GrahamC

There has been much chat over the years about replacement heads for Cox type 049 size engines and their performance.

It is often stated that using a standard 1/4"-32tpi glow plug replacement head is a sure way to kill performance. To that I will not argue but I do believe there is more to it than what has been offered in the past on this subject.

Two more recent such discussions can be found here:

Question - Mecoa Glow Plug Adapters

http://www.rcuniverse.com/forum/m_76...tm.htm#7656821

Galbreath Head / Nelson Plug - I'm a believer!!!

http://www.rcuniverse.com/forum/m_72...tm.htm#7282911


One of the biggest issues when starting to compare the standard Cox glow plugs to the various replacement heads that have appeared over the years is that these comparisons are ultimately apples to oranges. There has been little if any attempt to optimize size, shape, and volume of any replacement head to the standard Cox glow heads in order to get a fair comparison. Now, that is not to say that the makers of these replacement heads did not make an effort to mimick the Cox products as I am sure they did.

The waters get muddied even more when you start to try and compare the Galbreath head to Cox heads, to the Norve, to the AP, to the.... well, you get the idea.

So, how do we do as fair a comparison as possible? Well first order of business is to measure the head volumes so that we know what we are comparing. AndyW planted a seed in one of his posts quite some time ago in his description of using an eye dropper and counting drops required to fill the combustion chamber of the head/plug combination. I tried it but found it far to coarse a measure.

It took a bit of pondering to come up with a workable solution, at least one that gave reasonable and repeatable measure. I ended up using a 3cc syringe with a 20gauge x 1" needle as my tool with which to measure combustion chamber volumes.

First it needed to be calibrated. I filled the syringe to the 1cc mark, removed any air bubbles, and counted the number drops needed to empty the syringe. I repeated this 7 times, threw away the high and low counts, averaged the remaining 5 and rounded to the nearest whole number. In my case 250 drops = 1 CC

So, now I have a tool to use to make relative volume measurements of the various heads and plugs. Following the same process of measure 7 times, throw away the high and low, average and round to the nearest whole number I got these numbers:


Head / plug volume (drops)

AP061 17
Norvel 15
Cox #325 27
Cox 1702 22

Galbreath/Nelson 15
Ron Valentine (long plug) 35
Mecoa "Red" (long plug) 34
Mecoa "purple" (short plug) 35

Right away it is easy to see why there is such a loss when using one of the replacement heads which use a standard glow plug! They all have more head volume that even the old gold standard Cox 325! A sure bet to lose power. It is also easy to see why the Galbreath and Norvel type heads/plugs are an improvement over even the Cox High Compression 1702 head. The Galbreath/Nelson combination is less than half the volume of the Valentine or Mecoa replacement heads!

Another thing to note is that the Galbreath and Norvel style heads also have a squish band that is wider than the Cox 1702 plug. A "hop up" for the Cox engines about 30 years ago was to machine about .015 off the bottom of the 1702 plugs thus increasing compression and creating a wider squish band. This simple mod was good for nearly 1000 rpm.

I decided that in order to better compare the Galbreath/nelson combination to a head with a standard size glow plug, I would need to make of modify a suitable head. I chose to modify the Mecoa "Red" and "Purple" heads in order to reduce their combustion volumes to the same size as the Galbreath.

The mods where simple and amount to about 5 minutes of machine work in the lathe but that is after about 15 minutes work to make up a mandrel to hold the heads while the work is done.

The modifications made amounted to removing .045" from bottom edge of the head, removing the lower head fin and cutting a small shoulder on the bottom of the head so that it will seat properly in the cylinder.

After modifications the Mecoa head volumes now each measured 17.

The squish band of the Galbreath head is approximately .090" wide where the modified Mecoa head is now .100". So, the volumes are now closer but the shape of combustion chambers are still somewhat different. In any respect the running test will be a better comparison.


Fuel for the running test was Bryon 3000 r/c car fuel with 30% nitro and 11% oil of 50/50 caster and synthetic with added castor oil to get 26% nitro and 22% total oil

test prop was a APC 5.7x3

I had intended to run these tests on a Norvel .049 but fuel tank problems caused me no end of grief. In fact, two Cox reedies that I also tried also caused me minor problems and where not consistent enough probably due to having laid around since last year and being a bit gummed up. So, after a couple of hours of fussing, cursing, cutting a finger tip, and running nearly 3/4 of quart of fuel through some engines I got some usable results.

In the end I used the engine of my 1/2a reedy combat plane which I call "ole reliable" as this engine has always been just that. I did need to rotate the backplate in order to mount it to my test bench and failed to tighten everything up properly and had a bit of fuel leak but otherwise performed up to it's name.

This engine is nothing really special, just a bunch of old bits carefully assembled; twin bypass cylinder (not a TD), car crankase and large crank, large tank, nylon reed.

One this I also do is to use a wee dab of Permatex No. 133H Anti-sieze compound/lubricant on the plug threads. This is recommended by Larry Driskill on his website for the Galbreath/Nelson head and plug and I have extended it's use to the other plugs and heads. It does help to fill the plug thread area of the standard plugs and won't wash away with the alcohol based fuel we use.

Base line test was with Galbreath head and 4 head gaskets and 5.7 x 3 prop (also ran an APC 4.5 x 4 prop - 19.9k)

Weather conditions 18 deg C 45% relative humidity and barometric pressure 30.34 inches

Galbreath - 4 head gaskets 16.8K
Valentine - 1 head gasket 12.8K - didn't seem very happy
Mecoa mod "Red", OS LC3 plug 4 gaskets 14.5K
Mecoa mod "Purple", OS A3 plug 4 gaskets 14.7K
Mecoa mod "Purple", OS A3 plug 3 gaskets 15.1K
Mecoa mod "Purple", OS A3 plug 2 gaskets 15.5K
Mecoa mod "Purple", OS A3 plug 1 gasket 15.9K - seemed very happy and not overcompressed

Notes: The Galbreath head with 4 gaskets is pretty optimum for this motor and prop. I might have gotten a bit more by removing one but then it starts to get on the edge of being overcompressed. Surprisingly, the Mecoa head with one gasket sounded very comfortable, sort of like I hit a sweet spot with compression, fuel, prop and plug combination; very steady and smooth, smoother in fact than with the Galbreath head.

Conclusions

1) Yes indeed, as reported here and many times in the past - using one of the off the shelf replacement heads and a standard glow plug is a very good performance killer if in fact you can get the engine running well enough to fly with.

2) Of the off the shelf options, the Galbreath/Nelson combination is by far the best - readily available, inexpensive and performs very well.

3) A replacement head that uses a standard glow plug can be made to perform nearly as well as the Galbreath/Nelson combination. I could probably remove another .005" from the MECOA heads and still have enough of threads left to be comfortable. That would increase the width of the squish band and bring the head volume to the same as the Galbreath. With the extra .005, a wee bit more anti-seize carefully applied to fill a bit more of the plug thread, perhaps a hotter or colder plug and I bet you could have a head that takes a standard glow plug that would be within 700 rpm of the Galbreath/Nelson combo and certaily within a 1000rpm.

4) I suspect that results would be similar had I used a TD 049/051 or a Norvel 049/061. The standard plug giving way to the Turbo and Nelson plugs in the much high performance range above 20 to 25K.
So there you have it. Standard plugs are not as bad as all that, you just have to use them the right way.

5) The average home 1/2a enthusiast isn't able to make these mods to these common heads but I am going to pass on my findings to Larry at MECOA. Maybe if he enough emails on the subject he might just market a modified version of the conversion heads he currently sells.

Pictures, Pictures, Pictures!

First is of the test engine with the Mecoa mod "purple" head and OS A3 plug
Second is of the Mecoa "purple" head after modification on the mandrel
Third is of the Mecoa "Purple" head after modification showing a side view of the mods
Forth is of the Mecoa "Purple" head before modification on the mandrel in the lathe

cheers, Graham

Mr Cox

Nice tests!

Did you run a standard cox head also?

Another thing with these adaptor heads is that the lenght of the regular plugs are quite different, this alters both the geometry of the combustion chamber and the compression ration. I'm guessing the volume was measured using the OS plug?
Do you have a pict of the OS plug when inserted in the head?

There still seems to be a loss of 1000 rpm, as compared to the galbreth head, or did I miss something?

ffkiwi

Nice bit of work Graham-but you've only done half the job ;-) what about testing a range of different brand and types of plugs in these heads=for a start there are 3 different Nelson plugs-plus the Globee style insert one, let alone conventional 1/4-32 type glowplugs. What about for starters-OS #8, Enya, and K&B?

ChrisM
'ffkiwi'

GrahamC

I don't have any original Cox plugs any more so I couldn't run any comparisons.

Plug volume. I originally though that I would see a difference between long and short glow plugs with respect their internal volumes. There are differences between plugs of different heat ratings - hotter plugs tend to have a larger cavity than colder plugs. This is the reason I chose the OS LC3 and A3 - both about the same heat rating and after measuring their cavity volumes I found to my surprise that they where the same. So, the old idea about a long plug have more volume that a short plug is not really true. I spent some time measuring some old K&B plugs and found the volume of the long versus short to be the same. The only real difference would be the amount of metal (aluminum and steel) in the head/plug combo which could impact heat retention and cooling but to what degree I am not able to determine.

Head volume measurements where made with the OS plugs in place.

900 rpm in this test actually which is still pretty good all in all. A bit more optimizing and it might be closer. The goal really was to prove to myself that standard plugs could provide good performance but I really didn't expect to be able to better or even match the Galbreath/Nelson combination or turbo plugs as they do provide a much cleaner combustion chamber shape.

The last test with the Purple head and 1 head gasket even though at 900 rpm less than the Galbreath/Nelson produced a very nice run for this engine. Like I said - it was like I hit a sweet spot - very smooth and steady and sound just right; better than any other in fact even with the Galbreath/Nelson combo.

picture of comparison between Mecoa "purple" mod head with OS A3 plug and Galbreath/Nelson combo - not a great photo but better than nothing.

cheers, Graham

GrahamC

You are right of course Chris and half the job is being very generous. There are just so many combinations that a thorough test would take much too long and very quickly become rather tedious.

I thought there where only two heat ranges of the Nelson plug - Hot and Heavy Duty. I have some both and haven't any difference between the two in my tests. Henry does make a button for which you need a hold down clamp - might be worth getting a couple to try.

I have some Rossi short plugs "Very Hot", "Hot", and "Medium" that when time permits I will run some tests on. I haven't seen a genuine Enya plug for a great many years. I have a K&B to throw into the mix as well as some OS #8's. I will report some more results if time permits some more running.

This stuff is OK for the 049/061 size engines. But what are we to do when all the 020 (and 010) size Cox plugs are all gone. I have only a few spares left that should do me for a couple of years but what then?

cheers, Graham

ffkiwi

There are actually three Nelson plugs Graham-the std-the original, the HD (heavy duty) designed for high compression and a bit of nitro-mainly used by combat and F1C fliers, and the newer 'hot' plug (which is currently available from Larry Driskill) I agree-the tests would become very tedious but it MIGHT be possible-with a bit of serendipity-to stumble on a particular plug combination that reduced the rpm loss to a minimum. I could/would be quite happy to live with a 500-1000rpm loss but not a loss of 3-4000rpm
The British firm Model Technics=better known for their fuel -have done some pioneering work on glowplug heat ranges and produced two ranges of plugs as a result=they have IIRC, 6 heat ranges in each series (Firepower and Maxflash). Not that I'm suggesting that you do-but at least this would offer the experimental option of removing any other variables between different manufacturers.........
You can find out more at: www.modeltechnics.co.uk

ChrisM
'ffkiwi'

propjobbill

I have converted several .049 cox head s to glow plug . I haven't noticed more than about 300 rpm difference between heat ranges. I have found the the top rpm can be as much as 1,000 rpm or even more if you don't get a good leak free fit. I always take a little high temperature gasket maker and put it around the threads before screwing it in. My average rpm loss is about 500-600 rpm. With the nelson head I get just as good as new head and maybe 100=200rpm more.

The cox head will fit .049 and .061 Norvel. But I have never tried one on a Norvel engine. When I convert the Cox head I extend it about 1/64" into the chamber. I think the real problem is the standard glow plug allows for lower compression. The nelson glow plug adapter is made more like the high compression head by Cox. I think a plug with a small chamber for the heat element works better.

Nelson clams to work better because they seal at the bottom. And standard glow plug s seal at the top, which looses compression in the thread area. But if you look closely you will see the nelson head is shaved for higher compression, but the tread is longer raising the compression ratio. The problem with threading the conversion glow head in further is it alters the flow of air though the firing chamber. It is not possible to get the same compression with a converted Cox head . But because the compression ratio is lower you can raise the nitro content and get back the lost rpm from the conversion head .

I grind the top off the old head until flush with the aluminum, turn the head upside down use a punch to tap out the glow element from the inside of the head and it falls out easy.
to insure a good seal I fix the head at home and let the gasket material set up good. If a plug burns out I put a new conversion head assembly on. Take the old one home pull the glow plug coat the new plug with gasket material and let it dry until the old plug burns out.

To me at big loser is if you leak compression. The conversion head doesn't work as well but you may be satisfied with the slight lose if you increase the nitro a little it may even run better than the original set up on lower nitro.

One thing I do is use a flat head tool to square up the bottom of the seat. the picture is left to right; Cox standard head , Nelson head (notice it has a wider shallower base for higher compression), Inside view of conversion head , top view of a conversion head . Flat tool bit for squaring thread bottom from larger drill bit.

What I like about my conversion is there will be standard glow plug s as long as there is glow engines makers, and you are not locked into one supplier.

Comparing these heads on a sure start with a 5.7 x 3 APC prop I get about 15,000 rpm with standard head, about 15,200 rpm with Nelson head and about 14,500 rpm with the conversion with the same fuel. Thats prop speed of 42.67 mph with standard head at 11.36 oz of static thrust, and with the conversion head 41.19 mph and 10.72 oz of static thrust. These test were done on about 15% nitro with a drilled our back plate (venturi).

Mr Cox

So the magic might lie in the sealant then, have you tached with and without a sealant for the threads?

combatpigg

My Feedback: (3)

You might be able to match total volumes but the cavity in the glowplug is a dead airspace that only robs power, so until you can come up with a standard plug that minimizes it's own volume like the Nelson plug or Nelson GlowBee style plug, you're beating a $4.50 dead horse.

propjobbill

In my own mind I am sure that what combatpigg is saying is the problem. Standard glow plugs have too much air space for good compression. I have never tried short plugs, but to me it would stand to reason the short plug would increase compression; but only if you went deeper into the head. Otherwise a short head would leave avoid at the bottom of the plug and make matters worse.

Yes I tried the conversion heads that I made and gained about 500 or more rpm by using the sealant.

combatpigg

My Feedback: (3)

I just get irritated thinking about all the modelers who have been duped by MECOA over the years. Not just for these conversion heads, but they sell other substandard junk as well. If they were upfront about what they were selling, that would be different. Maybe their conversion heads are a good choice for Texaco?

SGC

Graham
The new Merlin glowbee plugs in turbo and std style plugs should offer a substanial boost to some of your tests. these will be on sale in April/May http://www.merlinglowplugs.com/Aircraft.html
Stewart

mikegordon10

Well GrahamC, Nice testing. Two things come to my mind. You must have way too much free time on your hands and I guess I'll just stick with the $4.50 Nelson/Galbreth Combo.

Remby

My Feedback: (3)

That is a lot of work and data, I thank you for your effort. I will add that the Galbreath/Nelson setup is so far the best replacement for power. I would add to what you have shown , be aware that using just one or two gaskets will impact the life of the crankcase assembly. It will up to the users to find a balance of power and performance, over higher wear of the crankcase. The gaskets (at three or four) brings the entire assembly into a good running engine, and keeps the engne's stress down for longest life.

Replacing a Tee Dee with another is only going to get harder as the years go by, it is up to the user to find a balance on power vs wear.

I find your four gaskets gets close to a stock Cox H.C. head compression. Thats great for sporty fun flying.

Thanks again for your efforts!

AndyW

My Feedback: (1)

The excess cavity in standard plugs does seem to be a large factor. Lite Machines made, for a while, a Glow bee style plug for their Norvel heli engines. I tried these for a quick, easy 600 or more boost. Low end idle suffered a bit, not a factor on the heli but in a light airplane, sometimes I had to kill the throttle to land. And I've always found that a stock, plug conversion nearly always improved throttling dramatically but at the expense of at least a thousand RPM. How this works is that a lowered compression ratio causes the need to lean out the main needle. Most engines go rich at idle, sometimes beyond the range that the throttle's airbleed can handle. Leaning out the main needle brings you into that range and idling and acceleration improves.

Also, I've found that heat ranges most often affect throttling as much as anything else. But that IS my obsession. [:@]

Got to try some Glowbee style turbos. What'll they think of next? Thanks Stewart, and thanks Graham, for the experimental work.

Mike, carpe diem.

lildiesel

I've been using the same bottle of Indian Head Shellac for what seems like forever. It makes for hard work to remove a plug but the sealing is great. Somewhere in the parts box is a Charlie's RC Goodies head with a lame OS short plug that may never come out...

GrahamC

Stewart,

Yes, I have just taken note of the Merlin plugs and will have to get some to try and what if any difference they make.

Too many combinations and too little time in search of that just right combination. And too little time.

cheers, Graham

GrahamC

I never have too much time on my hand.

This all started as my way of proving to myself that some of the old myths just were not completely true and that I did.

And yes indeed, the Galbreath / Nelson combo is still the best off the shelf bang for the buck.

cheers, Graham

lildiesel

Apparently so. Check out GrahamC's great thread about a Texaco project using a MECOA head:

[link=http://www.rcuniverse.com/forum/m_4085390/anchors_4085390/mpage_1/key_Cox%252C049%252CTexaco/anchor/tm.htm#4085390]texaco thread[/link]


I've run a MECOA and a Charlie's RC Goodies head over the years and won't complain. It's not like I want to be drilled into the middle of the circle by a Baby Bee in the early morning...

lildiesel

how do I manage to double so many posts... must be the compulsive editing.

iskandar taib

My Feedback: (5)

There's actually four. There's the two part "Glo Bee" style plug, with the flat spiral element. Mainly used for F3D (Pylon Racing). Same thread as the other Nelson plugs. Haven't tried one in my F2D engines as yet.



[link=http://www.pspec.com/details.asp?ProdID=37&category=7]http://www.pspec.com/details.asp?ProdID=37&category=7[/link]

There are also various Chinese and European copies - the Chinese ones are made by CS, I believe there are two heat range options. You get these from Billy Hughes (the F2A Speed flier). Adriano Molteni sells a European one. The Russians and Ukrainians were also re-elementing burned out plugs for a while. probably still do. I have a few of the remanufactured plugs (you had to trade in a "core" when you bought one).

Anyone contact Al Kelly (of Merlin Glowplugs)? He supposedly re-elements burnt out Cox glow heads (and other heads as well). If I ever get my Tee Dee .09 running I might ask him to do the heads when they burn out. Or send them to Nelson or someone to get converted to Nelson or Turbo plugs.

Iskandar

Yuu

Nice work, Graham. Beautiful presentation and method. I love the hypodermic drip measuring description, and the fuel mix description. But I got confused. First question: You wrote that you removed .045" from the bottom of the head; I assume you meant from the Mecoa head, right? Second question: You used Permatex anti-sieze on the 'plug' threads; but my Nelson plug seals against the bottom of the Galbreath head, and the 'plug threads' are not in the combustion chamber, so the anti-sieze is just to get the Nelson plug out of the Galbreath head, right? Third question: You use 'four' gaskets on the Galbreath head [?] and it runs best: but doesn't that 'Increase' the volume of the actual combustion chamber, thus reducing compression [?]' erasing the previous 'gain' in compression? Last question: Using anti-sieze on the plug threads; why not use anti-sieze ALSO on the Galbreath head-to-cylinder threads AND on/between the four gaskets... to reduce/adjust cylinder volume? None of my questions relate to anything about the 'squish band', which may involve some of the answers to my questions. Anyways, your report got me thinging again. Thanks.

unattainium

I have a Galbreath head and feel that it could be improved a little to make it more of a trumpet shape by having the head extend over the underside of the plug and only leave the element exposed.

A regular glow plug could be converted to a turbo type by machining a taper on the end and being very careful not to damage the element.

Adding head gaskets changes the squish band thickness and consequently is not the best way to change the compression ratio. A head for each C/R would be better or possibly havong spacer washers which covered the squish band.

Heavy anodizing on the combustion chamber would reduce heat flow into the heat and slightly improve performance by improving efficiency.

My guess is that the Mecoa head ran smoothly because of its rough shape which generated a lot of turbulence and discouraged detonation. Using a more regular head with an optimal squish band may provide the power of the Galbreath and the smoothness of the modified Mecoa.

Lapping the taper on the Nelson/Turbo plug and the seating taper in the head could ensure a good seal, a bit like lapping poppet valves in a four stroke except that a dummy plug with no thread would be needed for the head when turbo plugs were used. A custom made collet would do the job with the Nelsons...

Finally a tip about measuring combustion chamber volume we used to use back when i was involved in building high performance race engines. Make a plate of clear acrylic which covers the chamber and drill a hole in it. Clamp it to the head with some grease as a seal and use a burette to fill up to the hole and record the amount of liquid added. This method avoids meniscus and leveling errors.

I have several Medallion 09s and a TeeDee 09 and am short of good heads and have a project to make heads for them using Nelson plugs.

combatpigg

My Feedback: (3)

It's been awhile since I looked at shop drawings of tapered seat AN fittings, but I don't think they have a "lapped fit" with matching angles by design. I think the fitting is supposed to have slightly less angle than the seat so it bites into the seat when tight.

GrahamC

Iskandar, thanks for your comments. I am going to get a couple of the Nelson Glo Bee type plugs to try. I recently just recieved some of Al Kelly's Norvel type plugs (and head clamp) as well.

I was intending to do some more testing this weekend but it has been cold and wet - better suited to indoor work and some yard work between rain showers.

cheers, Graham