2017 4.0 with 250& miles: thinking cat converters are toast: if I dual it out & run w/o converters, do I have to re-tune? Don't need to pass emissions anymore.
Thanks!

 

2017????

In any case, no, no retuning required, Cat converters just clean the exhaust so are "neutral" as far as engine power, unless they are clogged up

You will get Downstream O2 sensor code(s) and CEL(check engine light) will come on and stay on because the Cats are no longer "working"

 

DUUUHHHHH!! -1997!!!!
Autocorrect; yet again!!
John

 

If clogged assuming they'd show a code on reader?
John

 

No, not if partially clogged, partially clogged cats or muffler limit flow at higher RPMs, if exhaust flow OUT is limited then new air/fuel mix IN is limited, so loss of power as back pressure builds up in the exhaust

You can test for that with vacuum gauge on the engine
Should see 18-21" of vacuum at idle
Raise RPMs to 3,000(approx.) and hold there, should see stable vacuum of say 14", number doesn't matter just stable, hold RPMs and wait 10 second, if vacuum starts to drop then exhaust IS clogged