The air intake system in most cars involves a plastic airbox that houses a rectangular filter. No matter what car you have, l bet you can find cheap filters and expensive filters—so you get to decide if the fancy one's worth the money. Since cars and conditions vary so much there's no stone-set definitive answer, but I think the results I got testing five different filters in my K20Z3-powered Honda Civic Si will apply fairly universally.
In other words, if you're wondering if a high-performance drop-in air filter is worth buying, this story will explain why the answer is "no." At least, from a power perspective.
For those of you who just want to know which filter is best, here you go: The horsepower and torque curves were all so close across the board that the difference is pretty much moot. However, look closely and you'll see that the Honda OEM filter had a slight edge.
Denso (used)—177.0 hp, 118.4 ft-lb of torque ($9.84 at RockAuto)
Honda OEM—176.7 hp, 118.4 ft-lb of torque ($17.74 at Majestic Honda)
Ultra 8—173.6 hp, 116.4 ft-lb of torque ($8.99 at NAPA)
aFe Power Pro Dry S—175.3 hp, 117.5 ft-lb of torque ($78.40 at aFe Power)
Spoon Sports—174.9 hp, 117.3 ft-lb of torque ($90.00 at Spoon USA)
Realistically, they're all so close that it's more apt to say "the filter isn't a bottleneck" in the stock air system, rather than "one filter is superior to another." Nevertheless, it's cool to log more empirical evidence of how aftermarket car parts perform.
Here are the filters I tested and why:
The Denso unit that was in the car—I'd bought it thinking Denso was the OEM supplier for this part. Turns out it isn't, but I figured we might as well test the one I drove down with
Honda OEM—gotta test the factory filter with the factory airbox
Ultra 8—this was the cheapest available air filter, period. My local NAPA Auto Parts had this (significantly) cheaper option than all the other auto parts stores around here, and the price on these even beat Amazon and RockAuto once you factor in shipping
aFe Power—an American tuner that's been making intakes forever. It makes a high-performance oiled air filter and a dry one (we tested the latter)
Spoon Sports—an elite Japanese Honda tuner. This part is technically for the eighth-gen Civic Type R, but it fits the Si airbox (would also fit in a Honda Element, if anybody cares)
I sort of fell into this test backward. My Civic had an Injen cold-air intake on it when I got it, and the car ran like crap. I got it professionally tuned with an ECU management device called a Hondata FlashPro which made the car run nicely—this is a critical step in car modding. But the cold-air intake system placed the filter down near the wheel and I was worried it would be dangerously susceptible to contamination from the elements. So I decided to build a stock intake (requiring a surprisingly protracted scavenger hunt for parts) and install it.
Still seeking optimization, I wrapped the stock intake in heat-reflecting gold tape and started researching air filters. I knew about oiled K&N filters and their claims of added hp, but I'm dubious of oiled filters for reasons I'll articulate later. What I didn't know, however, is that tuners aFe Power and Spoon Sports both sell dry drop-in air filters that fit my eighth-generation Civic and promise performance enhancements. "I wonder if they actually do anything," I thought, trying to convince myself to pony up $90 for the Spoon one because I really wanted a Spoon "part" in my car for coolness (hah, I know—I'm goofy as hell).
But then I realized: It would be cool to know the actual horsepower of my K20Z3 engine as a baseline against future mods, plus the tuner could optimize the car as-is. And since a factory air filter can be changed in seconds, it wouldn't cost much dyno time to try a handful.
In addition to the Spoon and aFe units, I thought it made the most sense to compare them against a factory OEM filter. Then for the sake of even more context, I figured, why not grab the cheapest filter available? In hindsight, I wish I'd grabbed a Wix and a NAPA Gold unit too, because those are my typical go-tos. But oh well, had to stop somewhere.
The Stock Filter Rocks
Turns out Honda's filter really is optimized for Honda's intake system. In this case, at least by this measure, the expected result came true.
Honda's factory filter for this car is made by a company called Filtech. That's a Europe-based supplier that furnishes many different types of industrial air filters as original equipment to various companies.
If you have a stock or lightly modified Honda, I'd highly recommend running this filter since it seems to be optimal from a performance standpoint and it's clearly capable of good filtration—eighth-gen Civics are pretty darn old now and I've never heard of reliability issues related to particle ingestion.
Test Car and Conditions
My eighth-gen Civic has a little over 100,000 miles on it, the valve lash was adjusted by a mechanic a few thousand miles ago, and I'm running fresh Eneos oil and reasonably new OEM-spec NGK laser iridium spark plugs. Fresh OEM coolant. The only performance mod is an A'pexi World Sport 2 exhaust—which I chose for sound, not speed.
We did our test on a warm November day at Evans Tuning in Pennsylvania (around 60 degrees ambient, dry air) using ProHub Dynamometers. Jeff Evans, the tuner who's worked with my car a couple of times now, indicated that this type of dyno would typically read on the lower side and that the margin for error would be about half a horsepower.
Takeaways for You
If you're running a stock airbox, splurge on an OEM air filter (or quality OEM-style brand) over the cheapest thing on the auto parts store rack. But don't spend crazy coin on a fancy-looking one. Yes, there may well be some situations and setups with which the aFe Power or Spoon Sports air filter can add some tangible performance benefit ... maybe if you had more aggressive aftermarket cams or something? But at that point, you're probably running a full aftermarket cold-air intake anyway.
Naturally, this result will be the most prudent for those of you running Honda K-Series engines, but since our research has substantiated common lore, I'm confident saying you're most likely going to get similar results with any nearly stock car.
Why I Didn’t Run An Oiled Filter
Oiled air filters, like those available from K&N, usually claim better airflow than stock with robust filtration. The basic idea with an oiled filter in general is that you can have a less restrictive media than a dry filter by relying on a thin layer of oil to grab the incoming dirt supplementing the paper and plastic. The theoretical appeal is that air can flow through and around oil more effectively than solid filter material.
Those who are skeptical of oiled filters often cite concerns that the oil will damage intake sensors, while supporters and reps from the sellers assert that they have done extensive testing and are ardent that this is a non-issue (in fact I just overheard a K&N guy say this at SEMA the other week).
I'm guessing what happens is that people buy these things, and then when it comes to re-oil them, do too much or too little, which does cause issues with intake sensor damage or inadequate filtration. The reason I personally don't want to deal with them is sort of related but pettier—I just hate oiling air filters. Also, I feel like I'd probably oil it too much or too little myself.
A lifetime ago I worked as a tour guide in Australia, leading groups of people on long off-road adventures with motorcycles and trucks in the outback. We ran oiled filters on our rental bikes, and one of my jobs was keeping those things clean. And holy hell did I come to hate that job. Dozens of motorcycles running hard through extreme conditions for eight hours a day? Yeah, we went through air filters like Derek Zoolander used cotton balls in that scene after his coal mining montage. The turpentine we used to clean them was stinky and the filter oil was disgustingly sticky—I worked it into the filters with my hands so I had to dive right into it, too. Never again.
But Wait—We Still Left the Dyno With New Horsepower
This little filter comparison test was only the secondary objective for my visit to Evans Tuning. The primary was to have a general health check for my Civic. Sure, it felt and sounded like it was running well, and I knew the Honda factory power claims, but what exactly was this car specifically putting out? The only way to know the true answer is to dyno it.
Another primary objective was to have the car tuned to run as well as it possibly could. Yes, you can tune a stock car. Of course, having no mods beyond a modest cat-back exhaust means the tuner won't be able to make mega power increase. Yet, they can refine the performance of the specific engine in subtle but significant ways—which will be the subject for discussion in an upcoming Project Car Diary entry.