Project S2000: Part 20 - Stock Air Box Modifications And Not Going Faster

by Khiem Dinh

Khiem Dinh is an engineer for Honeywell Turbo Technologies at the time of this writing.  All statements and opinions expressed by Khiem Dinh are solely those of Khiem Dinh and not reflective of Honeywell Turbo Technologies.

After testing the ram air/cold air NACA duct in Part 19, I noticed a trend in the intake manifold pressure where it drops as the engine approaches redline. A member of one of the forums had commented on some of his own testing showing that removing the lid of the air box resulted in less pressure drop. He had used a pressure gauge attached to the intake tube common on off-road trucks to gauge when the air filter is dirty. So this got me looking into the guts of the stock air box. Was the 90 degree internal tube a flow restriction?


Using my practically non-existent Photoshop skills, this overlay shows just how much space the stock cone air filter takes up in the air box. Yeah, the Honda engineers took up all the space they could.
With the lid cracked open, you can see how the internal snorkel aligns with only a fraction of the air filter. Is there opportunity to better use all that air filter surface area?
I stuffed the camera in the air box, put on the timer and closed the lid. You can see how the snorkel does sit a fair amount away from the filter.
With the lid shut, it seals up the internal snorkel very well with the internal divider wall in the air box.
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Wednesday, August 27, 2014 1:52 AM
That is the exact reason it takes an army of engineers to build a car. the airbox design has to meet strong requirements (NVH, performance, size, efficiency...), not just a "make it cheap to build" one, and this goes for every component used. The days people could get more horsepower by just hacking around is gone ;)

Wednesday, August 27, 2014 3:00 AM
There's a reason why my engine bay looks like this :)

Air filter boxes is just something the officials from the goverment can complain about (Tune, tax for bhp and so on)
and I don't spend money where money doesn't need to be spent.
Nick B
Nick Blink
Wednesday, August 27, 2014 5:24 AM
Just like the DIY intake on Project E26 323is... loud noises but major power loss. Those dang engineers that know what they're doing take all the fun out it. haha
Wednesday, August 27, 2014 6:12 AM
You should test one of the snorkels that's out there spoon/jay's/pwjdm/replicas. Please and thank you.
Dusty Duster
Dusty Dusterlink
Wednesday, August 27, 2014 6:53 AM
Most modern cars seem to have some dang good stock airboxes. Gone are the days of putting a piece of pipe and a cone filter onto your Honda Civic and getting noticeable power gains.
Wednesday, August 27, 2014 7:09 AM
What are you using to log data? I think you may have mentioned it in a past article, but I forget which one and where it was mentioned. I'm fairly certain it doesn't work off of OBD-II though, since that samples at some absurdly low rate like 1-3hz, IIRC.

Also interested in hearing a bit more about that pressure gauge that the other forum member used. Was it just a vacuum gauge, or one of these jobbies: http://assets.suredone.com/1704/media-photos/25886354-08-09-topkick-kodiak-duramax-lmm-in-cab-air-cleaner-filter-gauge.jpg
and how/where did he install the gauge?

Also, you're brave for hacking up your only airbox for science... I would have bought a spare or something.
Wednesday, August 27, 2014 7:17 AM
Nevermind, dug around a bit and found that it's a Hondata K-Pro.
Wednesday, August 27, 2014 7:31 AM
@griporslip, check out the previous Project S2k article where I made the NACA duct in the hood. I'd guess the snorkels have similar performance gains.

Monkius, yeah, I left the sample rate only at 5hz. I've had it cranked up to 25hz before. I don't recall the limit, but it can go higher. On hacking the stock air box, I figured there are plenty of stock used ones sitting in peoples' garages if I need to buy another one. The intake is now a slight bit louder, but for anyone unfamiliar with the car, they probably wouldn't even notice. It's the fun weekend/track car, so no biggy on the increased noise.

Wednesday, August 27, 2014 6:45 PM
Lol @ moar time for people to notice you.
Wednesday, August 27, 2014 7:45 PM
interested in your dyno spreadsheet. i was thinking about the same thing, and feel that doing coast-downs would increase the accuracy of the results. am i thinking right, have you considered it? i know there's a bunch of dyno apps for iphone, are any of them any good?
Wednesday, August 27, 2014 8:25 PM
Nothing to be gained from a K&N or similar filter?

What's the purpose of the first chamber of the airbox? Thought it was a resonator but it seems to be totally sealed off from the intake air.
Wednesday, August 27, 2014 9:16 PM
@slowpoke, check out http://www.virtualdyno.net/ You just need to be able to datalog from your car and you plug it into VD. It works pretty well.

Doing a coast-down test should allow for a pretty accurate cD calculation.

I made my dynosheet myself just as an exercise. I approximated the frontal area and cD of the S2k with the values being pretty close to what others have approximated. Of course, I've done my testing with windows up and down, and top up and down, which completely changes the cD. But when I do comparisons like this one, I use the same conditions. In the case of this testing, it was top and windows down. So, mine isn't completely accurate as it's not calibrated, but it should be precise allowing for good repeability and back-to-back comparisons.

@wrecked, the first camber is definitely some type of acoustic damping feature. With the wall call, there's a bit of a hum to the intake at pretty much all engine speeds. It's just not that noticeable on my car due to my Hasport engine mounts which add quite a bit of noise.

As for a K&N filter, they give up filtration efficiency to reduce pressure drop. In this particular testing, it just shows the divider wall and snorkel were not restrictions. The stock air filter is a huge cone with lots of surface area, so probably not a ton of advantage going to a K&N. So within the stock intake system, my feeling is the ribbed tube going from the fitler to the throttle body is probably one of the main sources of restriction. Maybe I'll make a replacement tube someday out of aluminum.

I did some testing way back in the day on my Evo comparing the stock air filter with a K&N drop-in. The K&N reduced the pressure drop by ~1kPa IIRC. That's about what I got with my NACA duct in the hood, so a good and measurable gain.
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