To upgrade a 1s to a variable system, the necessities are the aquamist high speed valve and the pressure switch manifold. This would make it a 2c.

The next question is how the valve is controlled.

The valve can be controlled via many mechanisms. One of which is the fia2 from aquamist which references the IDC of one of the injectors and then drives the high speed valve at the same duty cycle. With an fia2 it would make it a 2d and adds diagnostic functionality.

ANY aquamist dealer can provide this.