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Beat the Law

The truth about going catless

Text By Ryan McKay

The Problem
More exhaust backpressure generally means less horsepower, and a stock catalytic converter is often the greatest source of backpressure in an exhaust system. So it's no wonder the catalytic is often tossed in favor of an eBay "test pipe" when a performance-minded owner starts itching for a little more power. But there are a few downsides to de-catting a car - the big one being that it's illegal.

Hell bent on wrangling every last horsepower out of their steeds, power mongers often run sans-converter on the street; risking tickets, huge fines or even impoundment of their vehicles. But just how much power are they gaining, and is it really worth the risk? If the thought of surrendering your car to the local towing yard keeps you from yanking your converter, there still may be a way to make more power and not break the law.

Aftermarket catalytic converter manufacturers claim their products can reduce backpressure, helping to free up exhaust flow and make more power. They may have fancy CFM flow graphs and other fancy data to back those claims, but can these kitties really flow better than the OEM pieces? And, how do they compare to the good ol' test pipe, in terms of horsepower?

To answer these questions, we took a lightly-modded 1999 Honda Civic SI and did four back-to-back dyno pulls: one with the stock cat, one with a test-pipe, and two pulls with OBD II approved aftermarket high-flow cats. Read on, and see with your own eyes how powerful you can become by killing your cat.

The Test
Breathing through an aftermarket intake, header and cat-back exhaust system, our Civic mule put down a respectable 147hp and 105 lb-ft to the front wheels on K&N Engineering's dyno. Not too shabby, considering the car was still huffing through the stock OEM catalytic converter. Eager to see what we could gain by uncorking the exhaust, we bolted on the first aftermarket cats.

Aftermarket high-flow cats come with two different types of guts, metallic and ceramic. The ceramic type is less expensive to manufacture and will melt and fail at lower temperatures - not ideal for high performance applications. The metallic honeycombs can withstand the higher temperatures and richer air-fuel mixtures but are more expensive to make and subsequently cost more. But cost isn't the only factor to consider if you find yourself replacing the cat.

It's important to ensure that you have the correct type of converter for your application. If your car is OBD I compliant, you can use any EPA or CARB approved OBD I converter. If your car is OBD II compliant, your cat must be OBD II approved. As a general rule, any car 1996 or newer is OBD II and any car 1995 or older is OBD I. However, some cars were OBD II compliant as early as 1994, so double-check the underhood emissions sticker. The difference between OBD I and OBD II cats is bigger than you might think.

OBD II cats are required to remove about 90 percent of emissions, while the OBD I cats are only required to remove 70 percent. So, put an OBD I cat on an OBD II car and you might fail your emissions testing.

By Ryan McKay
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I'd like to see such a test on a moderately tuned turbo car with 300-400 hp. Isn't that what everybody is interested in? Mind you that the power loss by a cat does NOT go linearly with the power (because aerodynamic power loss at an obstacle (cat) is proportional to the 3rd (!) power of the gas (exhaust gas) velocity; the latter is not trivial to determine btw) so this test doesn't really help even for roughly estimating the power loss by cat/high flow cat for a performance car! It could be anywhere from 5-50 hp imo.

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