The 2.5L 4 cylinder was used in newer rangers and is a dual plug engine, so you'd either have to use the 2.3L's head and the bottom end of the 2.5L to use it in a '87 to '90 mustang, whereas it's a direct bolt in to the '91 to '93 mustangs(which i believe are exactly the same). The '91-'93 mustangs with the 2.3L had 105hp stock, whereas the '87 to '90 2.3's had only like 88 horsepower, i believe both engines made 135lb/ft of torque or maybe the earlier ones a little less. I believe the extra displacement would add like 10 to 15 horsepower, but really isn't worth the trouble. A turbo would be a much better swap though. Why are you thinking of building up a 2.3L (or 2.5L). It would cost about twice the cost of a stock 5.0L mustang to make a 2.3L as fast as a stock 5.0L mustang.
- Another note is that 4 cylinder mustangs came with weaker springs, weaker shock/strut valving, no rear sway bar, puny front sway bar, small radiator, weaker 7.5" rear end, non-heavy duty ball joints, smaller brakes, and incredibly weak transmissions (both the auto and manaul were different than the 5.0L although the manual was a weak duty T5 based on the GT's heavy duty T5
Good luck, if i were you, i'd either mod a 5.0L or get the 2.3L just for commuting and good mileage.
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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT