[Chris Williams]

It is largely at the event director's discretion:

Originally Posted by NHRA Rulebook

REPLACEMENT VEHICLES
The event director has the option of allowing a driver to utilize a replacement vehicle, but only under the following conditions:
1. The original vehicle is withdrawn from competition and cannot be
reinstated.
2. Replacement vehicle cannot have been utilized by any other
contestant at the same event.
3. Replacement vehicle must be fully certified and must pass
technical inspection prior to continuation of competition.
4. Driver must stay within original eliminator category and class
entered.
5. Sportsman categories: All previous event times are voided for
the vehicles and drivers involved. Changes must be made and driver must re-qualify during the normal schedule, as posted for the event. No changes are permitted after qualifying has been completed.
Professional categories: Driver retains qualifying times and standings as posted while driving the original entered vehicle. Replacement vehicles can be utilized during the course of any remaining qualifying sessions or for the start of elimination competition. Once eliminations are in progress - under normal scheduling - replacement vehicles are not permitted for subsequent rounds.
6. Checkout runs for replacement vehicles are not available.
7. Teams are limited to one replacement vehicle action per event.
8. A replacement vehicle change plus a driver change is NOT
permitted.