back to center.
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Old 01-27-2008, 10:52 PM
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David Ingham
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

You are right about the CA dissolving, ffkiwi. I put the cylinder in some of my hot fuel to soak, and it is becoming like soft rubber. At least it will be easy to take off.
Old 01-27-2008, 11:04 PM
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David Ingham
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

I tend not to take notes, but I think the engine must have lost compression by running on the un-doctored fuel with the two head gaskets. I think I actually added the extra head gasket while the tank was still full of the old fuel, and then leaned it down. I don't think a normally tuned cox engine will hurt itself, just from miss-adjusting the needle valve.
Also, my local hobby shop sells fuel mostly for cars, so this fuel may be a bit short of oil for my purposes.
Old 01-28-2008, 05:55 AM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

What about castor content? Couldn't see any numbers on that but castor is a must on these old style engines.
Adding that much nitro to a sport fuel would reduce the oil content which might mainly be synthetic in the first place...
Old 01-28-2008, 01:39 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

David I have just miked a brand new 020 head gasket out of the packet at 0.15mm or 0.006" . If you don't have a micrometer can I suggest you buy one of the electronic digital calipers that are around these days-these are good to about 0.0005" accuracy, flick between metric and imperial at the touch of a button and have a variety of modelling uses-more so than a micrometer. They are also dirt cheap-my local Mitre 10 hardware shop (I'm in NZ remember-so you won't find Mitre 10 in the US) has them on special for NZ $20 at present. You can probably find something similar at Harbour Freight.
Anyway I digress slightly-you'll also need a set of feeler gauges-just like we all used to have for cars when they had points! That is the easiest way of sizing the SPI gap. For example , just looking at a new TD 020 this morning when I was measuring the head gasket, the SPI is of the order of 1/32"-that is quite a lot-so in that case some 0.030 shim would be the answer for the 'washer' K&S does shim in a variety of thicknesses from 0.005" up-mainly in brass, but you may also find copper. Most model shops have a K&S rack for metal.
Cutting the washer is another thing of course, unless you have wad punches or the like-bear in mind its only the ID that needs to be accurate-the OD can be trimmed off with scissors or snips. In the unlikely event of your needing 1/32" or so (my impression-just from observation, not measurement-is that the ~1/32 I saw is an extreme case, and most are more likely to be 0.010-.015") you are probably better off making several thinner ones and stacking them. 0.032 brass is likely to be a bit tricky to cut a washer from accurately by hand.

I would reiterate the previous comment on castor fuel-it is essential for cox engines-mainly to protect the ball and socket joint-which takes a real hammering and needs the 'EP lubricant' effect of castor. Yes I know the little sods varnish, but that is the price you pay....

Actualy you have the opprtunity on this project, to see how much SPI influences the performance by progressively reducing it and test running on the same prop and fuel as you go........

'ffkiwi'

PS regarding nitro and cyano-the fact that nitro dissolves cyano has been a poorly kept secret since the early 80's-I keep a 35mm film canister of nitro not far from the building board for cleaning it off my fingers in the case of the odd 'oops'-its cheaper than buying the remover at the Hobby Shop.
Old 01-28-2008, 10:55 PM
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David Ingham
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

Do pharmacies still have caster oil?
Old 01-28-2008, 11:37 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

I have it ready to test, having nothing but my brass stock to measure with. (My cheap veneer caliper is no help.) Even if this try works, I may buy the tool you mention. (For ultimate precision, there are interferometers, by the way.)
Maybe I left my feeler gauges under the seats of cars I junked. (The one important electronic thing they put on cars is the ignition.)
One of my brass sheets from jewelery class is just about the right thickness. I made the shim with aircraft sheers, files and SiC paper, as I did the muffler top plate. It also is a nice fit. I made my best guess with what was available for the head thickness (I have more heads than head gaskets.) Of course I had to shorten the top of the cylinder, above its top fin.
I am testing on my porch and have had no complaints yet, but I can't test when my wife is home. I no longer have a garage.
Old 01-31-2008, 06:28 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

It ran at about 14,000 RPM with 20% nitro and 15,000 with 30%. Fuel, dripped on the head after it stopped, boiled vigorously, so I may need more compression to force a richer mixture.
Old 01-31-2008, 10:48 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

I found a micrometer at my large nearest hardware store, Orchard Supply, for $21. It is readable to about a ten thousandth of an inch but changed a ten thousandth while I held it. I is probably more stable after being in a room longer. I read a head gasket at three thou. and an older one at 2 and a half thou. So I took five thou. off the head and got it ready for the next test. (Some people use the term "mil", but I find that confusing because it sounds metric.) I also used my reset tool on it.
Old 02-07-2008, 04:27 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

Ok, I got back to testing. It did a nearly steady 17,000 RPM on my fuel mixture (around 30% nitro), with the muffler and the same 4.5D 2P propeller. So it is back to the same power as without the muffler. Fuel boiled vigorously on the head after it stopped, so it may be on the hot side? Maybe I need a couple of thou. less compression or more oil?
Still looking for caster oil.
It probably won't lean out to keep full power as speed increases, the way they do with sub-induction. It should behave more like a stunt engine, with the power dropping off when it flies downward. This way may be good for my purposes.
Old 02-07-2008, 05:11 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

you can order castor oil from sig it come in quart size
Old 02-07-2008, 05:42 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

I did find caster oil in the second drug store I looked for. I added half an ounce to the 8 oz. can.
Old 02-07-2008, 09:52 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

Hi Dave
I've been trying to understand this compression thing on nitro engines and I'm still not sure I have it understood. I'm used to car engines and it don't work the same. This link explains it, and from what it says you might not be going the wrong way. You might need less comrpession. Don't know if its right or not.

http://www.mecoa.com/faq/compression/compression.htm

Here is a link for overheating causes.

http://www.mecoa.com/faq/overheating/overheating.htm
There are many other interesting explanations if you to to the main page then faq.
Hope this helps. Terry
Old 02-07-2008, 11:11 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

Yes, as I remember, my brother who is an expert car tuner, says that one needs higher compression with more nitro, but I read the reverse not only stated but discussed on one of these model forums. I used 40% in an 049 reedie before, with a TD cylinder head, and with partial success. But that is not just higher compression, it has a different shape with more cooling aluminum between the filament and the main combustion chamber.
Trying those links.
What limits compression most on cars is detonation, combustion propagating in a shock wave, the local compression triggering combustion. (Apparently this not only changes the timing, but the energy goes out in the wave to burn the valves and pistons, instead of expanding the charge to push the piston down.) With glow, timing depends on how fast combustion propagates by heat transfer, Wikipedia calls it "deflagration". Maybe that is why it is not the same rule for both.
Old 02-08-2008, 12:28 AM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

Nice thread. I've found a Cox No. 3 muffler and want to use it on a reedie to fly at the nearest ball field during the summer months. This muffler differs from the QZ annular muffler by not having the black clip ring for opening and closing the muffler ring. The only opening is a 3/32" inch hole that I assume is supposed to be positioned over one of the exhaust slits (at least that's how I will put it on so I can prime with a hypo needle).

My only concern is with the fit of the black stamping that makes the seal on the top side of the muffler and this thread has given me some things to think about if I need to fabricate and install a better seal. Since I don't have any machining skills I'll be cutting and sanding phenolic resin sheet or scribing aluminum from the nearest K&S display. Anyway, all I want is an effective muffler so my son and I don't get any noise complaints. Nobody ever complained from the houses near the ball fields I flew control line on as a kid, but I don't expect the same kind of tolerance for an unmuffled .049 these days.
Old 02-08-2008, 12:02 PM
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David Ingham
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

With an 049, at least you won't need a shim under the cylinder. My Sure-starts, and maybe all other recent Cox 049 reedies have cylinders without sub-induction. I assume that later Tee Dees have large sub-induction like my old ones do. My most powerful non-sub-induction cylinder is from a Back Widow, but I don't know how that compares to the later engines. When I was last flying 049s, most of mine had mufflers.
The hypo sounds good. Another way to prime is to take the head off before each start.
Old 02-08-2008, 08:49 PM
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Default RE: Cox engine experience: hopping down for a muffler, compression, nitro

I've got what appear to be 2 Babe Bee piston and cylinder sets in the two slit exhaust, two bypass flutes, non SPI spec to use for my muffler project. In addition to the No. 3 annular muffler I've got a Cox car exhaust collector/header that might only muffle a little bit but will look pretty cool on one of the engines when polished. Both will get Sure Start style backplates so I can use wedge tanks on a Brodak trainer and a 1/2A stunter. BTW, I've almost turned a 1" OD 1/2" ID bronze washer into a new top plate for the annular muffler. The ID is now a pretty good slip fit 9/16th and I'm slowly working on the 7/8th OD by beveling one side and sanding down the other to match the inside of the muffler. The thinning from 16th to about 1/32nd will probably take the rest of the night. It's amazing what can be done with a flat file and a half round, good sand paper, and patience.

I'll see about adding pictures to this thread when the engines are built up and running.

The new top plate is finished. Nice tight press fit in the groove and thinned as much as I can with what's on hand. Final wet sanding should take it just about flush with the aluminum. Digital pictures tomorrow or Sunday depending on how long it takes to figure out how to get them up on this site.

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