You guys are probably going to kill me for having to go over this again but im still unsure. Im about to install my lpp longtubes on my 08 sierra. First off I want to run cats, I dont want rasp or smell, but I dont have to worry about any kind of emissions tests in LA. It seems like my two options are to weld in 2 3'' magnaflow spun cats into my y pipe OR weld a single cat into the exhaust system just behind the y pipe and tune out the rear 2 o2 sensors. What are you guys opinions on both the 1 or 2 cat system. Im sure the 1 cat system would be easier and cheaper but would I gain anyting by running the 2?

 

I'd run 2 if you've got California Emissions Standards. Better safe than sorry.

 

I have one 3" directly after my Y pipe and it still stinks. I would get 2 of the shortest high flow cats you can find and have them welded in after each collector.

 

I actually live in Louisiana not LA caliornia so no emissions here. Jrulz with the one cat you have did you see a drop in any your mpg? Iv been told with the one cat your truck will dump more fuel even if your retuned and better low end with 2 because of more back pressure.

 

I'm not sure since I did a cam and long tubes at the same time. I've never heard of it dumping more fuel. I did gain 2.4mpg out of the cam and headers over stock though.

 

Damn nice gains, whats your cam grind and truck specs? I noticed you havent added anything to your garage yet.

 

218/222 .544" 113lsa '03 reg cab 4x4 HD. Pulls nice. Still not fast enough but I'll take the big power gains and added mpg

 

The closer you have them to the engine, the faster they will light-off and start to fuction. Also, they require a certain amount of heat to fuction at all. Putting one too far back will cause the exhaust gas to be too cold.

I would put one directly after each collector. Don't worry about the "back-pressure" thing, you won't tell a difference. Plus, having 2-3" cats (one on each collector) can theoretically flow more than one 4" cat installed after the merge in the Y. Science Away.....!!!!!

 

Coban a few tuners just told me your exact quote. 2 will last longer and flow way better than one. So now just deciding if to do 2 or none. Thanks for yalls help guys.

 

i flanged in magnaflow 3" spun cats on each side on my lt's on the boosted av i had and they worked great but the guy that helped install them said they would not work properly if they had to be put in any further down stream because of lack of heat.