Measuring AFR with cats?

I could see that...The cats basically just change around the gasses. They convert the real nasty ones to not so nasty ones basically. Checking AFR at the tailpipes isn't very accurate since there could be small leaks in the exhaust system, and the change in heat will affect things also.

If you really want to know, plumb in a wide band 02 before the cats. It doesn't take much, just a bung a welder and a drill. I would think that about any exhaust shop would do it for you for less than $20.

MYSTICSVT said:

How much do catalytic converters affect your measured air/fuel ratio? My last dyno session with a wideband showed my AFR around 14:1 during WOT. The dyno operator said I'm probably pretty close to where I should be because my cats will show the AFR as being leaner than it really is. Any truth to this?

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A/f readings after the cats is off.Due to the process of getting rid of unburnt HC and producing more CO this throws off the reading.

I agree with samickguy15,just install a bung ahead of the cats and check it there.Or remove one of the o2 sensors before the cat(since they dont work at WOT anyway).

14.5 is about right at the tailpipe with cats.