Originally Posted By: Garak
This thread could almost be a sticky, thanks to the exceedingly useful and concise information here.
Maybe not quite so concise, but this usually gets cited sooner or later.
https://www.fueleconomy.gov/feg/pdfs/Air_Filter_Effects_02_26_2009.pdf
It gives some example restriction guage limits. I could be mis-understanding it, but it seems to disagree with the statements above.
A 2001 Chevrolet Silverado with 5.3L V8 (petrol) moved into the red at about 5.7 kPa (about 22 inches of water)
Two Dodge Ram pickup trucks with 5.9L (diesel) engines moved into the red at about 4.2 kPa (about 16.9 inches of water). i.e. Less than the petrol engine.
The paper says that the restriction guages read the pressure difference between the area behind the filter, and atmosphere.
The 10 inches of water guideline for filter change is an
excess, over the pressure drop caused by a new filter.
Unless newer restriction guages can be zero'd with a new filter, (?) which I suppose would be possible, then it seems they don't (or at least didn't when this paper was written) read this excess directly.
