I think I heard them say on All Access that they put a rev limiter on their engines to keep the speed down. Number one I don't think its helping with the 337 MPH they ran this weekend.And number 2 I think there are better ways to go about this. Those cars are pumping a lot of fuel on the top end.

1970 Duster
Edelbrock headed 408
5.984@112.52
422 Indy headed small block
5.982@112.56 mph
9.42@138.27

Livin and lovin life one day at a time