They kept the 13b engine, but they did a few tweaks to it. Since RX7's from 85+ have the 13b engine in it, they had to find a way to distinguish it between them.
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Driving definitions: Understeer is when you hit the wall with the front of the car and Oversteer is when you hit the wall with the rear of the car. Horsepower is how fast you hit the wall and Torque is how far you take the wall with you.