As diesel engine emissions standards become increasingly stringent, one of the most commonly employed method of emissions reduction by engine manufacturers is active control of inducted air and recirculated exhaust gas (EGR). Most often actuators such as an EGR valve and a variable geometry turbocharger (VGT) are used to manipulate the air flow through a diesel engine to provide the reduced engine out emissions. This paper evaluates four different multivariable controller designs for control of a diesel engine air path: three built up from Single Input, Single Output (SISO) transfer functions and one that is a fully multivariable design. Various performance metrics are analyzed to gauge the relative difference in performance capability while maintaining a simple controller architecture. As part of the analysis, the controller designs are simulated with a realistic non-linear engine model against a modified standard certification test cycle to give a sense of how the controllers would perform in a real-world application.