For all you people who think 240F CHT is too hot, look at page 33 of the new Fly RC magazine..There's a test by Andrew Coholic on the new Evolution 26...
CHT runs from 278 to 342....
FWIW, a converted G26 turns faster than the Evolution in the test..And is lighter...
And he calls it an EVO...Wait till Forgues sees this

My old Kawasaki 100 always tops 280F and it has over 6000 miles on it.

I have been told by a couple of manufacturers that most 2 cycle air cooled engine runs best and happiest at around 300.
I have also talked with a few people that run telemetry in UAV's and the HIGH 300's are not uncommon on twins that we commonly use, though they like to shoot for 300.

Ralph, You mean the Evolution 26 is not as strong as the G26? That can not be[X(]

I think that Andrew guy is the same dude that did a review of a H9 260 that only weighed 11.5 pounds with a Saito 220. He was obviously wrong on that so I wouldn't trust his stuffon this

Much depends on how the engine is installed, and at what mixture the engine is run.
FWIW:
I found cylinder temperatures around 150C (~300F) on most engines when adjusted for peak power. This is in close agreement with NACA technical notes on radial engines.
Power remains almost the same when tweeked to 180C (355F), which is getting quite hot.
Power dropped when adjusting to 120-135C (240-265F). This caused the spark plug to turn quite dark.
All this with non-cowled engines running medium sized props for the engine size.

Head temperatures near the plug boss ran some 15C (25F) higher.

I use an IR meter with open angle of 15 degrees (1:6), so at a distance of 6 cm I have 1 cm diameter measured plane surface.
Correlation to mixture setting is very good, so when setting the needles I now almost exclusively use the IR meter. Then I use the rpm meter to check rpm consistency.

All well and good
except
the method and timing of the measuring process can totally distort actual temperature readings
We all know that .
Assume :
The typical model, flown idled down and promptly landed
Asume one now takes the same measuring device and holds in the same position and reads at same point .
(This question for Pe-as he knows which point and distance he is measuring ).
Personally I have a devil of a time determining what figures to suggest as proper.
I don't have telemetry-- and on ground, constant full throttle running simply runs heat way above typical in air temps.
So --- the temps noted above----
are they from a thoroughly heat soaked engine -- or when?

With a properly adjusted mixture and well designed airflow, I know that even a pusher design gasser can run effectively and efficienly at temperatures of 120c and below. At mid range throttle settings that same engine can easily run as low as 85c. I also know that you're in trouble with eventual failure due to heat induced issues when you break 180c. 150c is a very good place to stop an engine and let it cool with a single, with 170c being about my personal max for a twin.

With the proper mixture and oil ratios (50-1) there's no reason that the plug will not last 50-100 running hours, even at the lower temperatures. There's far to much proof of that.

Pe, you market the engines that developed those numbers and I'm betting that you're aware of how much hotter us modelers are running engines that we should be.

Dick, and Silversurfer.
The reason I am conducting all these measurements is just because I have inspected a few engines which were run way too hot, resulting in failures of bearings and even a slight start of piston seizure damage (even with graphite coating!). In these engines the liner had turned blue (270C), and the anodize on the head was bleached.
I too had a hard time predicting some usefull data on engine temperatures, so I spent my holiday doing literature and engine-exhaust research with my newly aquired line of MTW mufflers. This work resulted in the above statement, and I now am much more confident (edit) when adjusting the carb of an engine.
It would be nice to have a sample/hold thermocouple sensor under the plug, but I do not have it. So I blast away full throttle with the 26/35/45 engines in it's safe operating rpm of ~7000 - 7500, until the temperature has soaked sufficiently and the temperature rise curve has all but flattened out. This takes about 1.5 minutes. After that I stopped to relieve the engine again lest it might come to harm. The highest temperature thus measured at lean setting was 200C. (Rotax quotes 220C as absolute max, measured with thermocouple) Best power ranged from 140 to 180C, power drop is present below 130C. This slight drop in power is what most adept gas engine fliers look for and accept as the best general adjustment. It is not rich enough to turn the plug black. It just is slightly darker. Plug fouling starts as low as 110C and down. (prolonged full bore static running up to complete heat soak). I took some plugs apart to inspect them in detail.
While not scientific, I found measurements to be sufficiently reproducible and not at all critical. On two or three successive runs, the measured temperatures were within five degrees of each other. Better accuracy would only serve academic purposes, which is not my objective.
An engine in flight runs cooler, except for prolonged 3D flight, where the plane also is stationary most of the time with an even lower airflow over the (cowled in) engine. The cowl through flow is directly related to pressure differences between inlet and outlet, which is relative to the square of the prop wash speed over the air extraction opening. Most (not all) model airplane designers never thought of cooling an engine, so very often 3D flight imho is grueling if no attention is paid to proper cooling air flow.
Dick, I know you run your engines richer. Additionally, NACA literature states that with altitude, the power-cooling balance tips towards better engine cooling, so your results may not match mine. Your engine temperatures up there will be lower than mine at sea level.

For the mathematically challenged...
180C is 392F
F = 9/5 C + 32
C = 5/9 (F -32)

Thanks for the input
The problem -
reading the plug/checking the piston skirt/ring etc., all show ME that best running temps are around 200F when measured after the engine has finished a flight and taxied back ,been shut down then measured
This is the procedure use by a number of fliers . It does allow for some temp reduction
Obviously the in air temps are higher
however - 400 F on any engine is way over the top-and 300F is in my book over the top when measured as most flyers measure temp - after running.
On water cooled engines when I was a kid , temps had to be below 212F (boiling) as the radiators were not pressurized
Now of course - pressure caps allow a significant rise , resulting in better efficiencies
but these temps are not applicable to our model engines -measured after the engine shuts down.
So - if someone is looking for an absolute number - be careful -it all depends on how and when you measure !

Here is the method I employ.

I picked up a temp gun that has a "MAX TEMP" option. I press this button and pass the gun back and forth over every area of the engine I can hit with it. The gun will then display the highest temp it found.

I do this immediatly after shutting off the engine and then a couple more times after that to see how saturated the engine temp was.

I run a ZDZ 160 Champion and my temps are typically between 180-190 F immediatly after landing. Occasionally on days over 100 degrees I have hit 220 -250 F.

I also check a friends engine. He runs a DA 150 and his temps are right around 120 - 130F immediatly after shutdown, but increases to 140 - 150F in the minutes following shutdown.


I go to great lengths to keep my engine cool since it only take once to heat damage it

Macanaw
David Fox

Dick,
You are measuring an engine that has been cooled well in the approach glide and subsequent taxi run. There is no way of telling what actual full throttle measurement was unless you use an on-board sample and hold system.
I know most modellers do it that way; that does not make it right. I also know, it is so useless that I see no point in doing it that way.
NACA reports on Whirlwind Radials mention 300F (~150C) as standard on cylinders under full load as safe max temperature. (valve housings in the head at 475F)
Like said, in the Rotax manuals, ~430F (220C) is mentioned as absolute redline using a CHT sensor under the plug. Normal operation is about 355F CHT
Our model airplane air cooled gas engines are no different than the full scale brethern, though the small ones should run slightly cooler because their size allows batter heat management.

Checking Temp after landing is worthless IMO, unless It is used as a reference point. I have an in-flight temp measuring device (the cheap one used by the R/C car guys) It gives me the maximum and minimum temp during the flight. My DA 50 has a maximum in-flight temp of 305°F on average. Very consistent with was it is being said here.
Andres

well --- our engines do differ--in that most of the gasoline specific purpose model engines are aluminum cast jugs with a coating only on the inner walls
On most big stuff they use steel barrels - whole different ballgame
aluminum looses 50% of strength at 500 F
On some small industrial engines they use liners and on some converted gassers - liners
these are much more rugged in withstanding cyl pressures at high temps .
Also engines such as the ROTO there are cylinder liners

very rugged, but heavier setup
checking after landing ain't all that bad -as long as the test is consistant
Look at it this way----
adjust mixture and prop load to produce a power curve that NEVER sags

Listen to the engine
Look at theplugs
also measure temp -however you choose to do it
If the sound stays right and the temp remains the same -you are doing the right thing
As for racing engines which keep moving - I would want an on board temp -and then I would push that engine to the limit - that is what racing is all about
I would blow up an engine at least once - find out what it could stand.

been there -but in cars not models except way back when on Quickies
I

I'm going to have to agree with Dicks' 200f number. And with 180c, or 392f, being much too high. As for using a heat gun "immediately" after landing, the engine has had the benefit of a reduced throttle setting and a light engine load for the duration of the approach. Either by themselves is enough to cool an engine substabtially in 30 seconds after either is initiated. Just reducing the throttle to idle from full on the ground can decrease an engine temp by 80 degrees or more in the fist 10 seconds. Add to that the dubious accuracy of any of the heat guns and you will find that even using one as fastideously as possible to be more of a "feel good" thing than anything else.

26 and 35cc steel liner; 45cc nicasil barrel. Rotax has nikasil barrels.
The steel liners will show a slightly lower cylinder temperature, but that is it.
Should an engine sag in the test, power is shut down immediately and the carb reset, just like you do. Power loss is a lean mixture telltale. A well tuned carb will climb up to best rpm as the engine heats up.
That is what I would call fully leaned out. Tuned like that, the engine will scream in a dive, and never enter a four-stroking pace unless throttle is reduced. A richer mixture will four-stroke when in a dive, and still burn the plug clean. This is about the range I used for my temperature measurements.

Yes - engine INCREASES rpm as it warms up - that is proper- but many do it wrongly - they set carb for max rpm in level flight or shallow dive.
One can tell who knows how to set their engine for best power.
Just by listening - -no need to even look.
Steel liners have a huge advantage at high temps -stability.
Aluminum castings can compress or distort as they reach 500F
The steel/ iron liner does not transmit heat as rapidly but the extremely dense surface and high temp stability are a plus - IF weight is not a problem.
Good ol General Motors once built one of the most wretched engines in the world - the "upside down " Vega 4
an aluminum block and an iron head
Shows what you can do with virtually unlimited resources and limited ability.
--

the last statement. Vast resources still does not fool people very long

I found the plugs.
The left one had run between 140 and 180 °C. The right one at about 130°C but is is too cold for the engine. It does show the wider black mixture ring though if you look for it.
Plugs 2 and 3 are correct heat range, but with the engine too rich (the black mixture ring has expanded nearly to the tip) at temperatures below 120° C

Just for an experiment, quite a few yrs ago, I took the shroud and fan off a 250 Rotax in an Elan to see how it would work as a freeair engine, just for laughs. Thermocouple under the spark plug. Winter around the freezing point. Idled around 300 F Going along with the clutch in ,around 15 MPH. around 350 F. Any more throttle than that, temp would go up like the second hand on a watch. Back off and it would go down just about as fast. I never let it get over 400F. As you say, on the ground tests are useless if you land and take the temp. Going by plug colour is still the best method of setting the carb.