I was just wondering what the air filter stops from going into the engine and how it harms it. I know that it stops some of the bigger debris like small pebles, dust etc. but what else doe it stop and what would be the cost of runing without it. i see a bunch of hot rods (nearly all of the open hood ones) running without them so what gives, do we really need them or could we just put a really fine screen in front.

You could run without a filter but why? Besides keeping crud from going into your engine a good unit can actually improve air flow. I have seen dyno tests that showed some filter units flowing better than running without a filter which doesn't surprise me.

The smaller particles (as small as .05mm) can do damage. They get sucked in without a filter, stick to things and become part of the oil. Its just an introduction of foreign material that is not healthy. You'd also get the occasional leaf, bird feather, or bug sucked in, not to mention mice, rats, birds, bees, and other bugs like dark holes.

The biggest problem in unfiltered air is dust and sand particles. These act like sand paper on the metal moving parts of the engine, causing accelerated wear. It only takes a tiny amount of dust every time you drive to shorten engine life.

Engines that do not run in dusty conditions (ie: most boats and snowmobiles) run happily without air filters.

That's exactly what i thought, thanks for the info.