BMatthews

That's amazing performance for a reedie. Congratulations! Who's got the champagne?

When I suggested adding a deck shim I assumed that the compression would be adjusted to match.... at least that's my story and I'm sticking with it...

I've got a 290 product engine that I modified to take a TeeDee carb on the rear. It ran fine with the stock suction sized intake but with that small an intake hole I doubt I'd make it up to as high as you. I ran it in a free flight model with an APC 6x2 that really let it sing. I needed the 6x2 because it was an overweight pig of a model at 8 oz. A british free flight nostalgia model called the Slicker by Bill Dean.

t_kwink

BMatthews,
Your mentioning of the 290 engine got me thinkin'. I bet if one were to take the stock killer-bee backplate (large venturi), removed the needle, drilled it straight through to .200", and drilled/tapped it for a tee-dee carb it would perform just as well as what I've got with a little tinkering. Plus, it would be all cox.
The biggest gains I've seen, by far, were from:
#1 The Norvel plug. I would have never imagined a gain of 300 or 400 RPM, and they're cheaper! Modifying a burnt cox glow head for this was simple, and you really need to look close to even notice.
#2 Decreasing crankcase volume. This seems to help it more and more, the farther you go.

My biggest dissapointment (so far):
#1 breaking the crank while changing a glow plug. I hate it when I'm stupid.
#2 I'm still struggling with the AP carb. Either I got a bad one, or they have extremely loose needle threads.

Japanman

Wink, you have got that thing really ripping! I never would have thought the standard cox reed setup would rev that high!

Stefan

t_kwink

I'm up late, and I can't sleep.
I am a 4 cycle guy trying to learn 2 cycle, so bare with me...

The intake port is not only a port, but also a plenum.
The diameter of this port will dictate both flow and velocity.
The optimum diameter will be determined by a balancing compromise between flow and velocity.
After the optimum diameter is achieved, the length will dictate the total volume.
The optimum volume, I would think, would be a result of the engine displacement and the RPM you are tuning for.

Perhaps I should make another backplate, with a small diameter port, and test it, and enlarge it in steps until I find the best diameter.

Then, maybe I should shorten it in steps until it performs best.

Japanman, what seems impressive to you? Is it the RPM that it is turning this particular prop (power), or is it just the RPM (durability)?

Japanman

Power- those sound like good numbers.

The induction tract has been tuned to be at a helpful resonance in motorbike engines but I have not read of people doing it with glow engines- though by the looks of some fora combat engines I`d say they have- but possibly without realising it. On early discvalve engines at the birth of really powerful two-strokes by the guy who first worked with resonance, in one experiment the discvalve seized wide open with the engine at peak power... but it didn`t stop or even loose power because the resonance of the intake tract was doing all the work!

Stefan

t_kwink

Man, that is so cool!

I think maybe I should order a few books on 2-stroke theory. Not neccesarily for the purpose of building some extreme engine, but just to learn something interesting.

Wink

t_kwink

I was looking at my killer-bee project again, and I noticed that the top slot in the cylinder is used for exhaust only, not sup-piston-induction. I also noticed that the bottom slot is only used for SPI and is never opened to exhaust. Would it be possible to make some funky kind of tuned pipe for a cylinder like this that clamped around the cylinder, and captured the exhaust from the top slits, but let the bottom slits stay open to the atmosphere for SPI?

Wink

t_kwink

I converted my killer-bee project over to use a tee-dee carburetor, and ran it again:

5.7-3 APC prop, 35% Norvel fuel, Norvel glow plug, 1 glow plug gasket = 20,000-20,100 RPM

A solid 200 RPM gain, and it held a needle setting perfectly, unlike the AP Wasp carb.
I want to try the G-Mark carb I got from Tim, but I need to start from scratch to get it to mount up, so I tried the tee-dee carb first.

Wink

t_kwink

I nearly forgot, I also put it on a scale today. Ready to go, with a cox spinner and prop screw, and the 5.7-3 APC prop and adapter installed it tipped the scales at 1.76 oz.

Wink

Tim Wiltse-RCU

Wink,

Man that rocks dude! After seeing the results you have gotten so far the heck with the G-Mark carb!

Later,
Tim

mr_leffe

Did the AP carb have a throttle?

t_kwink

Yes, the AP carb had a throttle.

mr_leffe

Did you do any low # tests?

t_kwink

It seemed to idle down O.K., but I didn't mess with it much. I was mostly concerned with full throttle power.

Wink

t_kwink

I pulled out my reed valve engine and ran it again tonight.

I ran it with a tee-dee venturi that I recieved on an e-bay engine that had no screen, I am not sure if it was enlargeed or not, but no screen. I also ran it with a KK fine-thread needle and body. It spun the 5.7-3 APC at 20,200 RPM, a very slight gain.

I then ran it with three different Tee-Dee piston/cylinder combinations ranging from new to blown-out. The best one only caused a LOSS of about 3000 RPM.

The tee-dee cylinders have WAY too much exhaust port area, and in my opinion are total, complete garbage for anything under probably about 35,000 RPM! I am ready to throw all mine in the garbage. They are absolutely good for nothing, and I find it embaressing that they are sold on an engine labeled as high performance. The killer bee cylinder seems to be the most superior cylinder cox has made, and I have yet to find anything that I can even use as an alternative.

I wish cox would sell the killer-bee piston/cylinder seperately.

Wink

DICKEYBIRD

Careful there Wink, you're gonna stir up Bill Atwood's ghost!

ps: If you're serious, gimme a day or so to send you a pre-paid postage box to chunk 'em into!

t_kwink

Oh well, the heck with swapping cylinders, I have one that works...

Here is the project I am currently most excited about...

A ball-bearing reed valve crankcase.

Wink

jetpack

My Feedback: (1)

Hello,

I sure enjoy watching the progress here, very good documentation. I was wondering since you have the camera working if we could see more of the reed backplate that you have machined. Is there a sketch of it? I am a home machinist and would like to try one for a project.

How do the needle bodies get removed from the plastic backplate? I have already thought of using my only backplate for a TD carb body conversion, but was unsure about mangling it so I never tried. It appears the throat body is just the perfect size to run the 1/4" tap into. Is there any other conversions to it, such as drilling it to .200 as mentioned?

I noticed you haven't introduced fuel pressure to the mix yet. Where would be the best location and what to use for a pressure take off is what I wonder about when looking.

Nice going on the idea of a ball bearing crank. I've thought about using bearings but also have thought about using a bronze bushing also. Aluminum as a base material for a journal never has really thrilled me, there are cranks to be saved. Was it easy to find a bearing with the right dimensions? I see they are sealed bearings, do they also close off air passage?

Japanman

Wink,
That looks like the car crankcase... Looks good.
I have been thinking I should do something to hold up my end of all my fighting talk about the speedy coxes.
I know what you mean about the exhaust port area, but here is something for you. several years ago when I got my TD I had it on an airboat running an APC 5.6X3 and had a few twin slitt cylinders lying around to experiment on. First I cut the bridge out, then I raised the exhaust port and wouldn`t you know it, it ran stronger than my standard triple milled, TD cylinder. Fuel econemy did suffer a bit though.
I kept the cylinders long after they died by means if a tuned pipe (it seemed to eat cylinders for breakfast!) and have them here. I`ll have to try them with a bumped piston just to confirm how they run.

J.M

t_kwink

Interesting about the cylinders, JM.

This is not a car crankcase, it is a standard airplane one. I used 5mm by 8mm bearings, and a 5/16 endmill gave me about .0005" press in the crankcase. I had to turn the crankshaft down, and just completed that today. All I need to do is assemble it...
Wink


p.s. Jetpack, I'll draw something up a little later and post it.

t_kwink

Jetpack, here are some more pictures for you. It just takes a factory tee-dee venturi and carb, a factory reed valve reed and reed retainer. The depth into the crankcase was and exact copy of a standard cox backplate, the hole is drilled all the way through at .200" and tapped just as deep as it needs to be with a glow plug tap. The mounting flange is .060" thick.

Wink

t_kwink

By the way, I assembled and ran the ball bearing engine last night. It ran, and ran well. Unfortunately, I think I damaged the bearings during instalation as they are really rough, so I'm going to raplace them and try it again before I post any numbers.

In the meantime, I plan to A-B test a cox crankshaft and a Davis Diesel "heavy duty" crank.

Wink

AndyW

My Feedback: (1)

Wink,

Just came across this thread and was mightily impressed by your work and workmanship. A few comments if I may.

About bearings. I was in the bearing business for over thirty years and it doesn't sound like you damaged the bearings. The roughness you are feeling may be from too tight a press fit in the case or the shaft or both. If the press fit is too tight, you take out the clearances in the bearing and what you're feeling is the balls skidding and scratching as they try to roll. Such small bearings need extremely careful housing and shaft tolerances. I solved this problem when installing the bearing in my Norvels. I got a bearing that was a correct fit to the crank, just a slide fit. Then, the case was bored out ever so slightly oversize for a slip fit. Then the bearing was JB Welded in place. I know it sounds Mickey Mouse but it works. The bearing needs to be scrupulously clean on the outside, acetone or lacquer thinner. A great deal of care also needs to be exercised to not apply too much J B Weld. You don't want to have any excess foul up the works.

About your throttle. Of course, you know that I'm going to insist that it needs an adjustable airbleed. The G Mark throttle is the best ever made for 1/2A and you really should give it a try. But,, IT NEEDS AN ADJUSTABLE AIRBLEED. Sorry, just can't help myself.

I agree with you that Cox cylinders have far more exhaust than necessary. This is/was true schneurle porting. Today's schneurle porting is actually a modified version of the original and has two bypass ports, one BOOST port and ONE exhaust port. I've made up cylinders with the latter and they worked well. I was concerned about distortion as the cylinders warmed up but this was not an apparent problem.

My feeling about bearings, as Jetpack may be suggesting, is that they may not contribute a lot to performance even though they ARE a cool idea. Our shaft diameters are small and the smaller a shaft is, the smaller the frictional area. Ball bearings are useful on larger engines/cranks for this reason. What we need is, maybe, bushings that are extremely hard, harder than the crank material.

Or, anodizing the case may be useful. Anodizing will be harder than the base aluminum, even decorative anodizing. Anodizing also has a porous surface that holds oil that contributes to friction free running.

But in any case, your rpm numbers are impressive. Congrats on some quality, innovative work.

t_kwink

Andy, the bearings were slightly more than a slip fit in the case, but not bad. I could press them in about half way by hand, and then needed to press them the rest of the way. I like the fit on the crank; I would call it "slight drag" as you slip the crank through. It probably has .0002"-.0004" clearance. The bearings I used were cheap car bearings made by duratrax? with some sort of fiber seal on them I have ordered, but not recieved, a few ABEC 5 stainless bearings from Boca bearings. I will try to open the clearance in the case a touch, if I can get it in the fixture that accurate again. It gets tough when you only want to bore a couple tenths. (at least for me)

I know what you are saying about the low bearing surface area of a small diameter shaft, but I think it's worth a try. I was thinking that it might just all be relative--meaning it's a small bearing surface, with small loads, but at the same time, a very small decrease in friction might have a significant effect. It did pick up a good amount of RPM, even with the rough bearings. I was also initially concerned about the extra rotating mass of the bearings (the inner race and all the balls). I know in larger automotive race engines, some times roller bearings are used on camshafts, but they are not used on some types of race courses that involve a lot of acceleration/deceleration and a lot of RPM fluctuation because the extra mass of the bearing parts can actually decrease the engines ability to accelerate, but on a race course that is run wide open, or close to it, the roller bearings will show an increase in power due to the decreased friction. This can actually be seen when a race motor is tested on a dynomometer. There are two basic types of dyno, a brake dyno, and an acceleration dyno. An engine with roller bearings will make less power on an acceleration dyno (where the engine spins a large flywheel, and the rate of acceleration of the flywheel is timed, and calculated into power) but will make more power on a brake dyno (where the engine runs a water pump or electric generator and the amount of water pumped or electricity generated is measured and calculated into power).

I was curious, how do these littly bearings react to side load? What I mean is, if I use just the inner race of the bearing as the thrust surface, will I hurt the bearing? The engine isn't capable of producing more than a couple pounds of thrust, but at that high RPM, could it damage them?

Wink

AndyW

My Feedback: (1)

Kwink,

Hard to tell if the fits are OK but it sounds right. But cheap bearings may be the issue here. The thing about ball bearings are that they're meant to take fairly heavy radial loads compared to sleeve bearings and they kind of depend on those loads to keep the balls rolling. At very high speeds and light loads, the balls can slide instead of roll. On one of my slip fit affairs, I found the crank polished where the inner race sat. In other words, the drag of the ball assembly was more than the drag of the inner race on the shaft. To prevent this I used red Loctite.

Side load capacity on typical ball bearings can be surprisingly high. I don't think that's a big concern here given that in the vertical a typical 1/2A at 25 ounces will impose no more than that when hovering, for instance. Twenty five ounces ain't a lot of load, even on very small bearings. Static load is one thing but dynamic loads are not an issue. But come to think of it, there are reasonable thrust loads so maybe the balls aren't skidding.

Oh, wait. You may know this but just in case. You have got to press the bearing in by applying pressure ONLY to the outer race. Any pressure (static) to the inner race can dent the balls and raceways. Again, static loads are an issue, dynamic loads much less so. I save the outer rings of kaffed bearings to use to press in bearings. Save your old ones for this purpose.

Great info on automotive race engines and it makes sense. Makes one wonder why the rods on our engines aren't just made bullet proof and be done with it.

And well, if with rough bearings you got a good gain, you're certainly on the right track. I did up a Texaco with a fat front end and we were kidding around about putting ball bearings in it and here you've gone and done it. Cool, I may give it a go. Wonderfully inspiring stuff on RCU.