The B21A1 is a 2.1 liter (2056cc), 16-valve DOHC engine. It came in the 1990 and 1991 prelude SI's. It makes 140 hp at 5800 and 135 ft.lbs at 5000. 9.4:1 cr, redline - 6500. It was a step up from the B20 motors, which make slightly less power (between 100 and 135 hp) . They are great motors, my friend has one in his 91 lude. It has 185K on it and still runs brand new - burns less than a quart between oil changes. If you can get a good deal on it go for it, it will be a huge step up from a d16.