What is the biggest engine you can fit into a Pontiac fiero?

There's no single definitive answer to the biggest engine you can fit in a Pontiac Fiero. It depends heavily on the modifications made to the car. People have successfully installed engines far larger than the factory options, but it requires significant fabrication and engineering.

While some have crammed in V8s like the small-block Chevy (most common), larger V8s (like big-block Chevys) and even some V6s require extensive bodywork, custom chassis modifications (including possibly a custom subframe), and significant alterations to the engine bay. The limitations are primarily related to space, weight distribution, and the structural integrity of the Fiero's chassis.

Therefore, you can't state a specific engine size as "the biggest." It's more about the extent of the modifications someone is willing to undertake.