The following codes should be entered as passwords on the title screen.

Code         Result 
NOBRAKES     Full engine power. 
HARDGAME     Makes the game harder. 
DNUMUAHCS    Ten times more energy, but only one life. 
DIEALIENDIE  All weapons on full power. 
JJRULES      Thirty-five lives, all weapons on full power.
WARHEAD      Makes missiles detonate when they hit a target. 
ELITEMASTER  Weapons are ten times more powerful. 
PETSKUMODE   Enables reverse gear. 

Thanks to Revolution readers fahad!