What does Catalytic Converter CPSI stand for?

Catalytic converters are a reality that our cars have to live with as long as internal combustion engines continue to dominate our roads. For decades car manufacturers adhered to emission standards that were mandated in developed countries. In the US, emission standards continue at the Federal and/or State government level. Now, more than ever, especially with stricter CAFE regulations, corporate fuel consumption average is heading up towards 35MPG in the next couple of years. Expect even more restrictions to be in the way of exhaust flow.

Having a better understanding of CPSI ratings should help you make better decisions for selecting aftermarket cats for your car. If your goal is to maximize engine performance while keeping emissions in check, read on.


Catalytic converters are used to reduce 3 harmful compounds that are emitted by an engine’s combustion chambers. Hydrocarbons in the form of unburned fuel, nitrogen gas, and a poisonous gas in the form carbon monoxide. In order to limit the amount of hydrocarbons, nitrogen gas, and carbon monoxide from exiting out of the tailpipe a catalyst is used. Most catalysts these days are ceramic honeycomb structures.

If you’ve looked around for performance aftermarket high-flow cats, most likely you’ve came across the term CPSI rating. Lower CPSI rated cats make exhaust systems sound slightly louder, and in some cases a little deeper. CPSI stands for Cells Per Square Inch. Take a cross section of the catalytic converter to look at the catalyst, you will see a bunch of honeycomb cells. These cells are passageways inside the catalyst that allow maximum exposure of the catalyst to the flowing exhaust for emission reduction. For instance a catalyst without any cells would have a 0 CPSI rating, which means the catalyst is a solid piece with no passage ways, which would not permit exhaust flow. Therefore a catalyst has to have a value greater than zero to make any sense.

The basic concept of CPSI is that the lower the number, the larger the space of the holes in the catalyst. Let’s take the Europipe for the Porsche 997 Turbo that we’ve reviewed previously. The high-flow cats have a 100CPSI rating, while the Porsche OEM catalyst most likely carries a 400CPSI rating. For the sake of comparison, a 100CPSI catalyst to a 400CPSI cat, the 100CPSI version has holes that are 4X the size of the 400CPSI cat, resulting in better exhaust flow through the cat, resulting in increased engine performance.

To further clarify, the higher CPSI number doesn’t necessarily mean that exhaust flow is higher. Higher CPSI means that there are more individual openings within a single square inch, providing more cell walls, translating to a smaller overall open passageway within a single square inch. That translates to better emission control because more of the polluting exhaust gases would adhere to the walls of these cells. On the other hand, exhaust flow becomes more restricted because there are more cells per a single square inch, which translates to having more cell walls and a smaller passageway area for exhaust to flow.

Image Credits: NGK Insulators.


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