on the last few flights I noticed a pre detonating condition on my OS 1.6 in a CAP 232.

10% Powermaster fuel w/ additional 3 oz. of KLOTZ per Gal., OS # 8 plug, Zinger 18x6 Pro, Brisson Pitts Muffler, Altitude 1200'. ( Note: no pump or other fuel delivery device).
Also noticed lots of oil on Bottom of fuse, whats the cure?

Hello!
The remedy is very simple!
You are getting preignition because of excess heat inside the engine!
Either open up the highspeed needle or idle needle somewhat (more likly the highspeed needle). And don't use too large fueltank (16oz is enough).
Also check your fuel...and glowplug....and that you have adequat airflow around the engine for cooling.


Regards!
Jan K
Sweden

thanks, I'll try open up the cowl some.This engine is totally enclosed with only the normal air intakes open plus some additional vents. also I installed a baffle to divert outside air over the cyl. head. I'm running rich now so opening up is not good.

I run the same engine in a Sig Extra 300. My Extra cowling may have a little more inlet area than yours, but not by much. The cowling exit area may be something to look at. You need 2 to 3 times the exit area to allow for effective cooling, from what I have read, and from my experience.

I purchased a Sig Cap 231/Saito 180 from a friend. During the summer months I had problems with overheating. I added a baffle similar to yours, and had to double the size of the cowl exit to cool things down. When I assembled my Sig Extra/O.S. 1.60FX I did the same. Baffle on the left, and large exit. I've flown the Extra in temps above 100 F, without a single problem.

Something else to consider. Did you use a large klunk and tubing throughout the fuel feed plumbing? This is an important requirement. The engine burns the fuel!

Hope this helps! I have really enjoyed my 1.60FX! It's absolutely the most dependable engine I have run to date. Nine gallons of fuel without a single dead-stick!

Rex Rutledge

[quote]


Something else to consider. Did you use a large klunk and tubing throughout the fuel feed plumbing? This is an important requirement. The engine burns the fuel!

No, I did not, I used standard fuel tubing, I think it's about 1/8" ID, and a reg. size Klunk that came with the 20 oz. Tank . She's not running lean and display's a rather heavy smoke trail, but that too could be because of the extra oil I add to my fuel.
I have yet to deadstick the Cap because of a empty tank and flight times are about 10 -12 min. I'm open to more suggestions???

Hi!
The pictures you show is in any way unusual and should not make the engine run hot...I have approximately the same size openings in my SIG CAP 231 EX powerred by a MVVS 25cc ABC (1.50) and I use 15% Motul "Micro"allsyntetic oil and 5% nitro, prop is a 18x6 Menz...and this engine has never quit for me during 3 years of flying.
Fueltubing is normal size (inner diameter 1,5-2mm). It is my experience from 30 years of flying that fueltubing size has no effect on engine behavior.
I still think you run your engine too lean.

Regards!
Jan K

Hello my friend from Svenska!
I'm still trying to solve the pre ignition problem and will try all and any modifications I'll have to make to achive that. First this morning I cut another vent on the cowl on the left side and enlarged the one on the bottom. I also found a restriction on the fuel filter ID diameter was just 3/32" which I enlarged to 1/8", which I think, will cause it to run lean,(the smoketrail has me puzzled) If too small a delivery fuel system : i.e. Fuel lines, filters etc. are too small for the engines demand, it may run, but run lean. But I'm no expert on that and need to investigate this even more.
Am I assuming correctly by saying that you run FAI Fuel without Nitro?, that would be my next step to switch to 5% fuel with about 20% oil just to see the difference.
Thanks again for your input on that subject.

Keep in mind that 2-stroke glow engines are compression
ignition types....like diesels. The most effective way to eliminate
pre-ignition and detonation in any engine.... is to #1 retard the
timing, and #2 lower the compression.

The only way to effectively retard the timing in a glow two-stroke
is to lower the compression. But, we shouldn't really have to do that.
The real problem comes from excessive heat. Too much heat in the
combustion chamber....no matter what the cause, or combination
of factors....and the "knocking" noise are a bad thing. []

We can create our own problems with our planes, by taking a perfectly
good engine....eliminating about 90% of the cooling, by wrapping a cowling
around it. Then we put a Pitts muffler on it....and just doubled the heat inside
the cowling.

Now lets put a nice big prop on that baby....a little more heat won't hurt....

And the fuel thing....some motors just like certain types of fuel/nitro/oil combos
and will run completely differently on one type or combo....than another.

The last denominater....the airframe. Not really designed with engine cooling as the
primary factor. Big fat fuse, big fat cowling....big fat wing. Less than great speed/cooling
....and, under heavy engine loads most of the time.

End result....knock, knock....[X(]

Your inlet and outlet cooling looks perfectly normal to me. Lots of theses engines end up in airplanes exactly like yours. I have been doing a lot of research on the 1.60 since I bought one (not yet ran). It seems that detonation is not too uncommon. If you're sure you don't have a fuel starvation problem leading to lean running, you could try to lower the compression by adding head gaskets.

The problem most likely is you have enough fuel coming into the engine. The problem is these engines are scavanged so well that the fuel may blow thru the engine and not be trapped into the combustion chamber as richly as it should. Thus your oily condition under the plane, granted the engine collects enough heat so that it probably takes having to goose the throttle to get cold fuel into it so it will idle down. Getting rid of the knocks or pings of detonation, is timing problem. My friends in the racing bussiness recommend that you go to a higher octane, or in our case use more nitro content. Also, decompress the engine as much as possible, using head gaskets or short plugs to drop the compression ratio. Lastly, make sure your fuel feeds are unobstructed as possible so the engine doesn't have to starve for fuel and remember most stunt aircraft like Caps, Extras and Edges don't fly around at a million miles an hour. Try different lower pitched props to load the engine correctly and get he desired 3d characteristics you want.
wra

Hi!
Well I use 5% nitro and 15% Motul "Micro" all-syntetic oil in my engines and so do most of my friends in my club. I have not run FAI fuel (80/20) in my MVVS .1.50 but assume that this will work too.
As I told you earlier I had this engine in a SIG Cap 231 EX and have had no propblems what so ever after 3 years flying.
I was a bit anctious first as the airplane didn't have any air exit behind the engine (and i have the MVVS steel silencer at 45 degrees angle just behind the cylinder and it gets real hot) but eveything has worked perfecktly so far even at temperatures at 30 degrees centigrade.

How much nitro do you use....0% (FAI) is not good as far as OS .1.60 is concerned.
5-15% nitro is better and does make the engine run cooler just because you get a broader needle setting.

Regards!
Jan K

Chemically speaking nitromethane is so stable, its flashpoint and heat of vaporization is so high that nitromethane is almost like water. According to the literature on the subject, you only get an increase of a couple of hundred rpms for each additional 5% of nitromethane in your fuel mix. As I have said before, two stroke engines are so well scavanged by the exhaust that they need more oil in the mix to provide enough lube, and also help with keeping the engine cooler. Correctly directed air flow is essential to cooling your engine properly. Remember you want to channel your air in one specific direction, so don't start cutting holes in your cowling nilly willy, you will create unwanted air eddies thus getting a non-functional cooling system. the small hles and or slits on the front of the cowl act as an air dam directing the air flow into the engine compartment. The exit hole should be 3 times greater in area size. Scoops or fairings on the intake or exhaust sides of your cowl can also help improve air flow cooling. Oil does some secondary cooling thru lubrication to reduce friction, high heat denatures most synthetics used in american fuels, this is the reason a small amount of natural castor oil is used fuels to avoid complete loss of lube at high temps. Whats this have to do with predetonation, you may ask? You are running a Diesel two stroke, with no way of changing timing without changing the engine mechanically, or thru chemistry and sometimes both are needed to solve the problem. Try restricting your muffler to slow the scavanging and getting more fuel to stay in your engine, then lower the compression will help even more, then find the proper prop and glow plug for your engine. Then fly the heck out of it!!!!!!!

Actually, first try the engine on a test stand or without the cowl. If you don't have the detonation problem, then you can be sure the engine's not getting the cooling it needs.

With ANY air-cooled engine, you have to make sure that the cooling air passes through the fins on the cylinder and head. Take a look at a full-size aircraft engine. No air going into the cowl is allowed to leave before it passes throught he fins of the engine. Baffling is used to direct the air where it needs to go. Take a look at some of the high performance homebuilt aircraft. A 180 Hp engine is cooled with two 3"x6" inlets. They can be that small because all the air going in passes through the cylinder fins.

In your model, you may have to build baffling to make sure that the air going into the cowl actually passes through the engine's cooling fins. If you can look into the air inlet and see the air outlet, then you can be sure that the air is taking the path of least resistance, and going directly from the inlet to the outlet, do not pass engine, do not collect cooling.

thanks to all that responded, I'm still working on the pre ignition problem as time permitts. Bax, you response is to the point, run the durn thing without the cowl and see what happen's, this would be the first step. I still think I ran this puppy too lean, trying to squeeze more RPM out of it.
I'll keep you guy's posted on the final results of this problem, tomorrow is the first day of Deer Season here, so it probably will be Sunday before I get to the OS again. ( Trigger Finger is itching)

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