I had to add a couple STC‘ed extras to cope with the heat. I went through pain and tears over trying to cool my left engine side. CHTs of over 400dF were the norm.
I know for a fact that although airflow over the cylinder fins only accounts for ca 10% of cooling, it can make a big difference. Correctly installed baffles are an important factor in any airplane, and more often than not old or worn baffles are the reason for overheating. The pressure differential between the upper and the lower part of the engine is not great anyway, so especially with the small intakes in the front, so any small hole in the baffling can effectively destroy the airflow and the air then circles over the upper engine side without being able to flow through the fins to the downside part.
Another item may be the fuel flow – check the fuel system if there is enough fuel being fed to the cylinders to cool them internally via incomplete combustion.
I do not make the statements about the DR400 cooling system without some knowledge of the basics, this is the second DR400 I have owned and both show much the same cooling characteristics.
Apart from checking the baffles the calibration of the CHT, oil temperature probe and EGT have all been checked.
The fact is for performance reasons the DR400 is close cowled and the cooling restricted to give just enough cooling at cruise speed and power. All aircraft design is a compromise and Robin decided that cruise performance was paramount , OK I have to step climb on a hot day but I will still lift more payload out of a shorter runway and cruise 15 kts faster than a PA28 with the same engine.
