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Re: How to improve fuel efficiency
There was an American study a while back. It doesn't. If the airflow is restricted the engine management just compensates.
Since reading it I haven't replaced an air filter.
Google "Effect of intake air filter condition on vehicle fuel economy".
Theoretically the engine management system should compensate (as the air reduces so the fuel content should match) but most studies will admit a dirty filter reduces acceleration so it it takes you longer to get to the best optimum economic rpm and road speed it is bound to affect fuel economy in some fashion. Most American test results I have read seem to apply to engines 2.0 to 4.0 Litre and above so it would be nice to see a similar test for smaller European engines. Most studies seem to relate to petrol engines and as diesels are known as free air breathers a dirty air filter will have a greater affect.
Also you have to remember once a filters pores start to block you get what is called break-through whereby the filtter begins to separate and allows bigger particles to enter the air intake which is not good for the engine with the risk of increasing engine wear which in turn will definitely reduce the engines overall peformance and fuel economy.
I try to take the overall view in that comparing the cost of fuel in Europe against the cost of an air filter any gains by having a clean filter are cost effective even if it only improves fuel economy by 1%. Assuming a filter costs around £20 and you change it every 12,000 miles the cost pales into insignificance against the cost of the fuel over that time. One percent of 12,000 miles is 1,200 miles which will cost the average motorist 150 litres of fuel or an estimated cost of £200.
No doubt if I have got my figures wrong please feel free to correct me.
Currently in madnoel10's garage:
Honda Civic 1.4l