As per the 2014 rule amendments:

Exhaust collectors/stacks must be securely fastened (i.e., metal connector straps, bolted, welded, etc.) to prevent loss of collector/stacks during competition. All removable multi-piece exhaust collectors/stacks must be securely fastened with either an NHRA-accepted header tether, or a minimum ½” (half-inch) stitch weld located on each primary tube to prevent loss of collector/stacks during competition. A current list of NHRA-accepted header tethers is available on NHRARacer.com.