What is the difference between a 318i and 318is, lets say between 86 and 91?
Any difference in engine size?

engine is the same, the is has a sport suspension and sport trim hence the "s".

actually, I believe the original E30 318i were sold only in '84 & '85, 2-door or 4-door w/M10 1.8L engines. the later E30 318is was '90-'92, w/M42 1.7L engine, 2-door only. The same car in 4-door was a '318i'......confused yet????

both the same cars except the "is" has a sportier setup, nice bolstered seats and stuff. oh and they come only in 2-door versions.