Air Filter Facts Part 9: Dirty air filters and performance
We proved conclusively that there is no measurable difference in vehicle performance (i.e. acceleration, horsepower, torque) between clean air filters. Is it possible that there could be more significant differences in vehicle performance between dirty (but not clogged) air filters?
As filters progressively accumulate dirt, their efficiency improves but their air flow worsens. In fact, Perrin Performance's pitch is that their filters boost performance after 10,000 miles: "We sell our filters by saying all filters when new flow about the same. But when dirty, foam continues to flow more air. In an example of an OEM filter being replaced with a PERRIN foam filter (on an STI), we see about a 3 wheel Horse Power gain. This is not much at all and not something you can generally notice. So its not that your stock filter when clean is super restrictive, but throw 10,000 miles at it, and the story changes."
We have already seen that, close to clogging, there is impact to performance. There clearly is a great difference between filters when close to clogging, as the capacity of air filters varies widely (later in this series of articles we will show conclusive proof of the widely varying capacity between filters). When one filter starts clogging when another isn't close to it, the clogging filter will certainly worsen engine performance, while the other one should be still able to ensure good air flow. In this case, the advantage will go to filters with higher capacity, assuming that they do not get swapped too early in their life cycle.
What happens when air filters are not clean any more, but not yet clogged, i.e. for most of their working life? Thankfully, we can look at the results from some comparative tests that we review extensively further in our series. The Spicer/Testand study compared multiple different air filters in a series of ISO 5011 tests, using coarse test dust. This is how the tested air filters compare in air flow restriction when accumulating dust:
If we look at the change in air flow restriction between filters, we find out that, among those tested, as long as we remain below the clogging threshold, the difference between filters remains roughly constant, except for the AMSOIL foam filter and the Baldwin filter, where restriction increases significantly faster than other filters.
We know that, around 2-5" H2O of restriction or 0.5-1.25 kPa - that is, for clean air filters-, XXX there is no difference between air filters XXX. We also know that, as you get to 16" H2O or 4 kPa, you may start seeing significant performance impact. Therefore, somewhere between 6" and 16" we should see the first signs of performance degradation - where exactly we don't know.
Initial restriction for a clean air filter at wide open throttle (here measured at 350 CFM) typically varies from roughly 2" to 6" H20. If we were to encounter performance impact, we would expect a given engine to see performance impact from all filters at the same restriction level, regardless of their initial restriction, but higher than 2-6" H2O. Let's take, for instance, 12" H2O as a hypothetical restriction level where we might start seeing performance impact. For this example, we see that the AMSOIL filter will get in trouble first, at 125 grams of dust, followed by the K&N and the AFE at 205 grams, the Baldwin at 220 grams, the generic (no-brand) filter at 300 grams, the Purolator at 315 grams, the UNI and the AC Delco at 340 grams, and finally by the WIX, performing very well dirty at 400 grams. But - the order in which we see these filters could be different if performance impact started at a different level of air flow restriction.
At what level do you see performance impact?
pic shows 10" H20 85-2444.jpg (JPEG Image, 382x550 pixels)
Donaldson filter service indicator PDF call?
AEM filter minder PDF : 10"H20...
We should expect each engine to have a different level of air flow restriction at which it starts showing performance degradation. This level will depend upon the sizing and design of the air intake, and the engine displacement. Unfortunately, we have no data to tell us at which restriction level engines start seeing performance impact. As can be seen from the graph, the set of filters performing well at a given restriction level varies widely with the restriction level: the filters starting with low initial restrictions typically seem to have low capacity, which results in their showing high flow restriction at low with relatively lower dust weight. As a result, without knowing at which level of flow restriction we see performance impact, it is difficult to rate filters on "dirty performance" - although we will try later in this series.
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