If you climb at low speed, just put in a notch of flaps, and the cooling increases. How much depends on flap configuration and how much a “notch” actually is. The idea is to lower the nose, moving the stagnation point further fwd so the pressure at the cooling air exhaust decreases. The Cherokee (O-540) had no problems pulling gliders at full throttle at 70 knots. The WT-9 (watercooled Rotax 912) will overheat on warm days with no flaps. These are just differences in how the cooling systems are set up. The Cherokee is originally an agricultural device, the WT-9 a fast cruiser, never meant to climb continuously with a high nose at full throttle at 70 knots.
@Peter: I understood that the reason Lycoming advises a max temperature change of 50 C per minute is due to the risk of sticking valves. Lycoming was able to reproduce valves sticking when shock cooling in a test cell. Can’t find the source at the moment.
Since the TB20 engine is de-rated from 300 HP to 250HP, running it at 75% HP should be equivalent to running it at less than 65% of its engineered maximum power, since it is really the same engine as the K series, say IO-540-K1A5, which achieves its 300HP through higher RPM of 2700, and slightly higher compression ratio, while still having the same TBO of 2000 hours (and same metallurgy and wear profile). In other words, in the case of the TB20 IO-540 you cruise never at more than 65% max power of what the engine is engineered for. At 16000 feet or above where you appear to cruise most of the time the effective power is less than 50% of max engineered power, or closer to where a car engine runs. I would therefore conclude, that practically speaking, the TBO of the 250HP IO-540-C should be 2300 hours, if the 300HP version has a TBO of 2000 hours. Right or wrong?
Yes it could be the valves. Or any other area where differential expansion is an issue.
I think this “de-rated” is not very meaningful if you are comparing two engines with a different CR. There is a corresponding difference in stress. For example I have it on good authority from an Exp engine builder that while my engine will normally make 2000hrs easily (8.5:1), if you put in 9.5:1 pistons but keep all else the same it struggles to make 1000hrs. The 9.0:1 pistons are apparently OK and the engine doesn’t need 100LL (it can run on 91UL).
There is an IO540 which does 350HP, too, AFAIK. Probably at 3000rpm+ 
I am sure there is no problem with running at 75% in cruise all the time, so long as the CHTs are OK. I am not sure they would be (on a TB20) and when running in an engine with new or re-honed cylinders (when you have to fly at 75% for quite a number of hours) you have to fly ROP. I did that in 2002 and again in 2008. But I would have to test it to be sure.
The CHT and cooling issues are an airframe design compromise. Minimizing cooling air flow to that required for adequate cooling at cruise power/altitude maximizes cruise speed. Cowl flaps when fitted help reduce the compromise at the expense of another system. Also, the higher the CHT, the lower the required cooling flow & drag. The higher CHT of an air cooled engine is a cooling drag advantage over liquid cooling which is limited to 100 C / 212 F or a little higher. Increasing efficiency requires stressing material more – which is often the case in any design situation.
Pressurised liquid cooling isn’t limited to 100C.
Welcome to the 1990es.
Perhaps a little information and data in your posts might be helpful Shorrick Making endless, bitter one liners does make one look silly.
That aside, I said “limited to 100 C / 212 F or a little higher”. Late model automotive engines operate in the range of 200-250 F, consistent with what I actually said, but you’re right in your implication that they’ve increased coolant temperature by about 30 F over the last few years to move it into that range. One of the motivations to do so was to reduce radiator size. An air cooled engine does the same thing only more so by eliminating the radiator and raising the cooling surface temperature to perhaps 390 degrees F. Obviously that creates issues with material strength if taken too far, but it provides a much higher temperature differential relative to ambient air, and more effective cooling for the same surface area.
What temperature and pressure do the Mercedes car-based aero engines run their coolant?
thanks all for your comments, and the added info regarding why certain rules apply to lycomings.
I am going to tackle replacing the baffles in the morn!
Will take pics and post them if there is any interest.
Timothy – have you checked your EDM is not out of adjustment?
Oh yes. This diagnosis has been going on for nearly 20 years!
I’ll gladly expand Silvaire. OM668s can run as high as 2 bars and 130C cooling.
Further to that, as I am sure you well know, “effective cooling” is a function not only of temperature delta and surface but also of specific heat. Liquid cooling has been operationally proved since the 40es to be vastly more effective at channelling heat away from cylinders in a more aerodynamic package. The only nag people could find about it was “it doesn’t deal with bullet holes very well” – which while true doesn’t apply much to GA operation.
So as an engineer you have to ask yourself whether you build a “simple” engine that is dependent on external factors like weather or airport infrastructure in order to reach design clearance parameters (truncated cone cylinders… great idea) – or a marginally more complex design that is self-sustainable in operation and does not impose performance penalties.
Well – you ask yourself these questions if you actually want to build a product that actually sells.
How many instances of water pump failure or heat stress induced internal failures on OM668 derivatives vs heat stress induced failures on air-cooled designs?
The Extra 400 has a Continental TSIOL-550 which is liquid cooled. It was expected to be available in the 400hp range but never made it to more than 350/325hp which is why the Extra 400 eventually failed. However, from what I hear, the engine is very reliable and easy to keep within its operating temperature range and generally a good engine. Even the primitive avgas engines can benefit from liquid cooling.
The OM668 aero engines have very favorable reliability figures. Initially there were a bunch of issues, mostly connected to situations where the ECU could lose power which is obviously fatal for those engines. That was addressed by extra redundancy.
Obviously the very crude setup in the DA42-NG is not an argument pro liquid cooled engines. That was a huge step back in engineering quality from the Thielert engines. With the latest DA42 VI, it looks a lot better.
