Dirty Filters May Impact Performance

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 10: Dirty air filters and performance

In our last post on air filters, we reviewed dirty filter air flow and concluded that there was possibly a range of air flow restriction which could impact engine performance. The air flow curves were characteristically shaped, most of them being exponential with dust accumulation, with the exception of a small number being linear. Can we corroborate this data? 

The Southwest Research Institute published a research report commissioned by AEM on similar tests:

Regrettably, the SwRI restriction tests occurred at a low flow level of 240 CFM, while the Oak Ridge National Lab tests occurred on the vehicle itself at wide open throttle, and the Spicer/Testand ISO 5011 tests occurred at 350 CFM, a better level to approximate true/ wide open throttle conditions. As a result, the initial restriction shown on the SwTI tests above is significantly lower than in other tests, and restriction under dust load is also affected. The SwRI tests also include filters that have been washed and dried five times: the changes in air flow between new filters and washed filters are interesting to note.

Nevertheless, the same curves show up in both graphs: similarly to the data in the Spicer/Testand study, there is relatively little spread in the change in air flow between filters until the clogging elbow, except, in this study, for K&N, which remains at very low restriction, increasing its advantage in air flow against the other filters until it hits its own clogging elbow. The AEM filter in this study appears to behave similarly to the AC Delco in the Spicer test, having a very linear curve on air flow vs. capacity. As the two tests use different reference dust (coarse test dust for the Spicer/Testand study., fine test dust for the DwRI study) and vehicles, it is not possible to directly compare them quantitatively.

Since we know that, when they reach clogging, filters definitely impact engine performance, it is fair to say,  at some point as we move towards the right in these curves, we will start seeing some performance impact. It it is possible that a small number of filters might do worse than others. No filter, however, appears to significantly improve performance as dust accumulates compared to the majority average. it is worth noting that capacity impact on performance appears very significant when you get close to clogging.

Conclusion

• There is no difference in vehicle performance between most dirty air filters 

• There may be some differences in vehicle performance between some specific dirty - but not clogged-  air filters

• There is no proof that there is a difference in vehicle performance between any two dirty - but not clogged-  air filters

• Filter Capacity may have more impact on performance than air flow over the life cycle of an air filter

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