I bought a pair in 1999 when NHRA said all cars had to have mufflers. They slowed my 12.5-1 446 with iron heads about .8 seconds in the 1/4. I think they said they were checking db's at the Indy points meet in 2000, they never released any findings, except to say the maximum allowable would be 105 db. The Stock and Super Stock guys ignored the rule, and it was never enforced after that. I ran the Pontiac Nats in 2000 wide open, no one said anything.