Your not answering all my questions. What is the deck hight? Is this a fixed compression head? Also 3% is to much boost.

I'm sorry, but I'm not quite sure what you mean by "deck hight"

The total stroke of the contra-piston is 3,5mm, resulting in a combustion chamber volume varying from 0 to 0.62cc

The deck height is the space between the top of the piston at the top of the stroke and the bottom head. do we have a language problem here? Is the head made exactly as shown in the drawing?

That's quite possible, because I'm from Holland and my technical English isn't perfect.

The head is made exactly as in the drawing, except for the cooling fins. They are 8mm deep.
The space between the top of the piston at TDP and the bottom of the head is 0.2mm. I thought that was called the squish band.

First you have to little space between the head and the piston at top dead center and second you have no squish angle. May I suggest if your going to copy our head you do better to take a closer look. also you might want to read our directions there really quite good as we refined them over a period of 30 years.

With all due respect, but I think those last comments are quite inappropriate;

1- You say I copied "your" head. Unfortunately I can't seem to find a conversion head for a MAC-10 KP2Y-C engine listed in the price-list on your site. If there was, I probably wouldn't have gone through the hassle of trying to make one myself.
2- When I made this head, I had never seen a model diesel engine, let alone one of your conversion heads in real life. I designed the head based on information and pictures I found on the internet (including this site)
3- As I don't own, or have ever owned, one of your conversion heads myself, I neither own, or have read, the directions that go with them.

I'm just a technically orientated modeler, who likes to experiment. That's why I thought converting one of my engines to diesel would be a nice challenge.

Don't get me wrong, I appreciate all the helpful comments, but I don't like being accused of stealing / copying someones ideas!

And besides, it's not that I'm trying to steal your business. Firstly, I'm only doing this for my own enjoyment and secondly I'm fully aware that the design and quality of this head is by no means good enough to be sold commercially.

Micropuller,

Keep on experimenting bro. Its a lot of fun and you might discover something.

After thirty years in this business, nothing afends me anymore. What we have here, is you may understand the English languge, but you do not understand our humor. That phrase about the Chinese copy is older than both of us put togerther and simply means you didn't get it right.
This will be my last comment to you I suggest you reread my prior remarks very carfully and take a good hard look at your friends OS 1.8 Davis head. By the way did you ever wonder why we didn't do that engine?

I think this is what upset micropuller "May I suggest if your going to copy our head you do better to take a closer look"

@FrancisPerson;
Thanks for the heads up! I'm definitely going to continue this experiment. If there's something I can't stand it's knowing that something should work, but I can't figure out why it doesn't. I set my mind to making my own tuned pipes some time ago, and after a lot of experimenting I cracked it! I now make pipes that weigh less and make more power than most of the commercially available ones.

@DDD;
My post wasn't meant to be offensive. I just didn't think it was fair when you said that I was copying your design: "May I suggest if your going to copy our head you do better to take a closer look"
I want to thank you for your input up till now, and I'll be sure to look further into the details you mentioned.

Cyril

I can vouch for what Mr. Davis said about the squish band needing a slight angle inward. Several years ago when I was younger and dumber I bought one of his heads for a Fox .60. Due to a MFG tolerance on the Fox cylinder that was off a little the piston hit the part of the head that protrudes into the cylinder. Well, Genius Hobbs hadn't heard of head shims so I proceded to sand the head down so the piston would not hit, and in the process removed the squish band and made head perfectly flat on the bottom. I ended up with an engine that ran hot and was all but impossible get running correctly. Lesson learned.

One thing I am not clear on here guys...

#1 - I thought our model diesels did not burn like the glow fuel, and the squish band (which basically is there to force the fuel and air mix into the combustion chamber) and angles werent important or as important.
#2 - relating to this, how many purpose built diesels have squish band style, domed combustion chambers? umm? Most I know have a slightly domed piston and a corresponding concave contrapiston or something similar.
#3 - I have built many of my own conversions that used a contra that was the same bore as the cylinder (like a purpose built diesel), therefore it was inpossible to have a squish band. They ran great. None ran hot.
#4 - in my own heads that use a contra which is a smaller diameter than the cyinder bore, I have NEVER used an angled squish band... and so far I am having no issues with overheating. Some of you guys have more than one of my heads - if there IS a problem with the design I would like to hear about it, but this is news to me... maybe this is particular to cetain engines?

In the glow ignition engine, the design of the combustion chamber, squaish band area, volume and angle will certainly affect the way the engine "burns" - but ignition is taking place at the coil of the pug. The design of the head can very much influence the way combustion takes place and a small change in the head design can greatly affect the performance.

In the model diesel engine, it seems that allmost "anything" goes (to a point) and as long as there is adequate compression to ignite the mix, the actual design of the head does not seem to have a great effect on the engines ultimate performance. Comments?

I too like to dabble in things mechanical, especially the model engine. I would encourage anyone to try out this stuff, it is fun! And satisfying.


AJC

I forgot to add - one trick I have used when I figure an engine it running too hot - try taking a blowgun attachment for an air compressor and aiming it right at the head (make sure not to influence the air into the carb) to force cool it. You can tell immediately if the engine cools off some and starts to run better. If it is an issue of inadequate cooling, you can always build an electric fan!

AJC

PS in my previous post I in no way meant to disagree or agrue anyone's opinion, just posing a few questions and observations of my own. We should all try and work together to make this fun and a learning experience. If anyone wants to know what I have done, experimented with, etc pertaining to diesel conversions by all means just ask. I will gladly share my limited knowledge with you

.......That's quite possible, because I'm from Holland and my technical English isn't perfect.......
[/quote] Sorry, can't contribute anything worthwhile to your diesel questions but can assure you that there's nothing wrong with your English, technical or otherwise.

Good questions lead to great discoveries.
You have already added much to this forum.

AJ, I could very well be wrong about the Fox .60 running hot because of what I did to the head. I've had no issues with your heads and none with any other Davis equipped engine. I now have another Fox .60 and another Davis head I got from a friend, I also have head shims to use if needed, Fox makes them. As my learning curve increases I've learned that the Diesels run much smoother and cooler if I back the compression down about 100 rpm from absolute peak rpm and run the idle needle slightly rich from the best idle.

Now I am interested in perhaps making a few various heads (with different size squish bands/contra sizes) and various angles just to see if there is any change in running temp, rpms, etc. It would make a great experiment!

AJC

My RJL 40 head had a concavity in the center of the contrapiston with a flat (maybe a slight angle, but I don't think so) band around it. Unfortunately the groove for the O-ring was too close to the bottom of the contrapiston and the flat band broke off and welded itself to the contrapiston and top of the piston. No damage except to put a twist in the rod.

Gene Hempel made me a new contrapiston with the groove for the O-ring higher and the contrapiston a little closer fit in the head. The bottom of the new contrapiston was flat except for a shallow conical dimple in the center. No reason for that except that it was there in the piece of aluminum we used, and I saw no harm in it. So far as I could tell, the two contrapistons gave the same performance.

It is my understanding that the fuel/air charge in a diesel fires very rapidly throughout the combustion chamber, as the major source of heat is compression which is uniform throughout the chamber. If so, then I wonder if shape of the combustion chamber would make any difference. However, perhaps there is a "hot spot" somewhere in the chamber which initiates ignition, not a slow burn like with a glow plug, but just a little less uniform burn than would be expected if the ignition heat were from compression alone.

Jim

I'n my work with automotive, ag & OTR 4 stroke diesels I've never heard of overheating caused by chamber shape. As a matter of fact, we have great difficulty making enough heat to warm a pickup cab. Charge motion is something we manipulate to gain performance targets. We is tumble, swirl or quiescent motions to control thing like idle quality, or high end power.

In a 2 stroke diesel with Schnuerle porting the charge motion would be a combination of swirl and tumble which I'm sure would make for very good combustion characteristics but I'm not sure what effect a squish band has be it flat or conic. Not being familiar with the engine in question, or to be honest Schnuerle ported diesels in general I can't really make any authoritative comment. However, with what I do know, I would say there is a chance that hot spots could develop in such an engine. The angled squish band would tend to direct the burning charge toward the center of the chamber, where you want it where a flat band would create a hot "band" if the squish clearance were too large. At least that's what I theorize. AJ has mentioned that in his experience chamber shape in our little diesels seems to make very little difference and I would have to agree that I've found the same thing. But in these high performance applicetions I'm not so sure we're not getting into the range of operation and combustion characteristics where chamber shape is becoming more critical.

Really I'm just thinking with my fingers but thought I'd share my thoughts and see what comes back.

Randy

I picked the picture who Micropuller had send to RCU about making own cylinderhead. You can see there are not enough cooling fins and not enough thinner cooling fins. A thick cylinderhead without or few cooling fins are good to save hot from combustion and difficult to reducing hot from the cylinder head.
In beautiful weather with sun can give warm asphalt who make warm air, there are not easy to keep engine cold.

Also increase thinner and cooling fins..

Jens Eirik

I can see either a dome or a cone for a diesel head working correctly but I would think if you ARE going to include a squish band it should have some angle to cause the swirl. It was determined in glow engines long ago that some angle was needed for best performance.
Might the use of a squish band be better for larger engines but not necessarily for smaller ones? Is it possible that the squish area will fire first or cause a more complete combustion on a diesel?
Hobbsy, do you still have that head? Could you add an angle to it and see if it runs cooler?

Geez, I seem to have a lot of questions and comments, but no answers...sorry.

George

I did a little search on squish band angles and found an interesting comment on a site dealing with rc boat-racing:

Squish Band Posted Thursday, March 8, 2001 by MartenDavis
In all the testing that I have done with the dyno, backed up with a huge amount of on water testing, I have arrived at a conclusion about the configuration of the squish band. To make maximum power, make the squish band FLAT. You will get some shake (like engine being out of balance with the flat squish band). Many engines come with 3 degree across the squish band. I believe that you are giving up a lot of low end power that way (launchability). A good compromise is to make the outer 1/2 of the squish band flat and the inner 1/2 with a 1/2 degree angle. These results are using the currently available pipes for the various engines (Irwin for the 21, Big Belly CMB for the 67) All other pipes that I have tried have also produced the same characteriestics, but with a very well optimized pipe there may be some change in this result.

And:

One of the most important items in the design of a good combustion chamber is the squish band. I believe that a flat squish band produces much more power than an angled squish band. The flat squish band head has a flat area (squish band) around the perimeter of the head which comes in close proximity to the piston at top dead center. This squish band is designed to keep the layer of combustion mist very thin, in order to let heat travel quickly from a hot piston to a cooler combustion chamber (head). The thinner this layer (the closer the head clearance), the better this heat transfer is accomplished. If your head has its squish band to far away from the piston at TDC and the compression ratio is high, you will get pre-detonation (knock). You can tell if this is happening by looking at the squish band. If it looks like it has been lightly sand blasted, it is pre-detonating.


I also found another interesting post:

I currently believe the reason the little engines can stand more nitro and higher compression ratios is far more basic. That is, they have a higher surface area to volume ratio in the cylinder. Assuming:
.21: bore = 16.4mm, stroke = 16.4mm, surface area halfway along the stroke = 845 mm^2, volume = 1730 mm^3, surface area / volume ratio = 0.488 /mm.
.91: bore = 27.05mm, stroke = 24.5mm, surface area halfway along stroke = 2190 mm^2, volume = 7040 mm^3, surface area / volume ratio = 0.311 /mm.

The .21 has almost 60% more surface area per volume. This means it dissipates more heat to the cylinder walls, head, and piston per joule of heat introduced from the combustion. Therefore, for the same % nitro, compression ratio, air/fuel mixture ratio, and charging efficiency, peak flame temperatures will be lower, and detonation will be abated. So, you can run the .21s with higher c.r. and higher nitro and still lean the needle down for good power before blowing the plug.

This would explain why larger diesel engines could get hot / overheat easier than the more common small engines.

You picqued my interest on squish bands so I looked up some glow engine reviews done by MAN. Squish bands ranged from 0 to 11 degrees on the engines where it was mentioned. It also explained that the boost port on some schnuerle engines direct the flow to the squish band. This prevents wetting the glow plug when idleing.

I guess it's whatever you prefer. I would prefer smooth running to max power. Remember these folks who race are usually willing to ruin an engine in one run in order to win. Perhaps you are in that category.

You might also want to ponder Mr. Davis' comment about WHY he didn't make a head for your engine. I understand that two basic reasons for not making a head are extremely low sales volume for an engine OR he doesn't think it will work well as a diesel. I understand he tests more engines than he produces heads for.

You mentioned that a tractor pull is done in a few seconds. Most diesels for airplanes are allowed to heat up before final adjustment. I think your final adjustment will have to come from a cold start setting. Using a prop to get your adjustments will be a little more complicated because you need to get a setting for immediately after starting and also one for after it has heated up in case you need to do consecutive runs.

Tractor pulling sounds like fun. Keep us posted.

George

George, this doesnt explain why my own heads, that have been made and run successfully on .10/.15 on up to over 2 cubic inches have all run fine. I made my first diesel conversion in the early 90's when I got my first lathe (I still have it, it was for an old Fox engine I had) and have allways made the squish band flat. None to my knowledge have run hot, or abnormally. Also, I made quite a few with the contra pison the same diameter as the cylinder bore, ie, no squaish band.

I dont personally think that the squish band plays a big role in the diesel as it does in the glow plug ignition engine. Bu like I stated previously the only way to test this is to take an engine, make a few heads with the only variable beig the squish band angle, and see how they run...

AJC

I presume in a head like the RJL head or the one illustrated by Micropuller, the squishband would be on the head insert going into the cylinder. Now if the contrapiston is up in the head or sticking out just a fraction, doesn't this change any effect the squishband might have? Of course in a situation where the contrapiston sits in the bore, the squishband could be put on the contrapiston.

It seems to me that one adjusts a diesel to run at a particular operating temperature. As the engine warms up you back the compression off as less compression heat is needed. In Micropuller's situation, where he needs maximum power for a few seconds, a head temperature measuring device, of which I think there are several available, might be very useful. It also strikes me that one would want to enter a high load situation a little undercompressed. All very interesting. I'd like to know more about Micropuller's competition event.

Jim