Strategies to keep cats from melting

[SSG 5.0]

Some interesting info I can share and I heard it earlier. I asked this old mechanic how can I tell if the cats are going bad. He said there are two ways to find out. Once the car is fully at hot temp, check the cats if they’re cherry red and do it at night. If it’s red, of course it’s going bad. The other is usually going to throw the engine light and the car will die if the cats are clogged up.

I wonder if these tuners turn that cat signal or sensor off before throwing that check engine light to prevent more damage to the engine?

Sponsored

 

[engineermike]

The coyote converter is 5.5” in diameter. The more I think about it, I highly suspect the gt500 and hellcat converters are probably the same material but larger diameter. In order to achieve 50% more area, the diameter would need to be 6.75”. Anyone have a hellcat or gt500 converter they could measure?

 

[engineermike]

I wonder if these tuners turn that cat signal or sensor off before throwing that check engine light to prevent more damage to the engine?

Could be.

I had a cat that was just bad enough to trigger the code, but it took 2-3k miles for it to set it off. I could look at the rear o2 voltage and it was apparent the cat was bad, but obd took a lot longer to recognize it. If we all made it a point to log rear o2 voltage we could probably see it coming before it’s too late.

 

[Torinate]

How do I log rear O2s?

What should voltage be and what does it show with cat damage?

 

[engineermike]

How do I log rear O2s?

What should voltage be and what does it show with cat damage?

I log with HPScanner. They are called O2 Voltage B1S2 and B2S2. The voltage doesn't matter as much as the switching. With new cats the voltage drifts very slowly over time. With aged cats, the voltage will toggle high to low (I think 0.450v is the mid-point) back and forth slowly, like 5-30 second swings. With failed cats, the voltage will swing very rapidly. The pic below shows a new cat (gray) and an aged but still functioning cat (white). The timespan is 30 seconds.

 

[MKL_DS]

The coyote converter is 5.5” in diameter. The more I think about it, I highly suspect the gt500 and hellcat converters are probably the same material but larger diameter. In order to achieve 50% more area, the diameter would need to be 6.75”. Anyone have a hellcat or gt500 converter they could measure?

I’ll try to climb under our redeye tomorrow and check it out.

 

[ihasnostang]

can someone explain to me how running rich cools cats? Yes fuel can cool air thru evaporative cooling but not after an ignition event?

 

[engineermike]

I struggle with that myself, but every single OEM cal I've looked at adds fuel to reduce cat temp. Here is a snipping from the article I posted before:

 

[Jaymar]

mixing outside air into the exhaust would seem like a solution (AIS), it would also flash-burn left over fuel to boot. But if there's too much of that then air temps go even higher.

Small block Fords and many others used to do that back in the 70's up to the mid 90's, it was called the air injector thermactor system and it was used to help burn off HCs inside the cat. Necessary for emissions at the time given the technology but it was awful. Imagine if you will a time when speedometers could only go up to 85 MPH and a 302 V8 made half the horsepower that a 4 cylinder Ecoboost makes today and still got shit mileage.

 

[Jaymar]

can someone explain to me how running rich cools cats? Yes fuel can cool air thru evaporative cooling but not after an ignition event?

I think the error in his reasoning was using the choke to create a rich mixture when that's not exactly how the choke works. I would guess the rapid rise in cat temp was from the many misfires that could be heard as an effect of the choke. A misfire or a late fire dumps a lot more gas into the cat than an ordinary rich condition with stable ignition.

That being said, running a rich mixture to cool EGTs is common in factory fuel control strategies as mentioned and piston aviation engines running back to pre-war times. The way it was explained to me (and I could be wrong) is that the rich mixture slows down the combustion flame front decreasing the cylinder compression during the combustion event and the remaining fuel left in the combustion mixture absorbs more heat energy with a lower rise in temperature due to its higher specific heat relative to the air it displaces. Along those lines, I suppose a water injection system would also help achieve the same results in lowered EGTs.

 

[Rothgray]

Just run some gutted magnaflow high-flows, and a MIL eliminator / mini cat on the 2nd O2. Leave the monitors on, just lessen their trigger values.

 

[Torinate]

I think the error in his reasoning was using the choke to create a rich mixture when that's not exactly how the choke works. I would guess the rapid rise in cat temp was from the many misfires that could be heard as an effect of the choke. A misfire or a late fire dumps a lot more gas into the cat than an ordinary rich condition with stable ignition.

That being said, running a rich mixture to cool EGTs is common in factory fuel control strategies as mentioned and piston aviation engines running back to pre-war times. The way it was explained to me (and I could be wrong) is that the rich mixture slows down the combustion flame front decreasing the cylinder compression during the combustion event and the remaining fuel left in the combustion mixture absorbs more heat energy with a lower rise in temperature due to its higher specific heat relative to the air it displaces. Along those lines, I suppose a water injection system would also help achieve the same results in lowered EGTs.

I’ve been wondering about a water meth injection kit and if it would reduce the cylinder temperature as well. At least enough to reduce the cat temp and maybe keep it under the temp threshold.

 

[markmurfie]

It's more than just how hot the exhaust gases are coming out of the combustion chamber. The reactions that happen in catalytic converters are exothermic themselves. That's why they are usually hotter than the exhaust pipe in front and behind them. They are most effecient at a stoich afr ratio. The more reactants going into them the more heat they are going to put off. COT protection will only take you so far, before a bigger catalytic converter is needed to maintain optimal heat range and efficency.

COT being so rich and far from stoich makes them inefficent and they them selfs stop producing soo much heat. Again this only gets you soo far, 800 hp just takes more oxygen and fuel than 700 hp. The catalytic converter has to deal with the extra emissions from that, and no matter how inefficent you make it, eventually the heats going to build up excessively and cause a melt down.

 

[J17GT]

Anyone know anything about these G-Sport by GESi cats? Appears they are built for boost and “EPA compliant” whatever that means. I wonder if they are too “high flow” and you’d still get a check engine light?

https://gsportbygesi.com/

 

[Rothgray]

Anyone know anything about these G-Sport by GESi cats? Appears they are built for boost and “EPA compliant” whatever that means. I wonder if they are too “high flow” and you’d still get a check engine light?

https://gsportbygesi.com/

basically the same thing as i mentioned above.. Buy those, punch them out, and run an L shaped MIL on your secondary o2. Have your tuner turn your OM monitors back on. the end.

Sponsored