Originally Posted by Resolve
Turbo just there to help effiecency of diesel.
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Not true. I have owned the turbo and non-turbo version of a few different diesel engines. For example: a W123 chassis Mercedes sedan with the turbo diesel has almost 65% more horsepower than the non-turbo version of the same engine. Some examples are more mild, like the 6.5L Chevy diesel, in which the turbo version only has about 20% more power... but the implementation of the turbo on that engine was pretty weak. Aftermarket turbo kits added more power than that.
What a turbo does, in essence, is make the engine behave like it has a higher displacement by cramming more air into the cylinders. This, in turn, allows more fuel to be injected. The up side is that it only uses more fuel when requested, unlike a larger displacement engine, which must inject more fuel all the time to maintain a stoichiometric ratio. Diesels have a much wider working fuel/air ratio, so they typically don't take much of a hit when a turbo is added, unless they are used at high load all the time, like a generator or a boat engine. Car engines are usually at a very low load, so they tend to benefit a lot from being turbocharged.