Re: QZ engine questions

KariFS Fri Jan 16, 2015 2:33 pm

I really enjoy this conversation Very Happy

Two-strokes are fascinating engines, my first ever motor vehicle was a moped, and with it I learned about wrenching, modifying, tuning and other things. The "rules of thumb" were that wide ports produce torque, tall ports give rpm. So I took a file to the cylinder and... Well, my top speed increased from (the legal) 40kph to over 60 Smoking

And that was all rpm, no change to the gear ratios. The head was also lowered a little and the gasket omitted. Acceleration was better than stock too, so there was no torque loss either.

I did discover that a fully open exhaust (header pipe only) produces mostly noise, and gives slightly higher rpm but only without load. Under load it was worse than stock. A two-stroker needs some back pressure to work well and with my moped a slightly modified standard silencer was the best combination of subtleness and power. In Finland all modifications to mopeds are illegal, so I rather kept the stock look. I had a standard carb too, but the main jet was bigger and induction silencer was ditched.

This discussion almost makes me wish that I still had that moped, I would definitely try the SPI concept on it, even though it was not a reed engine. Just a little hole on the piston skirt so that it meets the exhaust port at TDC...

GallopingGhostler Fri Jan 16, 2015 2:39 pm

balogh wrote:Indeed there is. Shown here are two genuine COX exhaust mufflers, the one on the right is with the pipe. The one on the left has a strange silicone cuff encapsulating the muffler with no opening left. Exhaust gases are pressed out between the cuff and the rim of the aluminum housing. This really is a choke, I wonder how the engine performance suffers with this type.

I got one of those on the Cox R/C Bee. The silicone stretches over the larger top diameter section of the cuffed throttle sleeve. The bottom has a slight air gap. As it runs, the silicone stretches a little, sort of "burbles". Thus, it is not a choke in the sense of hard materials. Seems to work effectively, although I have no tach to give quantitative results.

As with any muffling action short of a tuned expansion chamber muffler, there will be some RPM loss, often where not needed I'd simply pull it off. Then it operated as a standard throttle sleeve. That was easy as no tools were required.

Not sure why Cox discontinued it as it would seem cost effective, only machined metal part was the aluminum throttle sleeve with flanges for the silicon muffler cover. The silicon is very resilient and with exception of internal burn marks, is still intact and still works. Since these are no longer made, once mine fails, only solution is to replace it with Cox metal throttle muffler.

GallopingGhostler Fri Jan 16, 2015 6:54 pm

KariFS wrote:I did discover that a fully open exhaust (header pipe only) produces mostly noise, and gives slightly higher rpm but only without load. Under load it was worse than stock. A two-stroker needs some back pressure to work well and with my moped a slightly modified standard silencer was the best combination of subtleness and power.

Moped travels from idle to full throttle. Model airplane engines in simpler RC planes and CL run wide open. There, they put out more power without muffler unless a tuned pipe is used. Muffler tends to rob power, amount depending on chamber size.

I've removed mufflers off RC and CL where I didn't use throttle or had an RC carb wired wide open.

On my RC planes with throttle, I leave the muffler on as it needs the back pressure for a reliable idle.

Regarding your moped needing back pressure, when muffler is removed exhaust goes lean and carb must be rejetted. We have at least one with our motorcycle chapter, who has open pipes. His bike is loud, so loud that few like to follow him close. There, the previous owner rejetted. Although 4 stroke, with reduced back pressure, carb was jetted for enrichment.

Personally, I prefer the stock pipes on my touring bike, as it is more comfortable for riding distances.

Just too bad that prior to demise, Leisure Dynamics / Cox didn't come up with a muffled .061 reedie with .049 spaced mounting. Slight upping in size with modest weight gain would have redefined half-A flying, IMO.

OVERLORD Sat Nov 26, 2016 11:11 am

I'm rebuilding a QZ engine and I have a question concerning the type of muffler to use. I have 2 different types:
1. with a rectangular and a round opening with octagonal base.
2. with 2 rectangular openings with hexagonal base.

Which of those 2 is from a QZ? Or are they both?

QZ engine questions - Page 2 Imgp1810

Another thing, when sliding them on the same nr 6 cylinder, the cylinder screw thread protrudes on one and not on the other.

QZ engine questions - Page 2 Imgp1812

After looking further into this, I found out that the steel top ring for this type of mufflers is different. One ring slides over the bottom fin of the cylinder and the other doesn't. Maybe a difference to fit or the older nr 6 cylinders and the later unmarked slit exhaust cylinders?

How does the muffler needs to be mounted, with the round hole towards the needle? What about the other one?

To complete these mufflers I am looking for the steel regulator rings that fit onto the aluminium housing. Maybe someone has got them lying around?

Lieven

KariFS Sat Nov 26, 2016 1:35 pm

Lieven, it looks like both of your mufflers have been modified. I have 3 mufflers in different engines and all of them have only one rectangular hole that is covered with the steel part. Then of course the exhaust "stack" itself, which is not seen in your pictures... Do your mufflers have them?

The black steel ring can be installed two ways. If you have a starter spring, then you should install the steel ring so that the "lip" points downwards. Without the spring the muffler sits a little lower, and to eliminate the slack you should install the lip upwards.

OVERLORD Mon Nov 28, 2016 1:07 pm

KariFS wrote:Lieven, it looks like both of your mufflers have been modified. I have 3 mufflers in different engines and all of them have only one rectangular hole that is covered with the steel part. Then of course the exhaust "stack" itself, which is not seen in your pictures... Do your mufflers have them?

The black steel ring can be installed two ways. If you have a starter spring, then you should install the steel ring so that the "lip" points downwards. Without the spring the muffler sits a little lower, and to eliminate the slack you should install the lip upwards.

According to me, the mufflers are unmodified. The one with the hexagonal base seems to be fitted to earlier QZ's. The mufflers with the pipe are more of the QRC type engines. I've never seen one of them. The mufflers I have all have just a hole next to the rectangular opening. Concerning the steel ring, that's what's written in the Aeromodeller article. The author must have never tested that because it doesn't work. That's a general problem with journalists, in the end, they get so full of themselves that they start inventing!! When leaving the starter spring away, the best thing to do is to wrap a small electric wire around the base to immobilize the muffler.

KariFS Mon Nov 28, 2016 2:20 pm

Lieven, you are right on both accounts. I tested the ring thing with 3 different cylinders and it didn't work. And you also have a type of muffler that I have never seen before.

Sorry for spreading misinformation Embarassed

Anyway, about what you said about the wire wrapped around the base, made me look at the INSTRUCTIONS Affraid or WOW!

Ah, the dreaded I-word... Here's what they said:

QZ engine questions - Page 2 Image25

Both kits contain a spacer ring like the one you described, but the instructions of the standard muffler (#2630) instruct the ring to be installed above the muffler. That's how it was installed on my Medallion too. And also on Medallion, the muffler would not only rattle about without the ring but turn around quite easily.

The RC muffler throttle (#3615) has a ring too but it is supposed to be installed to the base of the cylinder, and it also has its ends bent so that it won't spin with the muffler.

None of the above helps with your original dilemma, of course

Edit: This shows why I thought your muffler with the small round hole may have been modified:

QZ engine questions - Page 2 Image26

I thought the rectangular hole was original and the small round one was where the exhaust pipe had been attached. It is a separate part and is sort of "riveted" on as you may be able to see.

roddie Mon Nov 28, 2016 9:34 pm

Just a reference post. My Cox Cat.#3615 muffler kit.

QZ engine questions - Page 2 Cox_ca10

Oldenginerod Tue Nov 29, 2016 2:19 am

The muffler Roddie shows looks identical to the one on my R/C Bee, except mine does not have the spacer ring. Actually, I'm not really sure it is a spacer, rather, I think it's a friction ring to firm up the rotating muffler. Mine is a little loose and could rattle around. As mine has a snap-starter spring, the friction ring would need to go on top as I understand it. I'd have to see it to know how that would work.

KariFS Tue Nov 29, 2016 2:35 am

Here's one more style of a muffler throttle:

QZ engine questions - Page 2 Image29

This one doesn't have the ring on the bottom, not sure if it should? The instruction sheet did not list the parts, I'll have to look it up. This one seems to be somehow pressed together, see the knurling(?) between the lock ring and the muffler chamber.

QZ engine questions - Page 2 Image30

batjac Tue Nov 29, 2016 8:01 pm

GallopingGhostler wrote:
Engine puts out 0.065 BHP at 15,000 RPM on 15% nitro. Peter stated a 6x3 prop was well suited as it fits well within the power band; 13,400 RPM to 14,100 RPM.

Hmmm.... I never thought about this. I've always pretty much just gone for max RPM and been happy with that. But I guess it would make sense to prop the engine to keep it in the middle or upper end of the max power band. I'm not sure that I'm interested enough to grab a bunch of props and test fly them on a plane in matching conditions to see how the plane performs while needling the engine to stay in the max power band. I doubt it would really make that much difference in how I fly.

I'll have to think about what times would be beneficial to stay in the power band.

The Thinker Mark

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