|Purchase a Cover
The Supra MKIV never came with a cabin air filter. However 93zupra on the Supra forums discovered that the airbox for the Supra is very similar to the Lexus SC300/SC400 which has an optional cabin filter. He discovered that the Supra blower box has provisions for filters, a cutout spot, and that the Lexus filters and cover are a direct fit. Here is his DIY thread on it: https://www.supraforums.com/threads/mkiv-cabin-air-filter-install-diy.1125508/
Likely Toyota had planned to include cabin filters (or make it an option) and scrapped the idea. Maybe it was to cut costs, or they figured most Supras are targa tops so what is the point of a cabin filter? I still wanted to run one in my car. The top stays on most of the time and I think it helps keep your air vents clean. Here are the OEM LHD Lexus part#s.
The issue is that the Lexus cabin filter covers are discontinued. They were never really sold as "just the cover" either. You had to buy the lower half of the blower box which we presume included the cover (Lexus parts diagrams leave a lot to be desired). After joining a few Lexus groups I was finally able to find someone parting out a Lexus that was willing to sell me just the cabin filter cover. They shipped me the bracket, the wing nut bolts and the square nuts. Here is a picture of it installed.
I decided to make my own model and come up with a 3d printed solution as well. I did something similar with the Subaru Impreza GC8 cover which is also discontinued.
I'll type out some instructions below with pictures, but to understand it even better I recommend watching this video I made:
Step 1: The first thing you need to do is cut out the underside of the airbox. The plastic is prestamped already. You will cut out the inside area of the lip. I scored it multiple times with a razor blade until it poked through. I didn't try to follow the circle area in the center of the cutout, I simply scored straight through it. Then I dremeled out the center and snapped the pieces out. I cleaned up the ends and center with a dremel/file.
Step 2: Install the filters. There are two identical cabin filters. The rectangular tabs on the bottom go towards the back of the car. The first filter can slide up into the opening and push it towards the firewall as far as it goes. There is a channel that will catch the front edge of the filter. If you look at the picture above you can see I already have the first filter installed. The second filter goes in after, but with the first filter installed you can't just slide it in straight. You need to bend the center at a 90 degree angle to get it started. Once half of it is in, you can straight the filter and it will lock in place. Continue to slide it straight up.
Here is what it looks like with both filters installed and the cover off.
Step 3: Install the cover. There are two plastic slots that secure the square nuts. Square nuts are ideal as they are less likely to spin, but I found that a normal 10-32 nut had enough width to it that it also works. The square nuts I stole from the Lexus are a very tight fit. They felt like they would snap the plastic if I tried to force them in. I shaved a little off the sides of them with a belt sander so they would slide in easy.
If you would like to purchase one, you can email me at email@example.com and I can ship you one for $35 shipped (assuming you are in US, overseas is $30 + shipping). I'll try to keep a few printed at all times. The only color I have is black. Keep in mind you'll also need to purchase a filter. I'm running the OEM one, but there are some aftermarket ones available (like ATP RA116 from rockauto) that may be cheaper or easier to obtain. I'll include some 10-32 nuts and the right size screw bolts need to install it.
When I made my Subaru cover I released the .STL file so that people could print their own, or they could buy from me. Unfortunately a big company (541 Motorsports) began printing and selling them on eBay. I think they have sold more than I have:/ For this reason I am no longer sharing any of my design files. No Exceptions.
Based on pictures, I believe RHD models will need me to print a mirror copy of the cover as the tabs need to be on opposite sides. I believe they will also need a different filter as the LHD filter bends left towards the center console when being installed and the RHD filter would need to bend the other way. Based on pictures online (shown below), I believe 88508-30020 is OEM part# for RHD, but so far no one has tried it. If you have a RHD (and are in the US) and would like to be the guninea pig... email me and I'll send you a free RHD cover.
Sure, just send me an email and I'll figure out what the extra shipping will be.
They are printed out of Hatchbox ABS filament. I use ABS because it can stand up to the hot temperatures inside of cars. ABS is difficult to dial in, but it is worth it. Easier plastics to work with like PLA and PETG could have shrinking/warping if exposed to hot cars.
I haven't had any issues with this in the Subaru world. With the plastic cover installed and my blower on high, I can't really feel any air escaping. Also if you look at the oem blower motor you'll see that it is also plastic on plastic. The factor blower motor doesn't use any kind of foam or seal. That being said, there is a channel built into the part. If you'd like you could install thin little piece of foam inside it. The plastic cover also lightly touches the plastic edge around the filters. If you wanted to put a thin piece of foam/felt on the top you could. I don't hear any rattles or anything with mine as-is.
Absolutely not. You'll see it removed in some of my photos. It was more of a "while you are in there" decision. I removed mine and cleaned it. With it removed you are also able to reach your arm up into the blower box. I took a wet rag to clean the inside of it. I used an old paint brush and some warm water to clean the blower motor. I held it upside down while cleaning to prevent water from running towards the motor. Check out these before and after pics: