We know that V.I.T gets effective after 40% of M.C.R. But why is that it is not functional below 40% though V.I.T will improve sfoc and gives better efficiency.
The reason is an increase maximum pressure. During slow running of engine, the lube oil film on the bearing is not enough to bear a heavy load as the rpm of the engine helps to make and maintain the oil wedge formation which takes the load. If higher pressure is created during firing then this weak lube oil film will not be able to take the load and hence bearing can get damaged. For this reason V.I.T is made operational after 40% of M.C.R.