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Swapping an 8.1L Big-Block in a 1973-1991 Square-Body Chevy Pickup

Posted in How To: Engine on April 21, 2017
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Photographers: Staff

Depending on where you live, big-block Chevy 8.1L engines can be found cheap. We are not sure why, but compared to the LS-series truck engines, these torque tanks are plentiful and inexpensive. This modern GM big-block is similar to the LS engines but also unique. And they are heavy, with iron blocks and heads. Most of the hot rod guys prefer the old-style big-blocks, and most of the off-road guys prefer the LS engines, so the 8.1Ls seem to get ignored. Plus, they don’t have a ton of aftermarket support in terms of upgrades. They are also not the best for fuel economy, so you’d almost wonder why we chose one. Well, they are abundant in 3/4-ton trucks, vans, Suburbans, Avalanches, and RVs from around 2001 to 2010. Some were even dropped in boats. And did we mention they are bad on gas, heavy, and cheap?

We swapped one in our 1986 Chevy military truck and couldn’t be happier. The engine runs smooth and is grunty. Well, we could be a little happier if we hadn’t found a rear main seal leak after we had the engine all bolted in place, but that is the gamble of just stabbing a used junkyard engine in your project. Overall, we are elated with our new-to-us big-block. Here are the swap highlights if you are contemplating one for your square-body GM.

We found an 8.1L out of a 2003 Suburban 2500 4x4 for under $1,500. It came with a 4L80 transmission, all used, but still healthy. We decided to swap it all into our 1986 Chevy K30 military CUCV truck we call the Alabama Army Truck (also known as the Alaska Army Truck or just AAT). We had built this truck in the past with an awesome supercharged 6.2L LSA engine that would roast the tires at will, but that engine got yanked and stuffed in our Ultimate Summer Camp Jeep, so the Alaska Army Truck has been sitting with open engine bay for a year now. The 8.1L would be a bigger, less expensive, and almost bolt-in swap.
The first step of our swap was removing all the wiring and boxing up the 4L80 transmission. The factory wiring harness was packaged up with the ECU and sent to Tilden Motorsports, where the harness was cut down to remove unneeded components and the ECU reflashed for our application. The 4L80 was shipped to Gearstar transmission for a stock-spec rebuild.
We said it before and we’ll say it again: Use plenty of engine degreaser and a nice pressure washer to clean your project truck before the engine swap begins. Starting with a clean truck makes working in an old engine bay that much easier. Now is also a good time to repair any rust and to paint anything you want painted that will be made inaccessible by the engine.
The 8.1L will bolt to GM transmissions like the TH400, SM465, and 4L80E. You can reuse those if they are already in your truck. We had the 4L80E gone through by Gearstar since the company can build automatic transmissions to live with very high horsepower and torque. Our build was pretty mild considering Gearstar’s options, but it will be adequate since our axles will have very deep 6.84 gears. Because our engine and transmission came together from the factory, no special adapters or unique flexplates were required.
Within the engine bay we have the Offroad Design high-clearance engine crossmember This ties into the factory engine mount supports found in the square-body Chevy trucks, and older small- and big-block engine mounts are available. Unlike the LS engines, the 8.1L big-block uses Offroad Design’s Heavy Duty Motor Mounts, which align with the older-style small- and big-blocks such that the engine was literally a bolt-in mounting affair. If your truck had a small- or big-block prior, the engine would be aligned to drop in and bolt to your transmission.
Dropping in a 760-pound engine can wreak havoc on the average engine hoist. Sure, a hoist can manage it, but if your friend or neighbor has a forklift you will be that much happier. We also recommend pulling the hood and the radiator and watching all those fingers and toes. Plus, putting the truck down on really short jackstands instead of fullsize mud tires will help as well.
We did have to do some frame clearance to the top lip of the C-channel to clear the exhaust manifolds. Firewall clearance was not bad at all.
When we installed the supercharged LSA engine back in 2013, we used a bolt-in replacement radiator from BeCool. We reused this with the 8.1L. The Tilden Motorsports wiring harness controls the dual electric fans, turning one on at 190 degrees and the second if the engine pushes up to 210 degrees. This is all controlled by the reflashed factory ECU. We also used the factory Suburban air intake to an Airaid filter and an Optima YellowTop to power it all.
Previously we used a fuel cell in the bed of the truck, and although it worked well, filling it was a pain and using the bed was stressful because we worried cargo would slide around and damage the tank. This time around we ordered up a 1980s Suburban fuel tank and filler neck from LMC Truck and installed an Aeromotive Phantom 17167 in-tank fuel pump. This pump mounts in a stock factory tank after a hole is cut in the tank, and is spring-loaded to adjust in height to the bottom of the tank. We always prefer an in-tank pump because it keeps the pump cool while submerged in fresh gasoline.
The Aeromotive fuel pressure regulator mounts to the firewall and is plumbed in with a Tilden Motorsports hard-line fuel line kit. The fuel rails on this 8.1L are returnless, so the regulator has the return line to the tank.
It’s nice to have access to the wiring and fuel pump without dropping the fuel tank, so we cut a hole in the floor of the bed of the truck. We’ll build a clean smooth cover plate so we can protect the pump from bed cargo.
The LMC fuel tank sits behind the rear axle and means our spare tire will live in the bed or on a rear tire hanger, but seeing as we are upgrading to 42-inch Maxxis tires, the spare would not fit underneath anymore anyways. We do need to invest time and materials into building a good skidplate under the tank to protect it when dropping off ledges.
The Tilden Motorsports reworked wiring harness supports a drive-by-wire throttle, so we reused the one from the donor Suburban. We like the simple mounting that the drive-by-wire systems offer for new engines. Just put the pedal where you find it the most comfortable, and plug in the wires. Yes, those are old cans bolted to the floor as cup holders.
When we had done the prior LS swap we changed over to Auto Meter gauges in a new dash panel. We are still using these and really dig the GPS speedometer that is nearly idiot-proof.
The Tilden 8.1L wiring harness is a cleaned-up version of the factory harness and plugs back in place easily. Just remember to connect all the grounds, or it won’t start. Ask us how we know.
One future upgrade we are looking at is an ECU mount from Mercenary Off Road. The ECU doesn’t have any mounting holes, so Mercenary offers an easy-to-mount case so that the ECU is held in, perfect for off-road driving.

Build Me One!

The folks at Tilden Motorsports build LS engines for hot rods and 4x4s and reworked both our wiring harness and ECU for the Alaska Army Truck. The company can also build you a complete 8.1L big-block for your project with upgraded cam options, heads, and intakes. Tilden recently built this 600-plus-horsepower, 700–lb-ft 8.1L for a customer.

Tell Me More!

Want to learn even more about the 8.1L? Check out this article from Car Craft magazine on the Hot Rod Network website. It dives deeper into the nitty-gritty of these engines and ways to get bigger power out of them.

Show Me More!

You can watch the swap and the suspension upgrades and the first adventure we took our 8.1L Army truck on Dirt Every Day. Search YouTube for “Alaskan Army truck adventure.”


Auto Meter
Sycamore, IL 60178
Be Cool
Essexville, MI 48732
Gearstar Performance Transmissions
Akron, 44308
LMC Truck
Lenexa, KS 66219
Offroad Design
Lenexa, KS 66214
Phoenix, AZ 85050
Suwanee, GA 30024
Mercenary Offroad

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