In my experience so far (limited, since I just have three engines tested so far with the on board telemetry) the temps cool off a little - about 20 or so degrees F - in the air. The last engine, the SAito 2.20 running an 18x8 prop for example ran about 25 F cooler in the air.

However this was in all three cases on my test plane which leaves the engine uncowled so the engine is getting very good cooling from the airflow of the prop and from flying through the air. It makes sense since the plane travelling at 70 MPH shoudl get plenty of cool air over the cylinder compared to just what the prop throws sitting on the ground.

As I use the RCATS system more and on a more varied selection of both gasoline and glow engines, it will be interesting to see the various readings.

AJC