All engines, including turbocharged engines loose output at altitude. It is usually more noticeable on naturally aspirated engines, but turbos suffer too unless they are designed to maintain a fixed level of absolute manifold pressure; however, they are usually set up to limit boost to a specified gage pressure (pressure relative to the surrounding ambient pressure) , which means absolute manifold pressure will go down with altitude.

High temperatures prevalent at high altitude in tropical areas further contributes to a lowering of inlet air density, so it's a double whammy.



Duke