Here's some clarification on the ratings... Bottom line is:

DOT is minimum requirement for safety helmet.
SNELL is the highest standard. SNELL SA is for autos, SNELL M is for motorcycles.

from caloutlaw.org:

"Helmet - What does Snell, ANSI and DOT mean?
There are three primary helmet ratings: Snell, ANSI and DOT. The standards are set to provide acceptable levels of head protection in the event of an accident. Snell rated helmets can be divided into Special Application (SA) and Motorcycle (M) ratings. The impact protection for these two ratings are generally similar. SA helmets are specifically designed to meet the requirements of an individual who is restrained within a motor vehicle and needs flammability and sharp object impact protection. M helmets are designed to protect individuals who are not restrained and may come in contact with many different shaped objects. Many clubs will accept either the Snell M or SA standard for autocross activities and non-racing track events. Some clubs will accept the ANSI or DOT ratings. High speed racing events generally require the Snell SA rating. Check with your club on their requirements and rating year. Beginning in January 1999 many sanctioning bodies updated their helmet requirements to the SNELL SA 1995 rating. The message is: If you're still racing with a SNELL SA 1990 or older helmet, it's time for a new helmet. The SNELL SA 2000 helmets are now fully available. In most cases these new helmets contain significant safety advances over their predecessors. "

From the NHRA FAQ's:

"Do I need a helmet, and if so, what kind?
A helmet is required for the driver of any car or truck running 13.99 seconds or quicker in the quarter mile, and for the rider of any motorcycle. Most drivers can use a helmet meeting SNELL K98, M2000, SA2000, M2005, or SA2005 specifications. Professional category and some alcohol burning vehicles require an SA rated helmet. It is important to note that the helmet rating must be designated on a tag INSIDE the helmet, or sewn to one of the helmet straps. The designation stenciled on the exterior of the helmet is insufficient for NHRA technical inspection. Also, some tracks require a helmet on ALL drivers, so check in advance."

---Ron