Official Novak Reference Thread!!!
This post serves as the master swap guide for installing Chevrolet and GM engines into Jeep TJ & LJ Wranglers.
You’re welcome to ask questions, share photos of your builds, or post tips and corrections below.
First, our detailed TJ conversion instruction package:
📄 Read Novak’s detailed TJ conversion instruction package (PDF)
⚙️ Factory Powertrain Overview
Engines
Transmissions
- AX5 / TF904 — light-duty, not recommended for swaps
- AX15 (’97–’99) — medium-duty, good choice for mild V8s
- NV3550 (2000–2004) — similar to AX15, robust and compact
- NSG370 (2005–2006) — six-speed, decent but harder to adapt
- 32RH Auto — okay, but GM automatics are stronger and better suited
Transfer Cases
🔩 Planning the Powertrain Conversion
Transmission Choice
Avoid AX5 and 904/30RH — they won’t survive GM torque. The AX15 and NV3550 are retainable with proper adapters. Otherwise, GM automatics offer superior performance and durability.
🧱 Automatic Transmission Options
✅ GM autos generally bolt directly to their matching engines — no adapter needed.
🔧 Manual Transmission Options
🔩 Transfer Case Choice
The NP231 and NP241OR are compact, strong, and fully compatible with GM power. Keep the Jeep case — it simplifies speedometer and sensor integration.
🔋 Engine Selection
Best Fits
- Chevy 4.3L V6
- Chevy Small Block V8 (Gen I–III)
- GM Gen III/IV LS & Vortec engines
- Buick 3.8L / 4.1L V6
Avoid big blocks — they’re heavy, tight-fitting, and excessive for the TJ’s chassis.
Rule of thumb: 3.8L minimum, 400 ci maximum.
💨 Fuel Injection vs. Carburetion
Carbs are simple but outdated. Modern GM fuel injection systems outperform them in efficiency, reliability, and tuning flexibility. Avoid “carb-swapping” a Vortec or LS engine — you’ll lose more than you gain.
🌎 Emissions Notes
Follow the same-year-or-newer rule. The donor engine must retain all emissions equipment and meet your state’s classification (light truck, SUV, etc.).
Keep your Jeep’s charcoal canister and integrate it with the GM system.
🧠 Salvage Pull Checklist
When sourcing your donor:
- Engine
- All accessory brackets (alt, PS, A/C)
- PCM / ECM
- Full wiring harness
- Matching transmission (especially for 4L60E swaps)
⚙️ Engine Placement Tips
Mock up the complete engine (accessories, manifolds, etc.) and test fit for:
- Firewall clearance
- Steering shaft alignment
- Radiator and fan spacing
- Hood and oil pan clearance
- Most installs center the engine or offset it ½" toward the passenger side.
❄️ Cooling System
Factory TJ radiators don’t cut it. Use a Novak bolt-in aluminum radiator designed for GM outlet locations:
👉 TJ Cooling Components
⚡ Electrical & PCM Integration
GM PCMs are reliable, self-contained, and easy to integrate. The key principle:
- “Make the engine think it’s still in its original GM chassis.”
- Supply power & ground to PCM
- Keep the Jeep’s relay/fuse block
- Use the GM DLC port for OBD-II diagnostics
- The Novak Crank Position Emulator enables factory TJ gauges to work with GM power.
🧮 Gauges, Steering & Suspension
- Use aftermarket GM-compatible gauges or run Jeep ECM in tandem with GM PCM.
- Steering typically retains the factory gear — just adapt the power steering hoses.
- Stock TJ springs are sufficient for most V8/V6 swaps (no lift required).
⛽ Fuel System
- Early TJs (’97–’04) use Novak’s FSK-9704 kit for GM fuel pressure compatibility.
- ’05–’06 models already run 60 PSI, no changes needed.
🔥 Exhaust
- Tight-fit, block-hugger headers work best.
- Use dual O₂ sensors and a clean Y-pipe merge ahead of the catalytic converter.
- 2½-inch tubing is ideal; keep the factory heat shields.
🔩 Driveshafts & Mounts
- Re-measure driveshafts after the swap — expect custom lengths.
- Use a simple, standard rear mount like the Novak #RMU.
🦴 Axles & Linkages
- Factory Dana 30/35 axles handle GM power for most street and trail builds.
- For shifters, the Novak SK2X
- simplifies linkage setup.
🛞 Brakes
- 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 TJ is more rewarding than a proper GM powertrain conversion.
The combination of modern power and Jeep capability transforms drivability while keeping the Jeep spirit intact.
Each swap is unique — but with solid planning, the right Novak components, and this guide, your conversion will be straightforward, reliable, and a blast to drive.
Have you done this swap in your TJ or LJ?
Share your setup, transmission choice, and lessons learned below! 👇
