Dual exhaust warning

[OutlawDrifter]

Originally Posted by GMCJOE

I heard the change in the exhaust note with my 08 gmc. It's definitely from the AFM. I am running a magnaflow, single in dual out. I had the muffler shop run 2 1/4 inch pipe off of it to keep the back pressure correct so it wouldn't affect the fuel economy. Plus I didn't want it as loud as my previous 2 1/2 inch that I have ran on my other trucks. Wanted to keep the drone/in-cab noise down on my long highway trips. Sounds great cruising or getting into the throttle.

the 2.5" pipe wouldn't hurt your fuel economy...period....it hasn't had any adverse effect on mine and it won't on any other stock 4.8l/5.3l pickup. you're stock intermediate pipe is 2.5" from the factory, so why would you go smaller, even if it is twice pipes. most single catbacks bump the tubing size up to 3" for a reason.

the majority of people are misinformed about "back pressure". the key principle in exhaust is scavenging. as far as drone, i ran a dual 3" system on a b&m supercharged 454, and it was quiet and had no drone in the cab. aside from the blower whine it wasn't too obnoxious, it was my sleeper. i had a dual 2" system on a old pickup with glasspacks that would shake your teeth out, you could hear me coming from a mile away.

forget everything you've heard about back pressure. i've put a lot of research into exhaust systems and still have a lot to learn myself.

[GMCJOE]

Originally Posted by OutlawDrifter

the 2.5" pipe wouldn't hurt your fuel economy...period....it hasn't had any adverse effect on mine and it won't on any other stock 4.8l/5.3l pickup. you're stock intermediate pipe is 2.5" from the factory, so why would you go smaller, even if it is twice pipes. most single catbacks bump the tubing size up to 3" for a reason.

the majority of people are misinformed about "back pressure". the key principle in exhaust is scavenging. as far as drone, i ran a dual 3" system on a b&m supercharged 454, and it was quiet and had no drone in the cab. aside from the blower whine it wasn't too obnoxious, it was my sleeper. i had a dual 2" system on a old pickup with glasspacks that would shake your teeth out, you could hear me coming from a mile away.

forget everything you've heard about back pressure. i've put a lot of research into exhaust systems and still have a lot to learn myself.

You still need back pressure for your engine to properly scavenge. And yes, the stock single exit is 2.5 inch. Entering the muffler is 2.75 I believe. So, if you install dual muffler with 2.5 inch pipe, you will have over-scavenging. Over-scavenging waste fuel. Been proven on many other forums with the 07+.

I average a mile to the gallon better in my truck compared to my buddy with the same truck only 3 years newer. We have the same CAI, both have single in dual out. His is 2.5in and mine 2.25in piping. He has a chip and throttle body spacer. Before his upgrades, we averaged the same fuel economy. After his exhaust he averaged 14mpg, down from his original 16mpg. Which is where I am at with my upgrades. He averages between 15 and 15.5 now.

Ultimately, I wanted to make my truck a bit louder and be heard when I get into the throttle. Not like my previous 2.5in system which was too loud after a bit and I experience the loss of fuel economy. From the research and talking to my exhaust guy, I found out why. I also have plenty to learn, and may not be completely right. This is just from my experience and comparison between two of the same trucks, and research.