L8S 6.6L engine specs (2027 HD trucks): what GM has confirmed
It is an enhanced 6.6 L gas V8. The RPO code L8S itself is Unconfirmed. It is the standard engine in the 2027 Chevrolet Silverado HD (2500 and 3500) and GMC Sierra HD. Confirmed so far: 6.6 L, 412 hp and 472 lb-ft (SAE J1349), built at Tonawanda, NY, and paired with the Allison 10-speed 10L1000. Unconfirmed: bore and stroke, compression, block and head material, fuel injection, valvetrain (VVT, AFM or DFM), redline, fuel, the rpm of those ratings, and the computer and harness. It is not the L78, the sixth-generation 6.6 L in the 2027 Silverado 1500 and Sierra 1500 (481 hp and 501 lb-ft). Initial L78 production is at Flint, with more capacity planned at Tonawanda and St. Catharines.
| Spec | Value |
|---|---|
| RPO | L8S (Unconfirmed) |
| Family | Unconfirmed. The 2027 HD trucks use an enhanced 6.6 L gas V8. It is not the sixth-generation L78 6.6 L in the 2027 Silverado 1500 and Sierra 1500 (481 hp and 501 lb-ft). Initial L78 production is at Flint, with more capacity planned at Tonawanda and St. Catharines. |
| Years | 2027 model year (available late 2026) |
| Displacement | 6.6 L |
| Horsepower | 412 hp |
| Torque | 472 lb-ft |
| Bore × stroke | Unconfirmed (L8T: 103.25 × 98.0 mm, 4.065 × 3.858 in) |
| Compression | Unconfirmed (L8T: 10.8:1) |
| Block | Unconfirmed (L8T: cast iron) |
| AFM/DOD | Unconfirmed |
| VIN 8th digit | Unconfirmed |
| Reluctor | Unconfirmed (L8T: 58X) |
| Throttle | Unconfirmed (L8T: electronic throttle) |
| Stock cam | Unconfirmed |
| Fuel injection | Unconfirmed (L8T: direct injection, with a high-pressure fuel pump) |
| Heads | Unconfirmed (L8T: cast aluminum) |
| Valvetrain | Unconfirmed (L8T: hydraulic roller lifters and VVT) |
| Power rpm | Unconfirmed (L8T: 401 hp at 5,200 rpm and 464 lb-ft at 4,000 rpm) |
| Redline | Unconfirmed (L8T maximum engine speed 5,600 rpm) |
| Fuel | Unconfirmed (L8T: regular unleaded, 87 octane) |
| Assembly | Tonawanda, NY |
| Transmission | Allison 10-speed 10L1000 |
| Intake | Turbo-Jet intake manifold |
| Oil system | Revised lubrication, engine-mounted oil cooler, digital oil-level readout |
Swap notes: how the L8S differs from the L8T
Rated output is 412 hp and 472 lb-ft, against the L8T at 401 hp at 5,200 rpm and 464 lb-ft at 4,000 rpm. The rpm where the L8S makes its numbers is Unconfirmed, so the gain is in the ratings, not a proven change in the rpm they arrive.
Named changes on the 2027 engine: a Turbo-Jet intake manifold, revised lubrication, an engine-mounted oil cooler, a digital oil-level readout, and updated emissions hardware, with more low-end pull. A swap has to leave room for that intake and for that oil cooler.
Still Unconfirmed. The L8T figure is context only: bore and stroke (L8T 103.25 x 98.0 mm), compression (L8T 10.8:1), block (L8T cast iron), heads (L8T cast aluminum), fuel injection (L8T direct injection), valvetrain (L8T hydraulic roller with VVT), maximum engine speed (L8T 5,600 rpm), fuel (L8T regular 87), and the computer (L8T uses an E93 and a 58X crank). Do not order L8T heads, cams, or a harness on the assumption that these carried over.
- No used supply yet. Both trucks are expected at U.S. dealers beginning late 2026, so there is no salvage long-block. The 6.6 L gas engine you can buy today is the L8T (2020-2026).
- Computer and harness: Unconfirmed. Nothing published names the controller or the crank pattern. A harness made for the L8T E93 and 58X crank may not match. More: how to wire your LS.
- Fuel system. Injection type is Unconfirmed. The L8T is direct injection and uses a high-pressure fuel pump. On the L8T crate engine that pump is mounted at the back of the engine, under the intake, and driven by a three-lobe cam, with a low-pressure supply feeding it. If the L8S keeps direct injection, a swap needs that high-pressure mechanical pump. Do not plumb it as a port-injection engine until injection type is confirmed.
- Transmission. The truck ships with the Allison 10-speed 10L1000. Older LS swap kits are built around a 6L80 or a 4L80E, which do not include the 10L1000 controller, wiring, or cooler. More: transmission guide.
- Not the half-ton 6.6, and not the 6.2. The 2027 Silverado 1500 and Sierra 1500 use a different engine, the L78: 481 hp and 501 lb-ft, sixth-generation. Initial production is at Flint, with more capacity planned at Tonawanda and St. Catharines. The half-ton 6.2 L engines are a different block again. More: L86 · L87 · L8T.
- Other known options: a 2020-2026 L8T from an HD truck, and the Chevrolet Performance L8T and L8P crate engines (same install sheet). There is no L8S salvage market yet, and no small-fabricator L8S long-block.
Trucks it is announced for
Not a junkyard source yet. If you find one, confirm it is an L8S from the truck's GM build documentation (its RPO list), not the seller's description. The 8th VIN digit is Unconfirmed.
| Vehicle | Years |
|---|---|
| Chevrolet Silverado HD/GMC Sierra HD | 2027 model year (available late 2026) |