Want a 6-speed automatic with overdrive behind your classic? TH400 to 6L80 is doable but complex. Here's what's involved.
Shop the parts for this fix
Or grab the individual parts:
Why this swap
- Modern 6-speed (vs 3-speed TH400)
- Overdrive lowers highway RPM 25-30%
- 5-10% MPG improvement
- Stronger than TH400 in HD form
- Read our 6L80 vs 8L90
What's different from TH400 swap
- 6L80 is electronic — needs TCM
- Wiring integration required
- Cooling demands higher
- Significantly more complex
- Read our TH400 to 700R4 swap for simpler alternative
Required parts
- 6L80 trans (from LS-era truck)
- Standalone TCM (US Shift Compushift, GM TCM with tune)
- Trans harness adapter
- New driveshaft (different length)
- Crossmember mods
- Aux cooler
- Read our transmission cooler installation
Mechanical considerations
- Bellhousing pattern: SBC/BBC small block compatible (verify)
- 6L80 longer than TH400
- Trans tunnel may need modification
- Custom driveshaft needed
Electronic integration
Option A: Standalone TCM
- US Shift Compushift, MSD Atomic
- $700-$1,500
- Programmable shift points and pressures
- Best for non-LS engine swaps
Option B: GM TCM with tune
- Use factory GM TCM
- Tune via HP Tuners or EFILive
- $500-$800 in TCM + tune
- Best for LS swap with GM ECM
- Read our LS swap transmission guide
Total cost
- Used 6L80: $800-$1,500
- Reman with HD upgrade: $2,500-$4,000
- TCM solution: $500-$1,500
- Adapter/wiring: $300-$600
- Driveshaft: $200-$400
- Cooler: $200-$400
- Total: $2,000-$8,000
When to do this swap
- Modern restomod project
- LS swap already happening
- HD use needing strength + OD
- Comfortable with electronic complexity
When NOT to
- Stock-restoration purist build
- Don't want electronics
- Use 700R4 instead (simpler)
- Read our TH400 to 700R4 swap
HD strength
- Stock 6L80: ~470 ft-lb
- With Sonnax HD upgrades: 650+ ft-lb
- Read our 6L80 wave plate
- Read our 6L80 input drum failure
Need 6L80 swap parts? Shop our 6L80 catalog. Rebuild kits, Sonnax HD upgrades, mechatronics, complete solutions. Free shipping over $70.
Related: