It needs to be for the engine you are using, they are all different.

As to temperature, yes, you can run an engine too cool, the nitro might not burn properly and there is a chance the engine will wear prematurely. I actually restict the water flow to my glow engine at this time of year, and my water cools my pipe before reaching the engine. As a guide, after a run, I can hold onto the the head for about 2 or 3 seconds before burning my fingers.

What kind of 7.5cc engine are you running that is making 5HP?

Glenn