A catalytic converter is an important part of a car's emissions control system, so replacing it with a test pipe is not a good idea. A test pipe does not have a catalyst and will therefore allow more pollutants to be emitted into the atmosphere. This is bad for the environment and can also lead to increased engine wear. In some jurisdictions, it may also be illegal to operate a vehicle without a catalytic converter. So, if you're thinking about replacing your catalytic converter with a test pipe, think again!