* Backpressure: The exhaust system is designed to manage backpressure. Too little backpressure can affect the scavenging effect of the exhaust system, potentially reducing engine efficiency and even causing damage over time. The Geo Metro's tiny 3-cylinder engine is particularly sensitive to exhaust flow.
* Increased noise: While not directly harmful to the engine, the extremely loud exhaust will be annoying and potentially illegal.
* Unmetered emissions: The catalytic converter plays a crucial role in cleaning up harmful emissions. Removing it (likely necessary with a straight pipe) will result in significantly higher levels of pollutants released into the atmosphere. This is illegal in most jurisdictions.
* Resonance: A straight pipe may create resonance frequencies that could potentially damage components, although this is less likely than the backpressure issues.
In short: While it might not immediately destroy the engine, running a straight pipe will likely negatively impact its performance and longevity, and is definitely not recommended. It's far more likely to cause problems than to provide any benefits. Sticking with the factory exhaust system, or a properly designed aftermarket system, is the best course of action for the engine's health and the law.
