Diesel engines are most efficient when fully warmed up and fed lots of cool air for combustion. Gasoline/petrol engines also perform better when warmed up but intake air differs in a Diesel. Many Ecomodders driving gasoline engines report much better mpg by using a “warm air intake”. They reroute the intake plumbing to pick up air from near the exhaust manifold so that it is warm coming into the engine. This may reduce peak power but they do most of their driving under low load/part throttle conditions. The throttle plate is mostly closed, the engine is working against high vaccuum, it is not running in its most efficient range. Warmer air is less dense so the throttle can be opened further for the same speed and they have lower pumping losses.
Diesels like lots of cool clean air. Since they don’t have a traditional throttle plate they can ingest lots of air and run very lean, using very little fuel, without creating vacuum. (The vacuum for the power brakes is from a vacuum pump driven by the camshaft). There is no need to route intake air from the exhaust manifold except perhaps to speed warmup. The downside is that extra oxygen can lead to higher NOX emissions so newer diesels have employed lots of technology to lower NOX emissions (and unfortunately reducing efficiency at the same time).
In general, DO NOT modify the intake for warm air in a diesel. Give it plenty of cool high pressure air from the front of the car (stagnation point). There are some intelligent tuners that sell smart tunes to decrease the demand for EGR that help the diesel run more powerfully, cleaner engines with less carbon buildup and return better mpg if that is your priority. Off-road or track use only of course.
