I;m not sure but I heard there is a difference between the 91 and 90 13b motor. Any answers? Something about the rotors?

 

91 and 90 are both S5. Almost posotive they are not different. S5 does have lighter rotors than S4 though.

 

S5 rotors have higher compression then S4. i think somehting like S4 is 9.4 to 1 and S5 is 9.7 to 1.

 

Read the FAQs
I am pretty sure that they are the same, unless your comparing an 88 to 89
Series 4 86-88 have lower compression rotors
Series 5 89-91 have slightly different intake manifolds and have higher compression rotors.

 

 

90 and 91 are identical as far as the engine is concerened. In fact the only differences at all are slight differences in model names and equipment.