Hi Folks
Well a bit of history here!!
In the beginning the engine manufacturer neded a place to run a tachometer - piston engine era - the front end was kinda busy with the prop and the back end was busy with the starter drive so they looked a a cheap way of running it and decided that the camshaft wasn't reallt being used so they hooked it up to that - runs at 1/2 engine speed in a 4 stroke engine.
Enter the gas turbine - the English engineers stayed with an actual RPM indicator in the beginning - ie RR Darts etc. - The US military decided to standardize the 3 phase tach generators and decided that 4200 RPM was going to equal 100%. This meant that a standard tach gen setup could be used for engines and gearboxes and that the engine manufacturer now had a standard drive system so that different engines would be able to use the standard tach system.
The reason some engines have a gas generator speed that is allowed to be above 100% is because of the drive train from the rotating unit to the tach drive. If my engine is designed to run and produce rated power at 37,500 RPM and my tach generator needs to turn at 4200 RPM to give me 100% speed indication then the gear ratio needs to be 37,500 / 4200 = 8.9285714 which is going to be imposible to get!! Soooo the engineers' have a choice - go up one tooth in the gear train system or go down one tooth in the system. The obvious way is to go 1 tooth less - that way at 37,500 RPM of the rotating unit the tach drive will be turning slightly faster giving you 101.5% perhaps. Pilots don't like not being able to get 100% so this way they think they are getting more - but they aren't.
If you notice that most turbo props with mechanical tach systems have a propeller speed that at the maximum RPM = 100% or 4200 RPM input into the tach generator. The instrument face can be calibrated in whatever units the manufacturer wants.
In addition some engines have a higher speed allowed as the engine is developed into different configurations - a good example of this is the old Bell 47 - the engine was run slightly faster for a bit more power and to make it easier to design the gear reduction unit to the rotor. The downside is a lower engine life.
Hope this helps a bit.
