The 318ti was equipped from the factory with two different types of engines: on the early 318ti (1994 〜 1996) this was the M42 engine; on the later 318ti (1996 〜 2000) the M44 engine succeeded the M42. Both engines are inline-4; all M44 engines leverage a dual-length intake manifold (DISA, short for DIfferenzierte SaugAnlage) while only some late M42 engines do. DISA increases the length of the intake at higher RPM. More systems which do this are called "variable intakes."

I have heard the M44 Engine runs best on 93 octane but truthfully this is the only gas I have ever pumped into the car so take that as untested fact.