Hi Dave,
In one of the Evo discussions, you said in part:

"...I left castor behind in the early seventies. If the engine is new or at least the piston and sleeve are new I see no need for bean oil. If the engine has already been been run on bean oil, you need to continue with it. I like between 15 and 18% synthetic. If you want to add some castor I'd limit it to 2-3%. Too much oil lowers the "octane" rating of the fuel and you have to start adding a lot of head gaskets to slow the burn back down..."

Then you asked us to be gentle.  Sorry, fella, no such luck!!   

Just kidding.  You caught my attention with this, not because of the all-synthetic, which is not unusual, but because of the low oil content and the octane rating/head gaskets thing, which is new to me. 

Would you care to go into more detail about octane and head clearance?  Is the use of "octane" metaphorical, since it's in quotes? 

Does too much oil lowering the octane rating apply to both castor and synthetic?

Finally, do you use after-run oil?

I enjoyed meeting you and the rest of the clan at the Brodak.  You talked with me like we'd been friends for 20 years--one of the best things about so many of the people in this hobby!  Only wish we lived closer. 

Thanks,
Kim Mortimore

Hi Dave,
In one of the Evo discussions, you said in part:

"...I left castor behind in the early seventies. If the engine is new or at least the piston and sleeve are new I see no need for bean oil. If the engine has already been been run on bean oil, you need to continue with it. I like between 15 and 18% synthetic. If you want to add some castor I'd limit it to 2-3%. Too much oil lowers the "octane" rating of the fuel and you have to start adding a lot of head gaskets to slow the burn back down..."

Then you asked us to be gentle.  Sorry, fella, no such luck!!   

Just kidding.  You caught my attention with this, not because of the all-synthetic, which is not unusual, but because of the low oil content and the octane rating/head gaskets thing, which is new to me. 

Would you care to go into more detail about octane and head clearance?  Is the use of "octane" metaphorical, since it's in quotes? 

Does too much oil lowering the octane rating apply to both castor and synthetic?

Finally, do you use after-run oil?

I enjoyed meeting you and the rest of the clan at the Brodak.  You talked with me like we'd been friends for 20 years--one of the best things about so many of the people in this hobby!  Only wish we lived closer. 

Thanks,
Kim Mortimore

Hi Kim. Octane and head gaskets..... The issue is that the oils have a lower octane rating then the methanol. The more oil you add, the easier the mixture detonates. There is as you know a relationship between compression ratio and the burn rate of the mixture. Higher ratio, hotter charge. If you lower the detonation point of the fuel past the point that the engine will accept without complaining you have to lower the compression ratio of the engine (add gaskets). Most of our engines have a squish band on the combustion chamber. That squish area doesn't function as squish (in small 2strokes like we use) with more than about 0.012" to 0.014" of clearance. I used to run X-40's in pylon with 0.005". I never shimmed the head higher to aliviate detonation. I would put in a short reach plug (increases the CC volume) OR........... lower the oil content of the fuel. I ran as low as 12% Ucon with no issues. The reason you don't want to jack up a squish band head too high is that the squish ares actually start to trap fuel and cause even more detonation. In most cases (dependent on air density and vol eff.) these tight squish clearances won't allow more oil. I've never had wear issues with the 15 to 18% so...... Yes the octane issue applies to both oils. Yes I use after run oil, if the engine is gonna sit for more than a couple weeks. I usually start with a good blast of WD-40 to float the water and alchohol away from the parts an then shoot in some after-run.

I also enjoyed meeting and talking with you. I'm always amazed to find how easily modellers drop into really arcane technical discussions that outsiders would just stare at and say wa??? Glad we all have SH to stay in touch.

Did some checking and you can assume about a 1 to 1 relationship between oil and octane. A percent of oil causes a single digit drop in octane. If your engine is at borderline detonation the oil level could be a tuning device...... or the reason you're at borderline..

It's easy to see why detonation might be a problem in high-rpm engines such as pylon racers.  Do you also encounter it in stunt?

We had a strange problem pop up at the Brodak.  A friend and I were flying Primary Force profiles with OS25FX engines each with over 2 hours on it.  They flew the planes beautifully at home in Calif, but when we got to PA, they would quickly overheat, run away and quit about halfway thru the tank.  We tried numerous fixes--no muffler, different props, plugs, tanks, more nitro, all with no joy. 

I have flown in Texas in hot, dry weather as a kid, and in Calif in warm-to-hot, medium to low humidity weather, and aside from an accidental, really screamingly lean run, I have never experienced this type of overheating before.  The combination of high heat and high humidity was all I could think of as a cause.  Any thoughts on this?

Thanks,
Kim

Hi Kim. Detonation is actually less of a problem on the high speed engines because the timescale, uh line uh horizon, (uhh nevermind) is so compressed. The cycle times as well as the vol eff/bmep are a lot shorter/lower while the burn rate of the fuel remains the same. Detonation is caused by fuel pockets "cooking off" before the main flame front (started at the plug) reaches them. Detonation requires time and heat so lower speed engines like ours that are running closer to their torque peak ( longer cycles times and higher vol eff) are more prone to it. Racing engines that are proped correctly are much closer to their power peak where mean cylinder pressures are lower (less likely to cook off the charge) which makes sense. The results of detonation are usually worse on the racing engine because the high rpm's means it happens more times. The detonation shows up in stunt engines as touchy needle valves. You know that "drift" that occurs when you are trying to dial it in and it either runs too lean or blubbers. A lot of people resort to ultra low compression ratios to cure it. I don't like to do that because I give up a lot of what the engine has to offer from a power perspective. Others do port timing modes that limit the vol eff of the engine which has the same effect by lowering the cylinder pressures by not trapping the charge. Another way to visualize it is think about your car. If the octane of the fuel is too low when does it detonate? It's always at low speeds especially around the torque peak. I want to think about the issues you were having at PA a little more. Good questions.

Hey, Dave.  It's interesting that detonation shows up in car engines as a very distinctive sounding "knock" or "ping", while in model engines is shows up as a touchy needle, which can be symptomatic of a number of different problems.  You mentioned that "Detonation requires time and heat..."  We had plenty of heat in PA. 

So an increase in oil causes a drop in octane.  At the same time castor has a reputation as a good cooling agent.  We were running Sig 10/20 half-and-half oil.  We also tried some 20% nitro on the theory that higher nitro runs cooler, but my friend abandoned it after one high-rpm run, without trying a richer setting. 

I take it that since you switched to 15-18% all synthetic you haven't experienced any particular overheating problems in stunt engines?  You mentioned no excessive wear problem.   

Thinking out loud mostly here......

Kim 

Answering quietly............. Detonation also shows up as a cackle or crackle sound in our engines. The knock or ping in a car engine is happening at a much lower frequency and an awful lot louder so it's easier to hear.

I was having some of the same issues that you were at Brodak and found when the sun got low and the humidity went down that my problems went away........The air density came back up. The next day the air was cooler and I had no problems. I was thinking back and wished that i had pulled out the head shim on day 1. The air was thin. Hot air has a tipping effect. It will take an engine that's close to having a problem and push it over the edge. When I was helping guys figure out calibration/performance issues at Ford, inlet air (into the cylinder) temperature had a large impact on the ability to even get a stable calibration. Changing from an aluminum to a plastic intake manifold could lower the temps by 50 deg F. The relevant point here is that most of the time when an engine is "working", it's close to the edge. I won't get  into the castor cools things off issue. I haven't used is for more than thirty years so I can only nod and shrug.

No overheating issues with the synthetic. I'm surprised or at least look for problems if I can't pick the plane by the muffler after it lands. No wear issues either. A couple of honest questions: Have you had any experience with pure syntnetic? Have you measured or at least noticed a temp drop with the addition of castor? Not trying to slay any bovines, just talking..

DaveA- we have noticed problems with too little oil in plain bearing engines like the LA series.  Standard RC fuel would run hot and make the engine runaway.  Adding oil, either castor or synthetic or both, up over 25% made the problem go away.  The higher oil also stabilized the 4 cycle running, changing a raggedy sounding 4 cycle into a smooth one.

One of the Expert fliers from So. Cal. said the same thing, tho less extreme, about the Brodak 40.  He had a very smooth 4-cycling B40 on a Classic plane at the state champs last year, and told us one key to smoothing out the run is to raise the oil content to around 26%, which left a couple of us scratching our heads.  A B40 of mine (early model) used to oscillate between 4 and 2 in level flight across a wide range of props and needle settings on 22% half/half oil.  Not sure if that was the same thing as a raggedy 4 cycle.   

I haven't tried all-synthetic myself.  One thing both camps seem to agree on is to stay with the type of fuel you start with.  I could designate a new engine all-synth.  Haven't run that experiment yet.  May want to get a round tuit. 

I agree with Dave, not trying to    here (much prefer to avoid that).  It's especially interesting to me when there are two different points of view even among highly experienced modelers, as there seem to be on the oil thing.      Kim

One of the Expert fliers from So. Cal. said the same thing, tho less extreme, about the Brodak 40.  He had a very smooth 4-cycling B40 on a Classic plane at the state champs last year, and told us one key to smoothing out the run is to raise the oil content to around 26%, which left a couple of us scratching our heads.  A B40 of mine (early model) used to oscillate between 4 and 2 in level flight across a wide range of props and needle settings on 22% half/half oil.  Not sure if that was the same thing as a raggedy 4 cycle.   

I haven't tried all-synthetic myself.  One thing both camps seem to agree on is to stay with the type of fuel you start with.  I could designate a new engine all-synth.  Haven't run that experiment yet.  May want to get a round tuit. 

I agree with Dave, not trying to    here (much prefer to avoid that).  It's especially interesting to me when there are two different points of view even among highly experienced modelers, as there seem to be on the oil thing.      Kim

Hey Kim, this has come a long way...... good thread.