90-99 spark plug question

hello guys. a buddy of mine has a 90 civic dx with a 99 dx engine. he kept the 90 ecu, dizzy (obd0). do we use the plugs that go with the 90 ignition system or plugs that got the the 99 engine?
thanks

use the plugs form the engine your using, i know that the plugs from the d16a6 and the d16y8 have a slightly diffrent reach and temp rating i belive, i dont rember about the d15 that was in the dx but the same probly still holds true. the gap should be the same in either case though, but go with the plugs from the 99 engine

ALWAYS use the plugs from the engine you're using.

Slight exaggeration here, but imagine putting a high comp small block in your friend's Honda, then using Honda's standard medium reach plugs with that engine... Honda's plugs are all based on Hemi-style heads, whereas the other is a 90* head (exaggeration begins here) the Honda plug would reach in too far to the combustion chamber, and the piston would hit it, damaging alot of shit.

Conversely, if the plug is too short, it just gets all coked up and fouls out quickly, since it's not centered where it's supposed to be in the cylinder... the idea is to have it centered in the combustion space, so that the flame front expands equally from the plug outward, with most of the force achieving a downward motion.

ah ok thats good to know. i though there could have been a issue from obd0- obd2. though the heat(gaps) might have been different on the plugs. thanks