* 301 cubic inch (4.9 liter) V8: This was a fairly common, smaller displacement V8 option.
* 305 cubic inch (5.0 liter) V8: A slightly larger V8.
* 403 cubic inch (6.6 liter) V8: A much larger and more powerful V8, available as an option.
Therefore, to know the exact engine size, you'd need to know the specific trim level and options of the particular 1981 Trans Am.