Ok, So I have 4 EF civics.
1991 Civic DX pm9-3 (311,000 KM)
1990 Civic SI pm9-1 (123,000 KM)
1989 Civic SI pm3-17 (302,000 KM)
1991 Civic DX pm9-1 (360,000 KM)
1991 CRX SI SE PM3-6 (399,000 KM) (Friends car)
All have the PM9 head except the 89 SI which is by far the fastest. I'm aware of a minor weight difference between the Si's but the 89 is much quicker than the 90 SI with less then half the km. I guess the head could have been ported and polished, maybe upgraded springs, but I want to know what the difference is between the PM9 and PM3. If head work has been done why did they choose a pm3? I've done some research and have come to the conclusion that the last number is the factory it was made in. Is that correct? Any info on the difference in the heads is appreciated. I'm aware the bare head is the same but what about the cam, valves, ect.

 

I know the 89-91 si cams are all the same

 

it could just be two engine that have worn differently. if you really want to compre go ahead and do compression and leakdown tests.