There should be an option for each engine to show there rpm. For each engine there is a setting that says "show rpm?" And when you enable it you will see in the flight at the bottom of the screen how many rpm your engines are running then the max rpm with the name of it (you can name the engine in the designer) like this.

 (ENGINE 1)           (ENGINE 2)

5681/6000 rpm 5680/6000 rpm

I would really like this since I use manual propellerPitch and Throttle it helps if i could see how many rpm my engines are running at.