Exhaust temperatures are some of the hardest parameters to measure and estimate based on the range of the signal and the environment that an engine exhaust system creates. Accurate exhaust temperature inputs in vehicle and engine control systems are important for performance, fuel economy, emissions and aftertreatment control. A turbine inlet exhaust temperature observer model based on isentropic expansion and heat transfer across a turbocharger turbine was developed and investigated in this paper. There are 4 main components used to model the exhaust temperature; an open loop exhaust manifold gas temperature mass/energy model, an isentropic expansion across the turbine, a turbine heat transfer model and an observer using the downstream turbine outlet temperature. Another method using only a reverse isentropic expansion model and heat transfer parts of the observer model was analyzed and compared to the observer model. The models were validated using data from two different engines that are representative of the end points of the range of diesel engine sizes and outputs, one sold in the European market and the other in North America.