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Hi, I have a 2011 Chevy Silverado 4.8L 1500 4x4.
I currently have a 4.5" zone suspension lift installed along with 33"x12.5"r17 mud tires with -18 offset rims.
I have a K&N Blackhawk CAI,
an aftermarket "high performance" Mass airflow sensor,
A Flowmaster Super 10 Single and dual out muffler,
3rd cat ( resonator ) removed,
A 2k stall converter and Corvette servo in the transmission,
And my engine is custom performance tuned to 89 octane.

I recently purchased a set of long tube headers and an off-road Y pipe (NO cats), which I'm having installed in a week and then I'm going to have the truck returne which I'm having installed in a week and then I'm going to have the truck retuned the same day to the headers and have the rear O2's shut off so the truck will run with no cats.
With all the modifications I have how much horsepower do you guys think I have already gained so far and how much more will I gain next week by putting on the long tube headers and getting rid of my cats (With a retune of course) ?
Will I hit 400 hp from my 4.8? Truck already has noticeably more power than stock so I'm very curious as to how much more power I will gain by eliminating the cats and having long tubes.
Also by the way getting an inspection sticker with no cats is a non-issue in my situation. As for the input I appreciate any thoughts you guys have I'm new to this form and I look forward to becoming a member thanks
 

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I think your mods are only good for low to moderate hp gains. The aftermarket MAF is a waste and gains nothing. It will cause problems down the road. The CAI maybe got you 10-15hp at WOT. The long tubes with no cats will get you some power, but you will not be anywhere near 400bhp. I would expect a cam swap and/or head work would get you there N/A, or forced induction. Those 4.8 motors can take a ton of boost...
 

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Throw some boost at it instead of nitrous.
 
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