Reviving A Classic: Installing A Modern GM V8 and 8-Speed In A Classic Jeep Grand Wagoneer

by Michael Satterfield -07/25/2024

Repowering our classic Grand Wagoneer was a bigger task than we originally had thought it would be. Unlike the previous version of the LS, the latest drivetrains do not require a simple plug-and-play installation. The 1987 Jeep Grand Wagoneer is a prime candidate for such a transformation, with its rugged charm and timeless appeal, crippled by its ancient and expensive to-rebuild AMC V8 under the hood. Our partners at Apex Auto Works recently took on the ambitious task of modernizing this iconic SUV by installing a low mileage 2018 GM 5.3 V8 engine and an 8L90 transmission, a swap that promised to enhance performance while preserving the vehicle's character. 

The 1987 Jeep Grand Wagoneer holds a special place in the hearts of many enthusiasts. Known for its wood-grain paneling, plush interiors, and off-road capabilities, this classic SUV symbolizes a bygone era of American automotive design. However, the original powertrain, a 5.9-liter V8 engine paired with a 3-speed automatic transmission, left much to be desired in terms of performance and efficiency by modern standards.

The decision to swap in a 2018 GM 5.3 V8 engine was driven by a desire to balance power, reliability, and modern engineering. The 5.3-liter V8, known for its robust performance and efficiency, produces 355 horsepower and 383 lb-ft of torque, a significant upgrade from the original engine. Coupled with an 8-speed automatic transmission, this powertrain promised smoother shifts, improved fuel economy, and a more responsive driving experience. These engines are inexpensive and plentiful as they are found in every full-size GM and Chevrolet pickup and large SUV; they are also less desirable for the aftermarket thanks to their challenging computer and wiring issues. 

Engine Mounts and Fitment

The first hurdle was the physical fitment of the new engine and transmission. The engine bay of the 1987 Grand Wagoneer was not designed to accommodate the more compact yet more powerful GM 5.3 V8. Thankfully, we could order custom engine mounts from BJ's Off-Road, which would at least get the engine and transmission mounted in place. But that was just half the battle; even with custom mounts, we still had to modify the engine compartment and add clearance around parts like the brake booster and fuel tank.

Transmission Tunnel Modification

The 8-speed automatic transmission required modifications to the transmission tunnel. The original tunnel was too narrow, necessitating some slight reshaping to create adequate space. This, however, was minor. While they were down there, Apex Auto Works reinforced the modified tunnel and patched some minor rust holes in the floorboard. We also had to have a local shop make custom driveshafts that would mate the GM transfer to the classic Jeep axles. 

Electronics and Wiring

This is where it got tricky; integrating modern electronics into a vintage vehicle is always complex. The team had to splice the new engine’s wiring harness with the Jeep’s original electrical system. This was straightforward enough, but to make sure all of our gauges worked, we went with a Dakota Digital cluster that worked with the modern engine but still retained the classic look, plus the GPS-driven speedo means we didn't have to adapt the original cable-driven speedometer. 

The team at Apex had to do several rounds of programming and testing compatibility with the engine control unit (ECU) and integrating the modern transmission’s electronic controls. Apex used a factory ECU to manage the engine and transmission, ensuring OEM levels of performance. While we started with an aftermarket harness, we quickly found that a lot of custom wiring would be required, and we also had to reflash the transmission controller to get it to work properly. Later we would find out that the transmission itself had an issue.

Cooling System

The increased power output of the GM 5.3 V8 necessitated an upgraded cooling system. We worked with Champion Cooling Systems to install a high-capacity custom aluminum radiator and electric cooling fans to keep the engine running at optimal temperatures. Custom hoses and fittings ensured proper coolant circulation. 

Exhaust System

The exhaust system had to be entirely redesigned. The team fabricated custom exhaust pipes from he factory exhaust manifold back to fit within the confines of the Jeep’s chassis. The new system accommodated the GM V8 and provided a throaty exhaust note that complemented the vehicle’s enhanced performance.

Brakes and Suspension

Thankfully, the Jeep Grand Wagoneer's braking and suspension systems are more than enough to handle a stock GM V8. The team installed new disc brake rotors, calipers, and pads to ensure reliable stopping power. New shocks, springs, and bushings enhanced the suspension to improve handling and ride quality. While it was all apart, they also installed new wheel bearings. 

Fuel System

The original fuel system could not support the demands of the modern V8 engine. To ensure consistent fuel delivery, we turned to BJ's Off-Road again, which offers a drop-in high-performance fuel pump; in addition to the high-flow fuel pump, Apex installed new fuel lines and an updated fuel pressure regulator. 

After months of meticulous work and countless hours of problem-solving, the 1987 Jeep Grand Wagoneer emerged from Apex Auto Works as a true modern classic. The modern driveline delivers smooth, responsive power and improved efficiency. The vehicle retained its classic charm while offering the reliability and performance of a contemporary SUV. 

Expect to spend between $10,000-$15,000 to do this swap, on top of whatever your used drivetrain costs. You could save money with a little more DIY, but honestly, without the Apex team and their computer wizards team, we would have never been able to get this amazing ride on the road.