5.7 vortec mpg improvements with exhaust

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Moofus02

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Ok it's been a few years since I drove this truck and it has issues with low compression on center cylinder on each bank but I've put 200k miles on it like that. The exhaust broke right below the manifold flange so I tossed it an in the scrap and put an aftermarket H pipe on with a duel in out muffler and ran the pipes out both sides behind the tires. 2 1/4 head pipes 2 1/2 out the back. When I did this I put 4 new o2 sensors in. My milage had almost doubled and I've been running it hard cause I like the sound. I'm shocked at the improvement. The only time it's got this kind of milage was getting on freeway and running steady until it needed fuel. I didn't think the stock exhaust was so restrictive. No cats now
 

L31MaxExpress

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Maybe a combination of exhaust and new O2s?
I dunno, my 99 Tahoe was a slug with the stock exhaust on it compared to the way it ran opened up. Those tiny 1-7/8" pipes choked the life out of the thing. I went headers and dual 2.5" with high flow cats to the magnaflow muffler and it felt like I had added a 75 hp nitrous shot to it. I also removed the massive TB lip on it. Then I added some timing to the Tahoe timing map that was very limp wristed compared to my 97 van timing map.
 

Moofus02

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I dunno, my 99 Tahoe was a slug with the stock exhaust on it compared to the way it ran opened up. Those tiny 1-7/8" pipes choked the life out of the thing. I went headers and dual 2.5" with high flow cats to the magnaflow muffler and it felt like I had added a 75 hp nitrous shot to it. I also removed the massive TB lip on it. Then I added some timing to the Tahoe timing map that was very limp wristed compared to my 97 van timing map.
Now that you mentioned it it does seem to run better now even with 2 dead cylinders than my Tahoe. I need to make something else run so I can cure this truck. When I get a chance I'm going to do a leak down on it to figure out the low compression. Then figure out what I'm going to do about that. I'm looking for parts for a computer swap and going to start searching the yards for a 4l80 auto tract core to rebuild. The 4l60 is good but has 300k on it and I've got a vortec in the garage that spun a rod. I think I'm going to just 383 that motor, kind of copy your build and swap motor trans transfer case and computer all at once. Set it up for e85 and coil on plug while I'm at it. I don't have to worry about emissions or visuals here obviously as the cats feel off lol. I have an intake question though. I know I will have to upgrade the spider and I'm cool with that but looking at the cost and some of the other options I'm curious how the marine duel plane port injection manifold compared to the factory truck intake. I can score a complete duel plane boat setup for about what a spider conversion costs. I think I read in a post somewhere that you used one of those at one point
 

Schurkey

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Two dead cylinders? Massive improvement by replacing the exhaust?

Your cats were plugged/melted from all the excess hydrocarbons dumped down the exhaust system. Good chance the old O2 sensors were faulty as well. Fix the dead cylinders, I bet the mileage goes up again--more efficient engine AND no false-lean O2 sensor signal to drive the fuel trims rich.

Yeah, the stock muffler is a disaster, at least on the big-block trucks (but probably on all of 'em.)
 

Moofus02

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Two dead cylinders? Massive improvement by replacing the exhaust?

Your cats were plugged/melted from all the excess hydrocarbons dumped down the exhaust system. Good chance the old O2 sensors were faulty as well. Fix the dead cylinders, I bet the mileage goes up again--more efficient engine AND no false-lean O2 sensor signal to drive the fuel trims rich.

Yeah, the stock muffler is a disaster, at least on the big-block trucks (but probably on all of 'em.)
That many miles like that the cats were probably melted
 
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