questions on 8.1 motor in 2004 Chevy truck

[Alan Roehrich]

I'm behind on my projects, but I'm actually developing a kit to convert the 8.1 to adjustable shaft style rocker arms, under the stock valve covers. If you're keeping the stock camshaft, you can use 1.8:1 rocker arms for a little boost in power. If you're changing cams, you'll probably want 1.7:1.

The intake manifold can be modified to solve the PCV problem, and also for about a 30HP gain, by cutting it open to remove a baffle or two and doing some porting. The stock PCV system inside the intake manifold is closed off, and the PCV system is run externally at the oil filler tube (you won't want to add oil with the engine running).

My personal truck will be getting a set of Hedman headers and 3" true dual exhaust.

[Chipper Chapman]

I had one with 280,000 Miles. It had been used hard its whole life. Gave it a tune up and it still ran like a champ and worked great. Sold it to a friend and they work it everyday still. I'd love to have another

[Dwight Southerland]

What's the fuel mileage pulling enclosed trailer?

[Alan Roehrich]

Dwight, I got 8-9 towing a very high profile 28' enclosed camper that had probably 2,000 pounds of extra weight in the trailer and close to 1,000 pounds extra in the bed, and three people in the truck, much of that trip was into the wind, and I was running 70 MPH, uphill and down. Truck pulled it and handled it with ease. This was before I upgraded to 19.5" wheels and tires, and better brakes.

There is more power and more fuel efficiency to be had from the truck. I have a friend with a chassis dyno and various EFI tuning software. I've been pressed for time, so this is moving slow, but I'm going to do some stuff and document each change as it happens. Since I'm keeping this truck, I'm going to go fairly far to see what can be done with it. I think over all there is 100HP and 100 ft/lbs to be had, and I think that if you do not use the power to show off how fast you are, the changes will result in better fuel efficiency, especially when towing. I know what some other guys are seeing with some work in the right places.

I think the single biggest problem is that the power band of the engine is too narrow, it runs out of power and useful RPM for the gearing it has. The one complaint I have is that it downshifts too early, and then runs out of useful RPM before it will upshift, so it has a hard time getting to that next shift to drop the RPM back down. So my goal is to increase the power available so it downshifts less, and spends less time in the lower gear. Using too much throttle and turning too many RPM is where the fuel goes.

I wish this truck was a 2006, as the 2006 has a 6 speed Allison. It's not a conversion that can be done easily and affordably, I doubt I will ever do it. I got a sweet deal on a one owner low mileage truck, so I settled for a 2003. A 2006 is a better truck to start with, but they 8.1L trucks get harder to find every day, especially the newer trucks, as the Duramax got more popular, and GM was intentionally killing the big block. I think it might be possible to make a 3.73:1 gear work well with the 6 speed, but I'm not sure it is a good idea with the 5 speed, my truck is a 5 speed with a 4.10:1 gear. I swapped to 19.5" Vision aluminum wheels and Hankook tires, which made a big difference everywhere, I think the RPM drop was worth at least 1 MPG.

A new truck like mine would be $70K, if you could buy it. I don't mind spending $2K or $3K on this one. It's still under 100K miles, and as Bridges says "that truck will go well over 200K miles, and when the time comes, we'll just make it better for the next 300K miles".

[Bob Don]

I tow with a 2002 Silverado 2500hd 2wd crew cab, long bed, tow package with Allison trans and 8.1 Vortec. I've got just over 100k miles on it (I bought it with 82k miles 4 years ago) I get about 10 mpg towing a 24' enclosed with a 3705# car plus tools, spares and a scooter. Other than downshifting for hills, you'd barely know you're towing anything. I get the piston slap when i start up cold like Alan said but it doesn't seem to have any ill effects. The engine uses about a half a quart of oil every 3500 miles but by then it's time for an oil change anyways. The only issue I had was a leaky trans cooler in front of the radiator.

[Alan Roehrich]

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.

[bigsixman]

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.