"Actually I wonder if it makes any difference to the transfer function if the engine is to run at constant speed, something like 60,000 rpm "

The simple alogorithm that I gave in my earlier post was based on trials conducted on a turbine running at 30,000 RPM. We were able to achieve a stability +/- 1000 RPM at the rated speed.

However the actual figure in your case will depend on the step response figures for the throttle mechanism and interial load on the drive shaft.

( I think we have gone too far away from 8051 and its proper that I stop here ! )

Raghu