i have a 1991 prelude with bent valves from a broken timing belt.
and i was told it's more cost effective and easier to buy a used engine
with a 9 month warranty. well the question is the b20 interchangealbe with
the b21. or do i need to alter the car at all for the b20 to fit?? why would i
ask cause i dont wanna spend an extra 200 dollars for an extra 100cc i could
tell i had or not.

 

the JDM B20A engine will fit ANY AND ALL 2.0Si models from 1988-1991. I know. I owned one. no modification required, except you have to re-use your original EGR valve and your intake manifold because of emissions.

 

^agreed. and reuse your exhaust manifold.

 

so the 91 prelude b21 and the b20 are exchangeable i wasn't too sure about
that that's y i asked. thx for the info. i appreciate it and thx again.

 

the b20a and b20a5 are exchangeable with the b21a1

if you're talking about the b20a1 in the 2nd gen prelude, then no, it isn't a direct swap, and it isn't worth the effort.