I once drove a K5 Blazer from the bottom of the country to Idaho and back. I did so with 39-inch tires, a 500hp big-block, three transmission speeds, no climate control, no top, earplugs, and a very loud stereo. The Blazer got an amazing 12 miles per gallon at 70 mph and I was happy the entire trip, rain or shine.
While I enjoyed the trip immensely then—and am in no way admitting to having softened up—I feel like I might not take pleasure in that same trip now. I'm going to blame it on the current price of fuel. Although, it may also have something to do with the 3,000 rpm my engine was spinning on the freeway and the ringing in my ears that stayed with me for months after the last trip. The three-speed TH400 transmission, combined with 4.88 gears, kept the big-block at a healthy roar through four states, and “fuel sipping” is not a term I'd use to describe a big-block with three speeds behind it. Of course, I wanted an overdrive gear.
Years later, after constant abuse and airtime, the back of the TH400 housing broke off completely. There is little doubt that the 240 pounds of my iron NP203/NP205 dual transfer case setup helped that happen—even with its own second crossmember—but the strength and multiple gear ratios convinced me to leave it in place.
The best way to improve my K5's performance, fuel economy, and drivability: add gears and lose pounds. The TH400 and NP203/NP205 doubler combination had served me well, but the 1-ton non-overdrive transmission combined with two iron transfer cases was weighing in on the high side of heavyweights.
While I could have just gone and bought a newer 4x4 for the cost of the parts I was about to install, I instead dropped in the "Ferrari of drivetrains" and obtained what I really wanted from the very beginning: a Gearstar 4L80E overdrive transmission combined with Advance Adapters' Atlas four-speed transfer case. The package equates to 16 speeds of power management that ended up dropping 100 pounds off the back of the transmission and improving fuel economy with the addition of a 0.75 overdrive gear. The combination was so close in size to the original TH400/NP203/NP205 grouping that we did not even have to change driveshaft lengths. The only downside (and why I didn't do this in the first place) is the cost. At around $7,500, it's not a cheap slush-box-and-T-case combo. Adding on, you'll also need a brain like HGM's Compushift II to manage the electronically controlled 4L80E transmission—something that tacks on an additional $1,000.
Can I justify the cost of parts in fuel savings? Certainly not. And I would never try to. But, I can justify the expense with driver happiness and improved performance in the vehicle. On top of that, the new transmission and transfer case are completely fresh and will allow me many more thousands of (enjoyable) miles before needing a rebuild.
Why a Gearstar 4L80E?
A 4L80E is a four-speed overdrive transmission that is the evolutionary counterpart to the TH400 three-speed transmission. Build both transmissions to handle 600 horsepower and Gearstar will charge you $2,200 for the TH400 and $3,900 for the 4L80E. I just added $1,700 to the drivetrain for one extra 0.75 overdrive gear.
Where is the logic? It's certainly not monetarily based (nor found in weight savings since the 4L80E is about 35 pounds heavier). But the superior functionality of the electronically controlled transmission with an overdrive gear cannot be argued.
While some may not prefer that the 4L80E requires a controller like HGM's Compushift II, I like the fact that it can be easily adjusted and reprogrammed via a hand-held display. Shift pressures, shift points, torque converter lockup, and the overdrive can all be controlled via the Compushift II, giving complete control over the transmission like I've never had before.
The advantages and benefits of an overdrive gear that lowers engine rpm and improves fuel economy are easily apparent.
Why an Atlas Four-Speed?
This is a valid question since I already had three transfer case shifters on the floor using an NP203/NP205 doubler setup, with four gear ratios (1:1, 1.96:1, 2.01:1, 3.94:1). I could have just swapped in the 4L80E transmission and been good to go. The doubler setup had served me well for years.
While the doubler setup is extremely strong and has only the cost of two used transfer cases and the doubling adapters, it also weighs about 240 pounds. If you're going fast and in the air when off road, then that is a lot of weight to be bolted up behind the transmission. Two of the four gear ratios are also extremely close (numerically), so it's really more of a three-speed T-case setup.
Advance Adapters' Atlas four-speed transfer case has recently been re-released to the marketplace after making some updates, and decades of transfer case evolution make the Atlas four-speed every bit the dominator that it is. No other T-case gives the combination of unbelievable strength, multiple drive options, user-friendliness, and light weight like the Atlas four-speed. It weighs only 135 pounds, has shift-on-the-fly capability, four gear ratios (1:1, 2:1, 2.72:1, 5.44:1), and has been reported to handle quadruple-digit power numbers while staying together and functioning perfectly.