Yeah 2.5hp out of the .21's is pretty common now adays. It's all in the rpm. These high rpm motors make crazy rpm but no torque. This is a rc car motor turned water cooled. 10mm carb throat is about what .32 and .40s use. I like how the water cooling goes around the glow plug. I wonder what temp it runs at?

I have Paris modified Picco .21's from 1994 or so, it's rated at 2.5 and that was a lot then.