I was recently asked this question by Randy M of Anderson, Indiana via the Contact Form.
When thinking about removing the catalytic converter, there are a lot of things to keep in mind. The very first of which is legality. In most states across the USA, removing the cat(catalytic converter) is illegal, and punishable by large fines. Even if it’s not illegal, it will often be impossible to pass a safety inspection in your state without a cat.
People with antique plates are often not required to have a cat on their car, as these older cars don’t come with them. This is why many people Mod older cars, so they can bypass regulations and avoid breaking the law.
Lets pretend you do have an older car that can pass for antique, for example, a 1990 civic that you’re looking to turbo.
Removing the catalytic converter on a NA(naturally aspirated car) would probably not add any horsepower, as the released exhaust is not excessive or restrictive.
Removing the catalytic converter on a Turbocharged car is absolutely essential, however. Depending on the size of your turbo, you’re going to want to run custom headers and exhaust from the turbo on back. Some people even use open headers to vent exhaust gases out quickly.
Trying to run a catalytic converter on a turbo’d car will have disastrous effects. First of all, you will more than likely damage the cat, as the amount of exhaust gas increases dramatically when adding a turbo.
On top of destroying your cat, the exhaust gases can create too much backpressure on the turbo, causing it to catastrophically fail, or become damaged over time.
If you’re running a standard turbo, the recommendation is often 3″ exhaust from the headers all the way back (or open headers).
Hopefully this helped, and remember to check your local vehicle codes to make sure you avoid doing anything illegal in your state/county.