Air filters on a dirt track car???

[dwilliams]

Some big trucks use externally-mounted "cyclone" filter housings. But you'd probably have to mount one on the passenger floorboard; they're pretty big... you could DIY something similar, though. There's quite a bit about cyclone filter design out on the web; woodworkers use them to control sawdust in their shops.

[ptuomov]

If you pull the engine air from the passenger compartment, it’s going to run some vacuum and dust will get everywhere. I think the cabin is designed to run slight over pressure to keep the dust out, at least in street cars.

The air-oil separator ideas like cyclones make a lot of sense.