Official Novak Reference Thread!!!

This post serves as the master swap guide for installing Chevrolet and GM engines into Jeep YJ Wranglers (1987–1995).
You’re welcome to ask questions, share photos of your builds, or post tips and corrections below.

📄 Read Novak’s detailed YJ conversion instruction package (PDF)

🧰 Introduction

The Jeep YJ Wrangler (1987–1995) is one of the most enjoyable Jeeps to work with today. Its generous engine bay allows for a comfortable, clean fit for GM Small Block V6, V8, and even Big Block V8 engines.

With improvements in power, fuel economy, reliability, and drivability, a properly executed GM conversion transforms the YJ into a whole new machine. Even the Chevy and Buick V6 engines make excellent upgrades from the stock options.

🕰️ A Little History

AMC designed the YJ in the early 1980s, with Chrysler bringing it to production in 1987 after acquiring Jeep. The YJ introduced a 6” wider track, heavier-gauge steel frame, galvanized body panels, and coil-spring anti-sway setup that gave it better road manners than its CJ predecessors.

While its square headlights sparked debate among purists, YJs proved themselves as tough, modifiable, and ideal swap candidates — with nearly 630,000 built by the end of the 1995 production run.

⚙️ Factory Powertrain Overview

Engines

2.5L I4 — AMC design, MPI, reliable but underpowered
4.2L I6 (258) — carbureted, emissions-heavy, through 1990
4.0L I6 — Mopar EFI, _180hp

Transmissions

BA10/5 (’87–’88) — Peugeot design — weak and failure-prone
AX15 (mid-’88–’95) — tough, smooth, overdrive — keeper
AX5 (2.5L models) — light-duty — not recommended
TF904 / 30RH autos (2.5L) — weak, replace for GM power
TF999 / 32RH (I6 models) — decent but inferior to GM automatics

Transfer Cases

NP207 (’87–’88) — early part-time chain-driven case
NP231 (mid-’88–’95) — strong, versatile, adaptable

🔩 Planning the Powertrain Conversion

Transmissions are often more central to a swap than the engine. In nearly all YJs, the AX15 is the only factory gearbox worth retaining for a GM conversion. All others (AX5, BA10/5, 904, 30RH) are too light-duty for even moderate V6/V8 torque.

🧱 Automatic Transmission Options

TH350 - Compact, strong, affordable - #131 Series Adapter
TH700R4 - Overdrive, great ratios - #171 Series
4L60E - PCM-controlled, ideal for Gen III+ engines - #4L61 Series
TH400 - Heavy-duty, great for towing and serious off-road - #141 Series

✅ GM automatics generally bolt directly to most Chevrolet engines — no adapter required.

🔧 Manual Transmission Options

AX15 - Good for moderate V8s or V6s - #GMAX Series
SM420 - Ultra-low first gear, extremely durable - #421 Series
SM465 - Slightly newer, common, strong - #461 Series
Ford T18 - Another solid option - #181 Series

Pair these with our HCR3 Hydraulic Clutch Retrofit Kit for a simple clutch setup.

🔩 Transfer Case Choice

Most YJs already have the NP231, which is strong and perfectly compatible with GM power.
Avoid trying to use Chevy NP231C or NP233C — they introduce speed sensor and tailshaft issues. Stick with Jeep versions for easier integration with your speedometer and upgrades.

🔋 Engine Selection

A properly sized GM engine gives you torque, reliability, and fun without overwhelming the YJ’s lightweight chassis.

Chevy 4.3L V6
Chevy Small Block V8 (Gen I–III)
GM Gen III/IV LS & Vortec
Buick 3.8L / 4.1L “90°” V6

Avoid Big Blocks — they fit, but barely, and add unnecessary weight.
Rule of thumb: 3.8L minimum, 400 c.i. maximum.

⚡ Computers, Wiring & Fuel Injection

Modern GM PCMs are simple and reliable. The key principle:

“Make the engine think it’s still in its original GM chassis.”
Keep the donor PCM, harness, and sensors intact.
Use the Jeep’s relay/fuse block and add the GM DLC port for OBD-II diagnostics.
See Novak Wiring Integration Options

❄️ Cooling System

Factory radiators won’t cut it. Use a Novak bolt-in aluminum radiator configured for GM hose routing:
👉 YJ Cooling Components

🔧 Steering & Suspension

Retain the factory steering gear
Use the GM power steering pump that comes with your engine
Factory front springs are strong enough for Small Block V8s — lift is optional, not required

⛽ Fuel Delivery

If your YJ is fuel-injected, the OEM in-tank pump may suffice. Otherwise, our FSK-YJ Fuel Pump Module provides the correct pressure for modern GM EFI systems.

🔥 Exhaust

Run tight-fitting block-hugger headers
Join both banks into a Y-pipe ahead of the catalytic converter
2½” tubing is ideal; keep factory heat shields

🦴 Driveshafts, Crossmember & Rear Mount

Expect to order new driveshafts after mock-up — there’s no “universal” fit
Use a standard, low-profile rear mount like the Novak RMU
Factory crossmembers usually work fine with new holes drilled for mounting

🛞 Axles & Brakes

Dana 35 and Dana 44 axles hold up well under GM power for most street and trail builds
If they were good before, they’ll be good after
Upgrade only for extreme use or larger tire diameters

🧭 Summary

No change you can make to a YJ is more exciting or rewarding than a proper GM powertrain conversion.
It’s a proven swap that dramatically improves drivability, reliability, and trail performance while keeping that classic Jeep feel.

Each build is unique — but with solid planning, the right Novak components, and help from this community, your swap will be straightforward, strong, and a blast to drive.

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Have you done this swap in your YJ?
Share your setup, transmission choice, and lessons learned below! 👇