Dave,

I'm not one of those "engine gurus," but here's my logic:

Detonation occurs mostly under heavier loads, i.e. when the engine is lugging a bit. If we increase the weight of the load (car,) we increase the liklihood that the engine will lug down sooner and longer. To me this indicates that higher CRs work better on lighter cars.

That's just an observation, and I have no way to relate weight to CR numerically. Of course, I could be totally out to lunch on this one.