Just wanted to give some feed back on replacing my stock cats on my '06 5.4with Magnaflow # 59956, 2.5" spun metallic cat. I had my tune turn off the rear O2s as they are not OBD II, but OBD 1 ( about $50 cheaper per cat). However they sound like i have no cats. You hear motor a lot more and thats with a stock 3" 2013 muffler/tailpipe i have. No drone and quiet when cruising, but hit it and it screams. I did notice my MPG picked up some. Pulls stronger lowend to top end. Able to turn over tires much easier.

Just wanted to give some feed back - going on 3 weeks since the install.

Rob

 

Sounds like a good switch! Are they still emissions compatible?