The owners manual calls for at least 91 a.k.i. optimally, and I wouldn't use less than 89. The engine is really not designed for "regular" which, in this country, usually means 87 to most people, although in forgivable circumstances 87 is tolerable, but a lot depends on the terrain where you drive, and the ambient temperature -- hot weather and hills both suggest using higher octane.

In any case, in our '93 cars (I have two '93s) there is no way to adjust timing or the advance curve. In older cars, you used to be able to do that by turning the distributor, and proper advance or retardation was enabled by springs and weights, and a vacuum diaphragm, in and on the distributor. But in our cars, the distributor is fixed (cannot be rotated) and empty, devoid of all the aforementioned mechanical gadgets -- it's even devoid of a crankshaft position sensor (hall effect pickup), which is replaced by a CPS on the bell housing. All of the timing and advance/retardation is calculated by an algorhythm programmed into the ECU, and therefore isn't adjustable.

Your only recourse is to consider the gasoline you buy (is it good quality), and perhaps whether your injectors are good (too lean a mixture, because the injectors can't deliver enough fuel, can cause knock).

Also, check the AMM, your O2 sensor, and even your coolant sensor (not the one to the gauge, but the other one to your ECU) -- they all feed information to the ECU with which it makes your timing decisions.

Good luck.