* A 5.0L (305 cubic inch) V8: This was the most common engine.
* A 5.7L (350 cubic inch) V8: This was a less common, more powerful option.
So there's no single answer; you'd need to check the vehicle's documentation (like the title or owner's manual) or the engine itself to know for sure which size engine is in a specific 1989 IROC-Z28.