After removing the OEM cat, we welded and bolted up an OBD II Magnaflow ceramic-core catalytic converter in place of the stock one. With the Magnaflow cat installed, the Honda turned the rollers to the tune of 150hp and 107 lb-ft to the wheels. The ceramic-core cat netted us an additional 3hp and 2 lb-ft to the front wheels. Obviously, this cat really can flow more than stock.
Next up was the metallic core Magnaflow OBD II cat. Because metallic cats can take more abuse and richer AFRs (and generally flow more), these are often the choice of the performance-minded tuner. After much cursing and a bit of charred flesh, we put the fancy metallic-core cat onto the Civic. Apparently, the metallic core unit does indeed flow slightly better than the ceramic unit. But when we say slight, we mean slight. The metallic core cat netted us a 1hp gain (to 151hp) and no increase in torque over the ceramic core. So, unless you are running a rich AFR (like in a boosted application) the extra cost of the metallic core might not be worth it. However, if you are running a rich mixture, the metallic cat is still your best bet.
With the testing of all three cats out of the way, it was time to unplug the exhaust and let her rip without that pesky brick in the exhaust. No matter how you cut it, a cat is a restriction and costs power, right? With the moment of truth at hand, we bolted in our test pipe - eliminating the cat entirely. On our final pull we stood eagerly by the dyno's monitor, waiting to see the results. Wait ... here it comes ... 152hp and 108 lb-ft - a gain of one measly horsepower and one lb-ft of torque at the cost of breaking the law.
The Lowdown
Now that we've armed you with the facts, you should be ready to make the correct decision should you need to replace your cat. Whether or not you choose to run a cat is between you and Johnny Law. But when you consider the potential gains versus the costs, it hardly seems worth risking getting caught. With a difference of only 1hp between a straight pipe and a 49-state LEGAL cat, it seems foolish to flaunt the law and pollute the air that we all breathe. Hell, if you are only looking for those kinds of gains, just loose 10 pounds -that alone will make your car faster than adding 1 measly HP.
The 5hp gained from losing the factory cat for the metallic one is substantial. If the engine were to later receive more extensive mods, the benefits would probably be even greater. What it all boils down to is that a good aftermarket high-flow cat can indeed flow better than a stock unit, and almost as well as a straight pipe. When you can have all the benefits of removing the cat without the risk of getting busted, it's kind of like having your cake and eating it too.
A Caveat
Before you go off all willy-nilly and replace your cat, there are a few things you should understand first. Contrary to popular belief, not all cats are created equally. The stock factory cats usually do the best job of cleaning up the exhaust, but often at the cost of high backpressure. Aftermarket cat flow and ability to pass emissions testing can be iffy because quality varies greatly. In an effort to save money, some manufacturers cut corners and don't include enough of the agents that actually catalyze and create the heat to burn up the pollutants. Install one of these cats and you might not pass smog. This is why it's important to pick one that is CARB or EPA compliant. These cats have been independently tested and should pass an emissions test.