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Old 05-22-2016, 11:08 AM   #11
Alan Roehrich
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Default Re: questions on 8.1 motor in 2004 Chevy truck

Bob's truck is typical of the bunch. That's pretty much how they all are, although that's pretty decent towing mileage. It would be interesting to know if the truck has 4.10:1 gears or 3.73:1 gears.

I think if you have a relatively light trailer that doesn't catch a ton of wind, the 3.73:1 will be fine, even with the 5 speed. I bought mine with intentions of a pretty good sized trailer that I'm looking at buying, I don't think the 3.73:1 gears will work with a 5 speed for a really big trailer, I think at around 28', you'll have enough weight and catch enough wind to make it difficult to keep the RPM down. They might not work at all for a really big trailer, it might not stay in 6th gear and locked up. That's the big deal for fuel economy, the truck needs to stay in the highest gear as much as possible, with the converter locked up. It is better to be in the highest gear and locked up with the 4.10 gear than be in a lower gear unlocked with a 3.73. It may be that for a bigger trailer, the 19.5" wheels and tires and a 4.10 are the ticket, that's why mine is set up that way.

The transmission cooler leak is not as common as the oil cooler leak on the earlier trucks. Most of the earlier trucks I've seen have had to have at least o-rings in the oil cooler lines, if not the lines replaced.

As I stated above, the 2006 and up trucks are the best, most have no EGR, they have the 6 speed Allison, and some other problems were resolved.

To go cheap and still get some improvement, with any year 8.1L, I think a true dual 2-1/2" exhaust with no catalytic converters, a 180 degree thermostat, and a really good tune will put you in a good place. With no cats, you can turn off the logic that adds fuel to keep them hot, the big block prefers to be cool, it can have more timing that way, which makes it happy, and the low restriction exhaust will reduce pumping losses. You'll see power and fuel economy without spending a ton, and it should do really well for a 24' or so trailer. A good shop should be able to do the tune and a dyno run or two for $250 or so. The thermostat is less than $20, and takes 15 minutes. With the exhaust, you have to be able to pass emissions without the cats, and take them off yourself. Fortunately, my GVWR exempts me from emissions here.


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Old 05-22-2016, 12:50 PM   #12
bigsixman
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Default Re: questions on 8.1 motor in 2004 Chevy truck

I have a 2004 GMC 2500HD 4x4 with the 8.1/ Allison and 3.73 gears. I did a Nelson Performance tune, 180* thermostat, different spark plugs and I went with 265 Michelin tires.


I pull a 24' enclosed on pretty flat going and it will get 7 1/2-10 mpg depending on the strength and direction of the wind. The truck will get 14-15 mph @ 65 mph by itself with no wind and no passengers. I have the 5 speed Allison and around here it holds 5th gear @ 65 mph when trailering. My friend has a 2006 Chevy with the 8.1/ Allison and 3.73 gears and he cannot hold 6th and we get basically the same mileage. The gearing thru the first five gears is identical in both designs. The 6th gear drops the overall rpm by about 350 rpm and it makes a difference when you can use it. The aftermarket makes a kit to convert a 5 speed to a 6 speed and it costs about $1,800.

A few years ago, he put larger aggressive wheels and tires on his truck and lost 1 1/2 mpg in the process. He put 265 tires back on it with the stock wheels and the mileage came back.
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Old 05-22-2016, 04:42 PM   #13
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Default Re: questions on 8.1 motor in 2004 Chevy truck

I don't think a truck with 3.73:1 gears can pull both 6th gear and a trailer, and almost certainly not with tall tires. I think the 4.10:1 and 6th gear with tall tires may work, and with some of the previously mentioned modifications, I'm sure it will. A 300-500 drop in RPM can be worth as much as 2MPG in the 8.1L trucks.

If you change to taller tires, not only is the speedometer and odometer off, but also all of the shift strategy and logic, and some of the tuning. It does make a difference to have one retuned and the speedometer buffer corrected. I noticed it immediately when I swapped those 19.5" tires and wheels on.
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