Hi, the proper max rpm under load for the TMD 22 Volvo as installed in the Amel SM is 2800. ( no load 4500) The turbo has what is called a waste gate which automatically controls the amount of exhaust gas going through the turbo as rpm increases. If this is stuck closed (will happen) the motor will rev much higher, even up to 4000 rpm and beyond. This is caused by over turbo pressure into the engine which will cause damage to the pistons and rings which in turn will reduce the power the motor can develop and result in lower revs and the need for either an overhaul or replacement. 

Regards

Danny

SM 299 Ocean Pearl