What to do with pre cat and after cat 02 sensors

[BeXtreme]

Do not agree on removing catalyst overheat mode. That is there to protect your pistons and exhaust valves as well. The added fuel is needed to cool them when the engine runs hard for a long period of time. It only richens it to about 11.8:1 at the most.

I will add that I run a fairly aggressive PE air/fuel for E10, around 12:1 at peak torque and 12.6:1 at peak HP. When it shifts to catalyst overheat it richens up the air/fuel and I pull 2-3* of timing out of it in the AFR compensation spark table at the richer commanded AFR in catalyst overheat mode. With the the stoich value at 14.1, I end up about 11.2:1 in catalyst overheat. Does not make a 2-3 HP difference to protect the pistons and valves as well as help prevent detonation when things really heat up.

Does the 0411 not have separate tables for that? Since catalyst temp is a calculated value that is using an algorithm based on a factory setup, it seems it would be completely inaccurate in a non-stock installation that doesn't actually have a catalytic converter. If you are just using it as an extra PE... that seems fine as long as you know what you are looking at and have the time and energy to properly tune your stuff. I'm not sure that's the case here though.

This has definitely gotten me thinking about my application though. So it looks like you are going very conservative on your normal PE, with a 1.12-1.17 PE ratio normally on E10, then you are using the catalyst overtemp tables to only get into a more conservative PE value of 1.26 when the PCM is calculating that you are really getting it hot(although it might not really be doing that since you don't have a stock cam/engine setup).

This might actually be more fuel efficient for normal driving as I'm thinking about this, because you would be able to set the PE to come in more often, but be less aggressive when it does in normal driving. Then when you are giving it the beans, it comes on full bore in that conservative protection mode of heavy overfueling to keep everything cool.

 

[L31MaxExpress]

Does the 0411 not have separate tables for that? Since catalyst temp is a calculated value that is using an algorithm based on a factory setup, it seems it would be completely inaccurate in a non-stock installation that doesn't actually have a catalytic converter. If you are just using it as an extra PE... that seems fine as long as you know what you are looking at and have the time and energy to properly tune your stuff. I'm not sure that's the case here though.

This has definitely gotten me thinking about my application though. So it looks like you are going very conservative on your normal PE, with a 1.12-1.17 PE ratio normally on E10, then you are using the catalyst overtemp tables to only get into a more conservative PE value of 1.26 when the PCM is calculating that you are really getting it hot(although it might not really be doing that since you don't have a stock cam/engine setup).

This might actually be more fuel efficient for normal driving as I'm thinking about this, because you would be able to set the PE to come in more often, but be less aggressive when it does in normal driving. Then when you are giving it the beans, it comes on full bore in that conservative protection mode of heavy overfueling to keep everything cool.

There is a table that IIRC relates catalyst temperature to airflow and commanded air/fuel ratio. With correct fueling and timing, EGTs do not change a lot on a gas engine even if the power is doubled. There is more exhaust volume but the actual EGTs stay about the same.

It does help fuel mileage a bit not that I expect decent fuel mileage in PE on a ~500 hp small block in a 7,000 lbs brick. It does prevent a slight loss in power though having to run a richer PE all the time though. GM started using added enrichment over time way back in the TBI era. The 93+ TBIs had a partial PE enrichment initially that decayed over time to full enrichment.

 

[GoToGuy]

You mentioned odd sensor setups. My 98 Tahoe 5.7, 4x4 4dr. Has dual cats and 2 forward, each cat; and 2 after each cat. It's a Calif model maybe that was the reason.

 

[99xcss4]

yeah 8 sensors sounds insane to me 350 should only have 4 and I think 454 is the same I had all ways thought that 4.3s and 305 were 3 sensors but that seems to not be the case even though they are a single cat that one if the weirdest things I have seen was on the sacrificial K-2500 that gave some of it self to build frank'n truck had the cat after the muffler just before the rear end it was a huge pellet bread loaf one

 

[99xcss4]

I feel bad for the blue oval people for a while starting at the end of the 90s their trucks have 4 cats do not know if the are still that way or not