Katman

Werner,

if i understand, you haven't constructed a baffle to direct the airflow over the head yet!!?? I'm not understanding how you're expecting it to cool properly. You must also ensure the exit is preferably 3x the intake air area. This is per the instructions.
I have a 60SP, in a H9 Mustang PTS. Cooling-wise w/cowl on, it was a total success(though on the ground just like a real P-51 it will start getting hot lol). You can get creative for inlet/exit air yet keep scale. I actually used the backward facing exhaust stacks(which act as suction) for more exit, using the stacks as a template and removing the cowl behind this where not visible. The baffle i simply built out of a clear(hard)plastic pomegraine juice bottle, which had the perfect contour to force most air coming in the small P-51 inlet up into the head. It was mounted into the cowl w/angled balsa blocks. I also opened up the cowl opening behind the spinner much as possible without being too obvious. And the inlet slightly, without affecting its scale appearance. The aluminum spinner/backplate is very important in my opinion for cooling, and acts like a heat sink. When i shut mine down it gets hot enough to burn you. I'd work around going without.
Now, having said all this my 60SP has failed under warranty and had to be sent off about a wk ago to UK. Please see next post for detail.
You did a very nice job on your plane, I hope my suggestions are of help. Please post how it works out for you!
Katman