* 2.5L Iron Duke (4-cylinder): A base engine option.
* 3.0L V6: A more common and popular choice.
* 5.0L V8 (small-block): Offered in various horsepower configurations, depending on the specific model and options.
* 5.7L V8 (small-block): A more powerful V8 option, often found in higher performance models.
To know the *original* engine for a *specific* 1986 Pontiac Firebird, you'd need to check its vehicle identification number (VIN).