Here's a copy of an e-mail from a couple years ago that may help...the ratings have changed (SA2000 now vs SA95 then but the info is still valid):
<< What are the differences between a Snell rated motorcycle helmet vs.
car helmet of the same rating?
The automotive helmets (SA95) are different from motorcycle helmets
in two important ways. First, SA helmets are designed to absorb
impacts whereas M helmets are designed to absorb one heavy impact. In a
serious car crash a drivers head will experience a series of minor hits
the car comes to a stop. Often the drivers helmet will strike the same
location several times. For that reason it is important that the helmet
remain intact until the car comes to rest. SA helmets are superior for
The second important difference between the two types of helmets is
SA helmets are more fire resistant than M helmets. The liner in SA
made out of Nomex. Even if the fire does not reach your head, the heat
generated by a fire can melt the liner of a motorcycle helmet. The
from the liner will then bond to your skin and be uncomfortable to say
least. Basically the SA helmet will buy you a little more time to get
you car in the event of a fire.
Since it's inception TCC has not had any drivers involved in a
fire. So to this point we probably could have gotten away with only
M rated helmets. However since the safer helmets are available at a
reasonable cost, that is what we require. As a driver it is comforting
know that you have done as much as you can to protect yourself while on
track. It lets you concentrate on more important things like passing
in front of you!
I hope I have answered your question. Please don't hesitate to write
if you have any more questions concerning road racing or Touring Car