My first silicon contra seal was made from a piece of 12mm ID x 18mm OD high temp (500 deg F) silicone hose or tube. I cut a 4mm slice off of it and grooved the contra to fit the silicone with a very tight sqeeze into the contra bore. I had to make a special tapered tool with a lot of lube to press the contra into the bore. I think this was my downfall on the first contra. I think with the 3mm groove in the contra and the tight silicone fit, the silicone wanted to go somewhere and was pressing up and down on the flanges of the contra, and it broke.

On the second contra, I made a special tool to hold the outside diameter of the silicone ring perfectly round and had a hole bored in the tool that I put another round cutting tool through to slice the silicone to a 1/16" or about 1.5mm wall thickness instead of the 3mm it origionally was. I then made a new contra with a 3mm wide x 1.5mm deep groove to fit the silicone ring. This groove is 1 mm in width smaller and 1.5mm in depth shallower, which should make the contra stronger. I machined the groove until I got a snug fit with the band into the contra bore but not near as tight as the first one.

I don't know what the benifits of the band vs orings would be but I do like the idea that any swelling of the ring will only help in the sealing capabilities of the ring.

I followed the diagram of the MVVS .49 plan very loosly in only really taking the concept from the plan. I decided that instead of streching a 8mm OD band over a 12mm contra, it would be better to have a band that was more of the correct size. My thoughts were that if the band got a nick or slight cut in it, it could tear and if the streched band contracted, it would begin to leak. With the band not having to be streched, the chances of tearing will be reduced. My contra bore started out at 0.710" diameter ( sorry, don't have any metric tools), and now is 0.736". I had to dress up the bore a coulple times between contras. I found a way to make a ultra fine feed on my lathe using a cordless drill as the cross slide drive and after boring, I use chrome polish to polish the bore to a mirror finish. With the fine feed, it only takes a minute to shine the bore. I have about 4mm of flange below the ring and about 3mm above the ring. This gives me considerably more meat on the contra to reduce the risk of the lower flange breaking again.

I did find that it likes about 3/4 throttle max. I was looking at a few older carbs I have and might put an older magnum 53 carb on it. It is a little less than 1mm smaller in bore diameter.

John, sometimes things happen that just baffle us. Martin knows this story. The first time I tried my high compression Saito .80, (15.5 to 1) I ran a 13x6 prop, same as glow, It hammered and knocked would only turn 5,500 and sounded like it was going to become a Black and Gold grenade. I installed a Bolly 13.5x8 and voila, just like you experienced, the knocking went away and it turned the 13.5x8 8,790 or there abouts.

Thanks Flyby for that very informative post.

My understanding is that O rings are very successful and long lasting (as evidenced by Davis Diesel, RJL, Andrew Coholic and many others), but it looks like silicone tube could be just as good. I'm tempted to try it.

Thanks,

Dave H

Getting back to the lower shelf cracking off on the contra piston, is there a stress riser of an included angle of 90 degrees machined in the silicon tube groove?

Maybe it's better to put a radius in it.

A gent in the Denver Colorado area made some Diesel conversion heads for TT, Magnum and ASP engines about 12 years ago. He passed them out like candy. He made the contra with a 1/32" gap all the way around it. The O-rings would blow out after about 5 minutes of operation. I have never had a Davis contra out of the head so don't know if he uses O-rings or not. I know his contras are a good, tight, precise fit. Yesterday I inspected the piston fit on the SuperTigre .90 I ran for John, I set my Torque It torque wrench for 10 inch lbs, the compression was soft, I had to go to 20 inch lbs. For a good seal. I did the "snug the bolt" and then turn the bolt an additional 45 degrees, i couldn't turn it 90 degrees. It took 22 inch lbs to move it from there. That's not scientific but it worked.

I did think about a radius both before it cracked and definintly after it cracked. I just did not know how to radius the silicone band. I did do an experiment after the crack and was able to get a radius in the silicone band by putting the silicone ring over a round rod and using my hobby knife tip to cut the 45 chamfer edge on the silicone ring while it was turning in my lathe. I however did not put the chamfer in my newest contra because the grove was so shallow that I didn't think it would need it. I will see how it holds up with a little more use.

I got my ACB this morning and will give the diesel one more try with 1.5% in the fuel. I am starting to think that this engine is not a good candidate for a diesel. I replaced the diesel head with the glow head and fired it up on 15% nitro glow fuel with the same redneck prop load I had on the diesel setup and got a full 2000 rpm higher with the glow than I could get with the diesel and the engine was a lot smoother and quieter running. It idled a little higher than the diesel, 2500 rpm vs the 2000 rpm. I was getting 9200 rpm with the glow vs the 7000 rpm with the diesel. We will give this ACB a try and see if it improves the power any.

I put 1.5% ACB into the fuel and it did run a lot better. Still 700 rpm below the glow on the same prop setup but it seemed to run pretty smooth. I measured in mass ounces because I didn't have anything small enough to measure .12 oz because I only had 8 oz of fuel left. I used my reloading scale and converted the oz to grains. Any way, I made a video of the engine running and me trying to tune it. Please let me know if you think I got it right or if I am doing something wrong. It is a long boring video, I start it two times and fiddle with the tuning both times. With the hillbilly prop load I have on it, it maxes out at 9000 rpm and idled at 1800 rpm. It transitions well. I am still using the 30% ether, 25% castor, and the remainder kero/heptane and 1.5% ACB. Heres the video if your board. I think I need a new main bearing as I get a little noise with both the glow and diesel at idle.

https://www.youtube.com/watch?v=xqGc_zIiQko&feature=youtu.be

The noise sounds good to me, it's definitely not over-compressed. If you cut the oil back to be the same content as that of the glow fuel you should pick up the 700 rpm. Even then you'd have to ask yourself what's the point apart from the novelty value?

It looks about right to me...
The rest will be fine tuning from there. The prop load is also important, as written earlier, some engines can act a little strange and be tricky to tune until you find the ideal prop for the engine. The pickup looks/sounds a bit laboured to me, either from a too rich setting or from a too high compression ratio (or a combination of the two).

In the early parts of the video it looks like you lean the engine a little and it starts to miss, rather than riching it back up from there you can keep the leaner setting and instead increase the compression ratio, as a test to see how the engine reacts. The two settings are linked, and one cannot first optimise one and then the other.

At 9000 rpm what is the point of having a boost port?

Ever tried blocking one off?

That is all it is right now, a novelty item. I started off with the dream of massive amount of power increase, and so much more fuel efficiency than glow but now after looking at my glow fuel price vs the diesel fuel price, the power being really close to the same and the fuel economy being about the same (1 oz/minute); I think I will fly the plane the engine came off of once and then convert back to glow and call it a "Been There, Done That" experience. A quart of fuel would last a lot longer on a smaller engine but with a big 91, it won't last more than half an hour.

To tell you the truth, I don't really know much about the porting on the engines. I got the basic concept but don't understand how improvements in performance are achieved by doing different modifications. Does blocking a boost port increase fuel efficiecy? I don't really want to do anything permenant because I am planing to convert back to glow. I have had no problems with the glow setup on this plane, I just thought I could get better power and more speed out of this engine as a diesel to begin with and now have found out other wise. The cost of diesel makes it cost prohibited for an everyday plane and although some may disagree, there is a bit more hassle with the diesel for me than with the glow since I am already set up for glow.

Please accept that the 'boost port' was never part of the original Schnuerle port arrangement but a later thought about using a late timed port to scavenge the parts of the combustion chamber missed by the twin loops of the main side ports.

And it was done for outright power.

The problem with some engines is that they are designed for a wetter methanol mix and high power outputs at high rpm's - so the pressures in the ports remain usefully high for that type of use.

Enter the model diesel that uses a much drier air fuel mix, runs happily at a lower rpm and is not really seeking great horsepower and the suffers cavernous ports with conversion engines.
You can well see that blocking off a third transfer port will direct the pressure stream to the remaining two ports, increase the pressure in those ports and lower the volume of the lower crankcase thereby increasing its pumping capability.

To do this you must remove the liner from your engine and fashion up a plug from softwood that fits nicely into the boost port's casting cavity, it will not come loose because of the curved recess that it sits into and should not burn either.
Oh, and it is only temporary and completely reversible if you do not like it.

For a more instructive lesson on this please refer to -

http://www.tulsacl.com/Engines.html

The idea on how to do this is illustrated well even though its aim is different.

Cheers.

Since most modern R/C glows come out of the box with the compression set low very low, there is considerable power (~ 10% at least) gain to be had free just by setting the squish height somewhere near optimum. It probably means shaving the underside of the head a bit. Further power can be unleashed by going to a Turbo or Nelson style bottom sealing glowplug. I don't know what this does to the idle but it's a lot cheaper than having to use premium priced diesel fuel.

There is a place and role for model diesels (I own at least a 100 of 'em), but cheaper flying isn't an attainable goal.