I'm rebuilding my 91 fc motor and the front housing is out of spec. I have an 86 front housing and I was wondering if I could use that? Mazdatrix website says its different and if so what is the difference?

The S4 and S5 front rotor housing are different, the timing of the spark plug location is different, unless you replace the housings as a pair it will not work as well, I was told, and the builders will have to expand on this, but you have to adjust timing for the different housings by a couple degrees, so if they are a mismatched pair, the timing of the spark between the rotors will be different, and the difference may throw the balance off.

NOPE!