d16z6 or d16y8

Application will be a 91 DX Hatchback

I have to keep automatic - even though i want manual so let's just leave it at that, it's a long story...my dad paid for this car originaly and he is on the title...you get my drift

What is the difference between the d16z6 and d16y8?

Which one would be better for my hatch to use?

Many people said that i sould keep the tranny that i got, just cuz it's there and I shouldn;t go looking for trouble there if it's working fine.

Would a new racing radiator help out my performance?