Will deleting the 2 cats cause any check engine codes or have any ill effects on the 5.4L 3V?

I was thinking of just keeping the 2 cats and do a muffler delete on my sons 08...

 

Unless you want to go custom tuned and technically be illegal, leave the cats alone on the 08. The 06+ only have two cats to start with and aren't like the 05 and down that had four. The older ones could remove the rear cats with no issues. The 06+ removal of the cats changes characteristics around the front O2 sensors which can lead to issues. Do a muffler swap and if more sound is desired go to aftermarket cats.

 

well I'm going to do some measuring to see what the pipe size is in front of (or) around the cats , next week-end there coming off > they will throw a code but I have my ways of fixing that so I'm not worried about it . I want the sound , plus one of my cats is reading low flow and giving me a code anyway , so there outa here as-ap .

 

The build up in carbon inside the chambers is what breaks them in, like it was said the only thing that happends is a pitch change and dampends the exhaust pulses