Catalytic Converter

Asymmetric Cell Structure for Square Metal Substrates – Why We Started Making Holes Different Sizes

If you cut open a normal catalytic converter substrate, all the cells look the same. Same size. Same shape. Same wall thickness. That's how they've been made for decades.

But we started messing with that. We made square substrates where the cells in the middle are bigger than the cells near the edges. Or the other way around. Depends on the exhaust pipe.

People think we're crazy. Then they test it. Here's what we found.

The Problem with Uniform Cells

On a square substrate, the exhaust doesn't hit the face evenly. If the inlet pipe is centered, the flow is faster in the middle. Slower near the edges. That means the middle cells work harder. They get hotter. They age faster.

Also, the middle of a square substrate is weaker than the edges. The edges have more support from the frame. The center flexes more under vibration. Uniform cells don't help that.

So we thought: what if we make the cells different sizes on purpose?

What We Did

We made square metal substrates with a asymmetric cell structure.

Design A – Big cells in the middle, small cells at the edges. The middle flows more because the exhaust is faster there. The edges flow less but have more cell walls per inch, so they're stronger.

Design B – Small cells in the middle, big cells at the edges. That pushes flow toward the edges, which can help if the inlet pipe is offset.

Design C – Cells get progressively larger from one edge to the opposite edge. For when the exhaust enters from the side.

We used the same foil thickness throughout. Just changed the corrugation pitch across the width of the substrate.

How We Made It

Making asymmetric honeycomb is harder than uniform.

Normal square honeycomb is stacked with a single corrugation pitch. You cut strips, stack them, braze. Simple.

Asymmetric? We have to change the corrugation pitch every few layers. Or we make separate sections and braze them together.

We built a special stacking fixture with movable guide pins. We can shift the pitch as we stack. It's slow. It's manual. It's expensive.

But for the right application, it's worth it.

What We Measured

We tested Design A – bigger cells in the middle, smaller at the edges – against a uniform cell substrate. Same overall open area (85%). Same depth (1/2 inch). Same outer size (12x12 inches).

Flow bench with a centered inlet pipe.

Uniform cell: velocity at the center was 25% higher than at the edges. Pressure drop 0.22 inches H2O.

Asymmetric (big center cells): velocity at the center was only 8% higher than at the edges. Much more even. Pressure drop was actually lower – 0.19 inches – because the bigger center cells flow easier.

So more even flow, lower backpressure. That's a win.

Thermal Test

We put both substrates in a hot exhaust rig. 600°C inlet, 10 hours.

Uniform cell had a hot spot in the center. The center cells were visibly darker. Infrared camera showed 60°C difference across the face.

Asymmetric cell had a much more even temperature profile. The bigger center cells flowed more air, which kept them cooler. The smaller edge cells had less flow but more metal mass, so they stayed at a similar temp. Delta across the face was only 20°C.

Less thermal stress. Less chance of cracking.

Vibration Test

We shook them on a table at 50 Hz for 24 hours.

Uniform cell developed small cracks at the edges – the frame interface. The center was fine.

Asymmetric cell had no cracks. The smaller edge cells (more walls per inch) stiffened the perimeter. The bigger center cells (fewer walls) flexed without breaking.

So it's also stronger.

Where We Use Asymmetric Cells

We don't sell these off the shelf. They're custom only.

Centered inlet pipe. Design A – bigger center cells. This is the most common.

Offset inlet pipe. Design B – small center, big edge on the side where the pipe hits. Or Design C – graduated size across the whole face.

High vibration. Any asymmetric design that puts smaller cells near the mounting edges. Stiffens the frame attachment.

Uneven thermal load. If one side of the engine is hotter than the other, we can put smaller cells (more metal mass) on the hot side to absorb heat. Bigger cells on the cool side to flow more air.

Real Example – Industrial Generator

A generator had an exhaust pipe offset to one side of the square catalyst. The uniform cell substrate kept cracking on the side where the exhaust hit hardest.

We made a graduated design – smallest cells on the hot side, biggest cells on the far side. The flow evened out. The cracking stopped.

The customer didn't believe it until they ran it for 1,000 hours. No cracks. They ordered 50 more.

Real Example – Race Car Exhaust

A race car builder wanted to reduce backpressure but keep the same outer size. They had a centered pipe.

We made a substrate with bigger cells in the center, smaller at the edges. Same 12x12 frame. Pressure drop dropped 15%. The engine gained 8 horsepower on the dyno. They said the exhaust note changed too – smoother.

They use our asymmetric cells on all their builds now.

When Not to Use

If your exhaust flow is already even, you don't need asymmetric. Uniform cells are cheaper and easier.

If your substrate is round, asymmetric is harder to make. The flow pattern in a round substrate is already pretty even. Not worth it.

If you need very high cell density (600 cpsi+), asymmetric is tough. The cell walls get too thin. You can't vary the pitch much without breaking things.

How to Order

Tell us your inlet pipe location. Center or offset? How far offset?

Tell us your flow rate and pressure drop budget.

Tell us if you have a hot side or a vibration problem.

We'll run a quick simulation. Then we'll build a sample. Test it on our flow bench. Send you the data.

If it works, we make a production fixture. If it doesn't, we tweak the design. No charge for the first sample.

Bottom Line

Asymmetric cell structure for square metal substrates isn't for everyone. But for engines with uneven flow, it's a game changer.

Bigger cells where the flow is fast. Smaller cells where it's slow. Or where you need strength.

We've done it. It works. More even flow. Lower backpressure. Less cracking.

If your exhaust hits your catalyst funny, give us a call. We'll fix it with holes that aren't all the same size. Sounds weird. But it works.