Thanks for the response. When you say open air, I assume you mean an external mount with no pipe. I would think the other key factor to consider in this test setup would be downthrust. I would suspect if the engine is in a model like the Kingcat where there is a significant downthrust, it would tend to register lower on the scale than an engine that is horizontally aligned.

If you are seriously concerned, you might want to talk to the manufacturer and possibly send the engine back to them for testing. Could be some other problem that they can address that is costing thrust.