The 1990 Mazda Miata has no cabin air filter. This means that bits of leaves or bugs can be sucked into the cabin when you run the heater or AC. The lack of filter also caused the blower motor to become full of debris. I got tired of this, so I designed a filter for my Miata. I had to remove a metal grille to get it to fit. A smaller filter might be able to fit in your Miata without modifications. Also the sheet metal that forms the cabin air intake isn’t manufactured with great accuracy, so my design may need to be altered for your Miata. This design provides a snug friction fit on my car.
After measuring and designing the filter mount, I printed a prototype in PLA to test the fit.
I grabbed some filter mesh from a hardware store. I think it was meant for home furnaces. After cutting it to the correct shape, I stuffed it in the mount and screwed everything together. The nuts and bolts are M6, though M5 should work. I needed 25mm long bolts, but 30mm or longer should be fine. They’ll stick out of the bottom, but that shouldn’t be an issue since there’s nothing in the vent that comes close to the bottom of the filter.
The mount is designed to fail gracefully. The holes for the nuts are against the metal body, so if a bolt comes loose, nothing should fall into the blower fan. I recommend printing the final product in glass filled nylon, as PLA will likely warp and degrade in the high temperatures of an engine bay.
Miata Cabin Air Filter Top
Top of the filter.
Miata Cabin Air Filter Bottom
Bottom of the filter.
After installing the filter, I’ve had zero issues with debris getting blown into the cabin. It’s a much more pleasant experience.