Catalytic Converters Explained: Cell Counts, High Flow, and Cat Delete

Understanding Cell Counts

This is where most people get lost — but it is actually straightforward once you understand what cell count means.

The honeycomb substrate inside a catalytic converter is made up of hundreds of small channels, or cells, per square inch. The more cells per square inch, the denser the substrate, the better the emissions filtration, and the more restriction it creates. The fewer cells, the freer the flow — but with reduced filtering capability.

Cell counts are measured in CPSI — cells per square inch. Here is how the most common types break down:

400 to 500 Cell — OEM Standard

This is what manufacturers fit from the factory. A 400 or 500 cell cat offers excellent emissions filtration and meets strict global emissions regulations. The trade-off is restriction — these units are dense by design, prioritising compliance over flow. If your car came with one of these, it is doing its emissions job exceptionally well, but it is not doing your exhaust flow any favours on a performance build.

200 to 300 Cell — High Flow

This is the most popular performance upgrade and what most people mean when they refer to a high-flow catalytic converter. With fewer cells per square inch, these units offer significantly better exhaust flow than an OEM cat while still retaining enough substrate to function as an emissions filtering device. A 200 or 300 cell cat will pass emissions testing in most regions, produces a noticeable improvement in exhaust flow, and on turbocharged cars in particular can have a meaningful effect on back pressure and turbo performance. For anyone building a fast road or performance car that still needs to be driven legally, this is the sensible upgrade.

100 Cell — Motorsport

A 100 cell cat is at the extreme end of the high-flow spectrum. The substrate is very sparse, meaning restriction is minimal — but emissions filtration is also minimal. These are primarily used in motorsport or track-only applications where emissions compliance is not a requirement. On a road car, a 100 cell cat may not pass emissions testing depending on your region and the sensitivity of the test. It will, however, offer the closest flow characteristics to a cat delete while technically still housing a substrate.

High Flow Cat vs Cat Delete

This is the question that comes up constantly in performance circles. Here is an honest comparison of both options.

A high-flow catalytic converter — whether 100, 200, or 300 cell — keeps a substrate in place. This means some level of emissions filtering is retained, the car remains road legal in most regions, and exhaust gases are still treated before exiting the tailpipe. The flow improvement over an OEM cat is significant, and the sound difference is noticeable — rawer, more present, with more crackle on overrun. For a fast road car, this is the right choice.

A cat delete removes the catalytic converter entirely and replaces it with a straight pipe or test pipe. The exhaust path is completely unrestricted at that point, which is as free-flowing as it gets. The sound change is dramatic — significantly louder, rawer, and more aggressive than any high-flow cat can achieve. On a turbocharged car, the reduction in back pressure is also at its maximum with a full delete.

However, the downsides are significant. A cat delete is illegal for road use in most countries. The car will fail any emissions test. It can trigger fault codes on most modern cars unless additional tuning is performed to suppress the downstream oxygen sensor readings. The smell from unfiltered exhaust gases is also noticeable — particularly at idle or in slow traffic.

For track-only cars, a cat delete makes complete sense. For anything that uses public roads, a high-flow cat achieves most of the performance and sound benefit without the legal risk, the fault codes, or a failed emissions test.

The Bottom Line

If you are on a stock or mildly modified road car, a 200 or 300 cell high-flow cat is the ideal upgrade — better flow, better sound, road legal, and a genuine performance improvement particularly on turbo applications. If you are building a dedicated track car, a 100 cell or cat delete is worth considering for maximum flow. And if your car is completely standard, understanding what your OEM 400 or 500 cell cat is doing helps explain why a well-chosen replacement makes such a noticeable difference.