What the diff. is between the motor in a 91 Standard Hatchback and the DX Hatch?

different options.and engine pakage,price.

Most likely to get that kind of HP gain, it's probably a difference in either the heads, cam timing, or even just the computer programming used to control the engine functions.

There is a restricter plate in the in intake.  If you swap intakes, the horsepower will be _nearly_ the same as they are both D15B1s they are pretty much the same.  The cam is different as well, but it doesn't account for but a few horses, so I personally don't think it is worth the work digging that deep unless you plan on doing heavier mods.

hah! my DX has a ZC with 35,000 on it!

that's obviously not stock though