Physics engines are concerned with determining standards for the physical reality the characters inhabit. Dungeons & Dragons, GURPS, Shadowrun, and Battletech are all good examples of physics engines because the characters will have stats like Strength or Agility or Intelligence which give in-game mechanics for the limitations of those abilities. A character with 14 Strength can simply lift more than a character with 9 Strength in D&D, and much of the gameplay resolves around how much damage that stat applies to your weapons. Characters with high Strengths have to take the front lines of combat, characters with high Reaction should become Riggers in Shadowrun, and characters with high IQ should pick up lots of mental skills in GURPS.
Narrative engines use abilities to push the story forward, or give the players authority to dictate a new direction for the action to take. How much damage a character does when they punch somebody is usually an after thought.
In a couple of weeks I'll be attending PAX East, and I made plans with my friends there in Boston that I would GM a one-shot of Dungeon Crawl Classics (DCC) for them. At the time the plans were made I was a few weeks into running my DCC game and I was pretty excited at the prospect of playing an OSR game. DCC is also a physics engine. Over the last few months I've been playing a couple of role-playing games (mainly Apocalypse World) that give players a stronger narrative control over the action that appears in a game. It's difficult for me to actually get my brain to go back into an older physics engine style of game without feeling compelled to hack it into something I want to play. My appreciation for old school renaissance (OSR) games is still strong, but I think my main difficulty is with the old pass/fail binary mechanic. I really like the idea of a partial success too much and I want to incorporate it into all future games I might GM.
I know my friends are eager to try DCC as well and I've been thinking about this a lot. I could run the game in one of two ways:
1) Physics: straight by the book
in most situations they would have a target number and rolling a d20 they would add an attribute bonus, the result is a binary pass/fail outcome, and I would roll dice for my monsters, traps, etc.
2) Narrative: variable difficulty resolution where I don't roll dice, except for damage
the normal d20 rolls would be replaced with a set difficulty of 12 and 18
an 18+ is an unequivocal success where they would always have a choice as to the results of their success
a result that falls between 12 and 17 is a partial success where I offer a choice between two options: accepting a good failure, a bad success, or a tough choice
a failed result of 11 or less means something bad happens or an opponent sees some form of success
The adventure I plan on running is pretty straight forward with few monsters, though the monsters are real killers, so I'm thinking of applying the narrative rolls to non-combat resolution and then doing combat by the book. Or maybe I'll just give people the option from the beginning of the game to use narrative rolls instead of physics rolls.
In any case, the concept that I learned from Apocalypse World of "announcing future badness" sticks with me, and as a result I don't think I can ever use Perception checks ever again. It's the most versatile ability that a GM has and I think should just be used as a principle of GMing, being able to say that a thing is about to happen unless the player takes action without explicitly stating "Hey! This fucker is about to brain you if you don't prepare to defend yourself."
Calling for a Perception check in D&D was always a simple way of saying "Hey! There's something here." to the players, but a good role-player who failed their roll would continue on into the danger or simply not find the clue, neither of which is interesting. Now I've come to regard Perception checks as useless. Why call for a Perception check at all when it's too ambiguous as to what the check could be for? Either there's something dangerous and you give the players an opportunity to discover what it is and react to it, or there's something hidden to be found and they find it.