The photos below show two very different air filters. The small air filter on the left is from a Nissan 1.5 dci ( turbo diesel ) engine, while the filter on the right is a brand new air filter for a 1.9TDI VW/Audi engine. The Nissan filter was taken out of a vehicle with approx 50,000 kms on the clock. It was completely overloaded and almost at saturation point, as we are trying to show with the new filter that is next to it. It was very obvious that this filter had never been changed. Now apart from Nissan’s obvious error in designing a stupidly small air filter ( actually its Renault that made that mistake as the 1.5 dci engine is a Renault engine, Renault have a 43.4% stake in Nissan) for a turbo charged diesel engine the biggest cost to the customer is not the air filter ( approx E 17.50 depending on the model ) but the substantial increase in fuel consumption and the unnecessary load put on turbo.
The best way to describe this is to imagine yourself going for a jog. While you are jogging someone is cycling beside you, and for every intake of breath you try to take that person is restricting your airflow by gently putting his/her hand over your mouth. You will still be able to run, but you will have expended almost twice the energy to 1: overcome the restriction of your airflow and 2: to get to the finish line.
This principle can be applied directly to turbo charged diesel engines. The engine is over fueling to try and drag more air through the overloaded air filter.
At V.A.S.S. Motor Works we fit air filters as standard to all turbo charged diesel engine at every service. The extra cost of the filter is completely negated, multiple times over, by the fuel saving until the next service and also improves the life of the turbo itself.