Here's a link to the NorCal Shelby club website aptly named "Sound Advice."
Sound Advice
This is lifted directly from the "Open Track Rules" section of the same website:
"Mufflers are required on all cars. The exhaust system must be able to meet the sound limit for the venue: Laguna Seca - 90dB(A), Infineon Raceway - 103dB(A), Thunderhill - 105dB(A). No open exhausts will be allowed. Note: For Thunderhill exceptions to this rule may be made on a case by case basis, at the discretion of the chief steward or event organizer. There are no exceptions at Laguna Seca or Infineon Raceway."
Thanks Joe for taking a shot at this. I know the Kirkhams would do a wonderful job.