i pulled my s5 TII motor and the housings on the left is the front housing from the original motor and the one on the right is one i bought to replace the rear housing. in the pics are the only differences between the 2..besides the knock sensor on the front housing. i was wondering why the area around the "13b mazda" is cutout on the left housing..but is not cutout on the right. (if that makes sense) the bottom pic is just another difference i noticed...

 

s5 housings had thier knock sensors located above the trailing spark plug hole and the s4 did not.

 

Different castings...I think the major key difference is the distance between the spark plugs. They should be the same for S4/S5. I'd ask around some more just to be sure.

 

you said the original one of your TII is on the left right? that is weird, i have both a 1987 TII and a 85 gsl-se, the housing on the left looks like my 85 housings, and the one on the right looks like the regular TII housings. Maybe a previos owner has done some engine work???

 

????

S4 and S5 have totally different plug timing's in the housings