Lil Red - Duramax 6600/Allison Conversion/Swap & OBS/NBS Dash Conversion

[More Power]

Some time ago, I announced here in this thread that I was having Kennedy reprogram Lil Red's ECM to help the truck be a little more "daily driver friendly". I was told that I could have two programs available, and toggle between them using a switch. I asked JK for one of the two programs to be "stock". John sighed a little at hearing the word "stock", but did as I asked, though he added a little twist to the definition of the word "stock".

When delivered, John said that he programmed the "stock" switch setting would be a "stock/fuel economy" setting. This is what I thought I had been using since the ECM was returned to me - a little more than 4 years ago iirc. The truck was already pretty spunky, I thought, so why add more power if I didn't need it. Plus, I wanted the new head gaskets and injector cup reseal plenty of time to "settle in" before putting the wood to it.

I had installed a toggle switch on the bottom rim of the dash way back when... not long after JK first began offering EFI-Live programs and I began using one of his ECMs with his big power programming. I knew (or was convinced I did) that pushing the switch toggle away from me would select the "stock" setting and pulling the switch toggle toward me would select the "fun" position (a +100 horsepower setting). Murphy said I had a 50/50 chance of getting it right, if I ever forgot...

Since replacing the head gaskets in Lil Red (in late August 2021) till a couple of weeks ago I left the switch in what I thought was the "stock/fuel economy" position. I thought I didn't need the +100 position. All of the fuel economy reports I've posted here in this thread these past 4 years were actually with the switch in the +100 position, unbeknownst to me. Lil Red is 2500-lbs lighter than my crew cab, so I thought the spunk was due to that... till one day... a week or so ago, I thought I'd try the fun position... To add a little spice to a YouTube video...

Much to my surprise, I definitely noticed a change in performance, but it was opposite to what I was expecting. Long story short, I had been running in the +100 position all this time - for at least the last 4 years. Funny story actually, but after considering my mistake for a few minutes a question formed in my mind about whether fuel economy would improve if the "Fuel Economy" switch position was active... So I filled up about 10 days ago, selected the "stock/fuel economy" switch position, then drove the truck another 375 miles over about a week or so - some local and some a little longer distance driving. The prior tank in Lil Red, with similar driving produced just over 20 mpg, which I thought was pretty good for a local biased tank of fuel.

What was the result of that tank of fuel? 375 miles, 15.708 gallons = 23.873 mpg.

So maybe there is something to the "Fuel Economy" label... I'm looking forward to some Interstate driving to get a better comparison, but for now, it looks like the improvement could be at least 2-3 mpg better... when using the "stock/fuel economy" power setting. Would an improvement of 2-3 mpg be a worth losing 100-hp?

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A YouTube viewer asked recently what rear axle I was using in Lil Red. This photo shows that the truck is running a 6-lug GM 9-1/2" ring gear 14-bolt semi-floating rear axle. This axle replaced this truck's original 10-bolt - that GM used in most of their 1500 series trucks/SUVs. I was told by a drivetrain expert back when this truck was in the development stage that the GM 9-1/2" semi-floating axle's input torque rating limit would not likely be exceeded by a programmed Duramax because traction would be the limiting factor.

So far, this axle has been great! I've been running Amsoil synthetic 75W-90 in it since the 500 mile break-in period. The aluminum cover adds another quart of capacity and the fill/magnetic drain plugs make service easy.

More information about this axle along with a complete description for how it was built can be found at the following ink:
https://www.thedieselpage.com/reviews/randys.htm

[arveetek]

I'd like to have that diff cover! I installed a 14-bolt semi-float axle in my Tahoe a little over a year ago. Mine also has the factory G80, but unfortunately the governor gears were stripped out and it is not functioning. I have been having trouble sourcing replacement parts. I'm perfectly happy with the G80 for my uses, and a new locking diff of some sort is rather expensive.....

Casey

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I'd like to have that diff cover! I installed a 14-bolt semi-float axle in my Tahoe a little over a year ago. Mine also has the factory G80, but unfortunately the governor gears were stripped out and it is not functioning. I have been having trouble sourcing replacement parts. I'm perfectly happy with the G80 for my uses, and a new locking diff of some sort is rather expensive.....

Casey

Casey,
The pictured axle was built at Randy's Ring & Pinion facility in Everett, Washington using an Eaton Posi differential along with new 3.42 Yukon gears, bearings and seals. I supplied the original axle, from a 1997 heavy half ton K1500 (I've forgotten the RPO code). I bought it from Spaulding Auto in Spokane, Washington in 2005 - an auto recycling center for (iirc) $250. It had a G80 locker and it appeared fine otherwise. For me, the GM 9-1/2" was the best choice for Lil Red, when considering cost, torque rating, and that it was a direct bolt-in. Randy's may have a used G80 they'd sell for a good price. In hindsight, I guess I should have kept the original differential.

The G80 can be a great choice for a limited slip/locker. You just have to go easy on it during the lockup phase. Lots of throttle/wheelspin before it locks can cause the friction plates to be slammed together hard enough to crush them. That's what happened to the G80 in our 6.5 Power Project (not me driving it ). Once the friction plates get crushed, all that loose material (abrasive slurry) wipes out the gears and bearings.

I learned recently that Spaulding now has a franchise here in Missoula, Montana, and is where we bought a replacement tailgate for Sarah's Blazer - just last week. Still need a rear hatch and front bumper...

The Mag HyTec diff cover makes service a lot easier (and looks cool), but isn't really necessary. Increased capacity and cooling becomes more necessary for those trucks that tow heavy. Dodge learned back in the day with its hi output Cummins that their rear differentials needed more fluid capacity, cooling and synthetic lube to survive those long hard tows in the summer heat.

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I received the following message the other day asking how to overcome the problem of the Duramax oil pan interfering with the OBS 4WD front axle mount bracket (on the passenger side). The stock Duramax oil pan won't fit into the 4WD OBS chassis without a mod - a fairly easy mod...

OBS Duramax swap
From Abe P on 2025-07-21 21:56

Hello, I bought the Duramax conversion motor mounts a few years ago from the diesel page, I'm finally able to start my project, now that I mount the engine to the frame the oil pan is sitting against the front axle bracket, how do you deal with this?

The above image details the problem. I pointed the camera straight down while standing inside the engine bay (front clip was off the truck). This is the rear portion of the left side (passenger side) mount for the 4WD front axle. The rear of the truck is toward the top of this photo. The red you see in this photo was one of the legs of my engine hoist (that is parallel to the floor).

I snapped the above photo during the very first trial fitting of the LB7 into the chassis. This interference was a problem, but the first step in the solution was pretty easily dealt with using an air chisel and angle grinder.



The first thing I did was remove the outer brace, which itself was the largest contributor to the interference. I considered that the brace was important, but... just how important, I wondered? This truck has been driven for over 14,000 miles with the Duramax, and has seen many 1/4-mile launches in 4WD, some of those were with up to 600+ horsepower. That front axle bracket hasn't budged. However, if this truck was intended to be used in driving conditions that required a lot of 4WD driving, I might add a new, though flatter, more closely fitting bracket brace - just in case. I don't intend to do that with Lil Red, but it's always an option that can be added later. Of course, those with 2WD OBS trucks/SUVs don't have this to deal with.



The above shows the scratch that appeared on the oil pan after the above trial fit. That showed exactly where the interference was.



This was the final step in the solution. I cut out the area using a cut-off wheel, making the cut large enough to provide for a little extra clearance to allow for engine movement. A flat piece of sheet steel (thickness matching the oil pan) was shaped and then TIG welded over the hole. The TIG weld cleaned up nicely, and then the oil pan was painted black. Looks good. Works great! No leaks. And, very little was lost in the way of oil pan capacity. Of course, this modification works with the motor mounts we offer, and this oil pan mod info is included in the printed Duramax Conversion Guide linked below so you can show your welder exactly what you want.
Jim