Check my thinking on this...

[Kevfra]

Comments please - I have four O2 sensors, located just before and after the first set of cats. On most header systems these cats are eliminated, and on most headers there are four bungs to receive the O2 sensors. Sometimes - often I hear - you still through a CEL because there simply isn't enough difference in readings and the computer has a hissy fit. But even if you don't throw a code here is my question:

What is the computer doing to your mixture?

Seems to me the system will be reading more hydrocarbons without the benefit of a cat, and seeing more hydrocarbons it is going to try to run the engine more lean - cut back on the amount of gas in the mix. That's not good. Comment on that thought please.

Next, I have seen the mod using a spark plug fouling adaptor to remote the sensor away from the exhaust stream but still give it enough sniff to be happy. But again - the sample is going to be different than it would be with a cat in the loop, and the computer is going to make an adjustment, good or bad.

I'm wondering if anybody has actually been able to get a read on what the computer is doing in the case of headers, and if it is a healthy response or potentially harmful response. For example there are some folks that have experienced premature cat failure on the second sets still remaining, and they attribute this to hot gas coming through the headers and burning out the cats. Seems to me a cat makes the gases even hotter than straight pipes - that's how it works - and the exhaust without primary cats should be cooler if anything. But what if the computer made an adjustment to lean the mixture - that would make the exhaust hotter and possibly cause the damage some have seen.

Comments?

Final thought. The second set of cats have a bung not being used. When installing headers why not use O2 sensors up front as normal, but extend the second set to the secondary cats, rather than to the second set of bungs on the headers? This gives you the closest to expected sampling that the computer would be looking for. Might have to extend the wires but that's not a big deal. Anybody tried this?

These things keep me awake at night...