An open-face helmet is much easier to see out of than a closed-face one when autocrossing. If you have your own helmet, make sure it has a Snell, or SA (automotive) or M (motorcycle), rating with the requisite sticker. If it doesn't, then you probably won't be able to use it. If you're in the market for a helmet and only plan on autocrossing, you can get by with one rated for motorcycle use but if you're serious about motorsports then buy one with that SA rating. Wear it around in the store with the chin strap secured for about 15 minutes to make sure it's comfortable and fits well. And if you wear glasses, make sure the helmet fits with your eye wear in place. A helmet can save your life, so don't cheap out on it.