Engine tune is a lot different. There is a short interval allowed in FIM between the down and return runs and a rider needs to wait at the end of the track on the down run, be escorted back to impound, get released from impound, be escorted to the other end of the track, and wait to make the return run. There is not much time for screwing around so the bike and rider are set up so no refueling, maintenance, etc is needed between the runs. This requires a much gentler tune than a drag bike with an emphasis on reliability.