What’s the difference…….

The Tee Dee design - rotary valve - lends a much smoother and wider channel for the air-fuel mixture to enter the crankcase, and the mixture is also better atomized by the 3 micro orifices on the venturi, than, through the single orifice of a spraybar in a reedie. Reed valves represent a flow resistance, and may also not seal completely when the descending piston pressurizes the crankcase.

In addition, Golden Bees have a single by-pass port in their No 2 cylinder, while Tee Dee-s have dual by-pass ports, further widened by 2 boosters per by-pass. The charging of cylinder is much more efficient in a Tee Dee than in any reedie like the Golden Bee.

So, in my opinion, the reason of performance difference between Tee Dee and reedie engines boils down mainly to the amount , and level of atomization of air-fuel mixture entering the cylinder...the high compression glow head of Tee Dee-s is a further icing on the cake.

deleted, double post, my apologies..

Thanks for the explanations gents.

joel45acp wrote:Thanks for the explanations gents.

This is a great resourse for general information about all Cox engines..  It will take you quite a while to get through it. Bookmark it.

https://en.wikipedia.org/wiki/Cox_model_engine

The nice thing about the Cox .049's is that one can swap parts to do their own performance enhancements. Swap out the single bypass Golden Bee piston/cylinder with a Tee Dee piston/cylinder, then you can have power approaching the Black Widow with the beauty of the Golden Bee tank.

I bought from Cox International a replacement R/C Bee piston, cylinder and rotating exhaust muffler kit several years ago (now sold out). I intend to replace the Medallion .049 single bypass cylinder with SPI (sub-port induction - the gap exposed by piston skirt lifting above the exhaust port allowing a brief puff of fresh air to augment the fuel-air charge for greater power), with this to convert it to R/C. The factory older Medallion R/C's had their power slightly reduced by the exhaust gases within the muffler recirculating back into the combustion chamber through SPI, diluting the fuel-air charge.

The dual bypass non-SPI R/C Bee cylinder will enhance power with the extra bypass and no SPI. If the muffler clears the venturi, then I'll have a higher performance Medallion that will exceed the power output of the R/C Bee (or Dragonfly). Also by experimenting with various prop sizes, fuel nitro levels and glow head types, one can optimize the engine flight characteristics for best performance in their particular plane.

So, these tiny engines allows one to experiment without breaking the bank. When I was in college 40 years ago, half-A R/C flight with one and 2 channel airplanes allowed me as a college student to fly R/C on a college budget. I had a lot of fun doing it, plus they were transportable on my motorcycle to the public flying field 15 miles away. A quart can of fuel lasted me for at least a half season. I could buy 2 or 3 half A props for cost of 1 larger engine prop.

Hi joel,

I keep things simple.
Just get it running, and get the needle valve set. Take your time. Sounds good? Throw a rag in the prop.
Fuel up. Fly it.
This is the beauty of Cox engines.
The difference between a single port BabyBee and a hot TD is not that much. (relative to fun)
Hauling a plane up with a Cox .049 engine is the fun.
If you are lucky, you will attract others.
Otherwise, a single port BabyBee is just as good, for having fun.
Your results may vary,
Dave

There are two major sources for COX engines, parts, bits and bangles, and all things fiddly that most of us like.  If you've not yet found them, here are the links.  Both owners are top notch and we are all appreciative of their efforts to keep the COX brand alive and well.

EX Model Engines

COX International

EX Model Engines is US based; COX International is located in Canada

Sorry for the short answer. I like this site:

https://www.mh-aerotools.de/airfoils/cox_frameset.htm

the main thing that makes a TD go faster is the rotary valve crank and tbe unimpeded fuel flow. If you put a slitted cyl on a td CK there is not a lot of difference. Any good 2 stroke performance tunning book such a Jennings will tell you the first and biggest power gain by modification is to increase the exhaust port a lot. and to raise the port and widen it. The only thing in normally aspirated engines that goes better than rotary valve is piston ported induction and cox never did that. reeds are a middle of the road, reliable good mass market solution that produces plenty of power and good midrange and are reliable and easy starting. On a reed engine with SPI the SPI closes the reed early as it reverses rhe pressure in the CK. far from convinced about SPI as no one else has ever bothered. i think if you got a slitted cyl and machined the bar out but didnt lower the port it would perform better on a reedy than SPI does. I think the main reason cox produced the slitted cyl was to get better engine life as it stops the piston wobbling in the cyl. I think they brought this out about the same time japanese gp bikes were going to bridged exhaust ports for this reason. the high comp heads and things help but the core reason a TD makes more piwer is all the benefits of a rotary valve induction which is well documented in 2 stroke performance tuning books as producing much more power than a reed induction system. but outright power is not everything. reeds start easy....... they have a very strong place for most uses.

Yabby

Yabby wrote:K. far from convinced about SPI as no one else has ever bothered.

Yabby

This statement is utterly incorrect-and totally in error-SPI has been widely used since the 40s-in both spark ignition, diesel and glow engines-in fact one of the 1940s ignition engines used only SPI as the air inlet, with the fuel being provided by a separate channel. Diesels in particular used SPI extensively from the late 40s and all through the 1950s-most notably Elfin in the UK, and Webra and Taifun in Germany-though neither were the first in their respective countries to use it. Mills Bros AFAIK were the first to use it in the UK-and gave ED a nasty shock -resulting in a quick redesign of the ED MkII to create the Comp Special-which is just a MkII with deeper exhaust ports-plenty of SPI, and a sensible head!

Both Elfin Taifun and Webra used SPI in quite considerable amounts-ie the whole depth of the exhaust opening-not just a few thou.....as did Frog when they started using it-in fact the significant improvement in power of the Frog 150R compared to the earlier models was largely due to the introduction of significant amounts of SPI in that model (and a slight contribution from the lighter piston that resulted...)

SPI only started taking a back seat when mufflers started to make a widespread appearance-and it was noteworthy that Cox learned early on-with the QZ model, that SPI cylinders and mufflers don't make a good combination. DC however failed to learn the lesson with the Wasp-which suffered severely when muffled.

There were good design and engineering reasons for using SPI in the first place-especially with front rotary diesels-it allowed the gas passage and intake port to be kept relatively small-thereby improving suction and handling, and markedly reducing the chance of crankshaft breakage-which was always a risk-especially with the quality of steels immediately postwar-if crankshaft bores were made larger in the quest for more power-and the mere presence of a port creates a stress point and weakness in the shaft.

ChrisM
'ffkiwi'

Thanks, Chris on the SPI history for model engines, especially the diesels.

Regarding SPI, I kind of gathered that for example, the Medallion .09 R/C with rotating exhaust throttle muffler, although reasonably lightweight, would be better suited as a hop up for a .049 reedie plane. Peter Chinn had tested the earlier exhaust throttle couple with rotating venturi spray bar with flat sides to alter idle mixture. It scored 0.14 BHP on 30% nitro. It had SPI.  

This was a far cry from Enya's .09-III TV, which did 0.16 BHP on 5% nitro.  

Too bad parts are no longer available. It probably would have been a very sweet engine with a later .09 TD non-SPI cylinder and rotating exhaust throttle muffler.

ffkiwi wrote:

Yabby wrote:K. far from convinced about SPI as no one else has ever bothered.

Yabby

This statement is utterly incorrect-and totally in error-SPI has been widely used since the 40s-in both spark  ignition
ChrisM
'ffkiwi'

I stand corrected as to the use of SPI by others. But along with many others not necessarily of this forum I dont agree that its quite what its cracked up to be on a glow engine. I guess I have not seen it used on modern high performance two stroke engines sych as bikes, boat outboards,....... Maybe it has been and is called something else as I couldnt find it. It certainly is not the same as Piston porting.  On a reed based engine It would seem to cause the reed to close early as the pressure /vacumn whichever that causes the gulp of air to come in is going to close the reed. I also couldnt find it in any two stroke performance tuning manuals as a performance technique/modification.

GaryB
Yabby

Probably why we don't hear about SPI on 2 cycle bikes, is they require mufflers, which negate SPI benefits.

I suggest that SPI in our engines has come about through three separate evolutions...the first is by accident-when the production tolerances stack up in a way that allows the lower edge of the piston skirt to clear the lower edge of the exhaust port as TDC approaches....this is likely to amount to only a few thousanths of an inch worth of SPI-and is a fairly random production occurrence; secondly by experiment-if people increase exhaust timing by increasing the exhaust port depth (note-NOT by shimming the cylinder..which would reduce/eliminate any previous SPI...!) then SPI may be incorporated and the result show an improvement over the earlier model....you quite often read in reviews "the exhaust ports in the current model are deeper than before" etc-or by deliberate design-either by arranging the cylinder geometry/rod length/gudgeon location to achieve this, or by relieving a portion of the piston skirt lower edge....and again this too could conceivably occur by accident -such as in a rear exhaust engine where the rear of the piston is scalloped or relieved to stop it hitting the backplate at BDC....in some cases people would have noticed a performance increase, and looked into it further.

I am not aware of any modern schneurle engine where it occurs other than by accident....and the point about mufflers is key. There is ample evidence-the aforementioned DC Wasp being one good example; the Super Tigre G20/23 being another:

http://sceptreflight.com/Model%20Engine%20Tests/Davis-Charlton%20Quickstart%20Wasp.html

http://sceptreflight.com/Model%20Engine%20Tests/ST%20G.20-23.html

take a look at the power curves in both open exhaust and muffled condition-and note the massive % power loss in both cases.....due entirely to having SPI-and hence drawing in burnt and hot-exhaust gas rather than fresh air-when a muffler is fitted....not only is there less oxygen present-but the gas being drawn in is now hot rather than at atmospheric temperature-so dilutes the fuel
charge density as well-a double whammy.....whereas with a well designed engine and muffler you would only expect to lose 10-15% off the open exhaust output under normal conditions.

ChrisM
'ffkiwi'