From 2011 rulebook:
"The Vintage Oval Track class is for old-style race cars with pre-1948 design engine blocks, i.e., no modern overhead V-8s or blowers are allowed. This class is for vintage engine, old-style open wheel, rear drive, dirt track and Indy, one or two seat cars, with a tapered tail and cowl. "
Thats why it is vintage class. You really need a rulebook.
DW