Hope this helps, from a post I made in the bracket racing section:
According to the 2006 NHRA rule book in General Regulations section 10:7 Helmets & Goggles:
"As outlined under Class Requirements, drivers in all classes, including motorcycles, must wear a helmet meeting Snell or SFI Specifications. Snell K-98 is acceptable in place of any Snell M-rated helet.
SFI Spec 31.1A = Snell SA, open-face helmet
SFI Spec 31.2A = Snell SA, full-face helmet
SFI Spec 41.1A = Snell M, open-face helmet
SFI Spec 41.2A = Snell M, full-face helmet
Drivers of NHRA Lucas Oil Drag Racing Series and E.T. cars (13.99 or quicker) must use a helmet meeting Snell 95, K98, 2000, 2005 or SFI 31.1, 31.2, 31.1A, 31.2A, 31.1/2005, 41.1, 41.2, 41.1A, 41.2A, or 41.1/2005 Specs."
"Modifications to helmet/visor/sheild are prohibited. All helmets must have the appropriate certification sticker affixed inside the helmet"
Everything else in the section refers to open bodied cars and funny cars.
Steve