Because the way the motors in the 88-91 Prelude "lean" back, dropping in a b16 motor is not a wise choice.
WHY?
Oiling will be a problem.
Vtak y0.
Hood closure and oil pan scraping is another.
as my good friend Eludethis noted, the transmission.
There arent many people who do this swap...it will cost you well over $5000.USD to fab up motor mounts, cut/weld to make a transmission work, then axel issues etc etc.
PLEASE.
Do not insult the 3g Prelude.
A b16 may have nice high-end power, but it's only a 1.6L motor. NO TQ.
It belongs in a bubble or a CRX.
The 3g Prelude is a cornering MACHINE. Imagine a b16 powered Ba4. You hit a corner and NOW you have NO TQ to push it around like you did with the b20a5/b21a1.
The beauty of the b20a5/b21a1 matched with either the d2j3/d2a4 is the amazing LOW END power the motor/tranny produces.
This is why 3g Prelude owners keep coming back for more...it's a special car that is hated on and not understood by many.
DONT INSULT US and the legacy of the Lude.
^_^