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GMC LMM specs: 403 ci (6.6L) 8-cylinder, 4.055 x 3.898 in (103 x 99 mm) bore and stroke. Rated 250-365 hp, 2007–2010. Used in the GMC Sierra HD. OHV.
The GMC 403ci (LMM) is close to square: 4.0551" bore against 3.8976" stroke, within 4.0%, which splits the difference between a high-revving layout and a torque layout. The 3.8976" stroke puts mean piston speed at 4,000 ft/min around 6,150 RPM, which is where a production bottom end is generally considered at its limit. A rod ratio of 1.646 is in the range most production engines fall in. Each of the 8 cylinders displaces 50.4 ci (825 cc).
It is one of 9 engines in the GMC 403ci family, the largest at 403 ci. 6 factory variants share these dimensions, rated from 250 to 365 hp across 2007–2010.
| Displacement | 403 ci / 6.6 L |
|---|---|
| Bore | 4.0551" |
| Stroke | 3.8976" |
| Cylinders | 8 |
| Configuration | V8 |
| Rod length | 6.417" |
| Rod ratio | 1.646 |
| Bore/stroke ratio | 1.04 |
| Block material | cast iron |
|---|---|
| Head material | aluminum |
| Valvetrain | OHV |
| Fuel type | diesel |
| Advertised HP | 250–365 hp |
| Years | 2007–2010 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 2009 | LMM net | 365 @ 3200 * | 660 lb-ft @ 1600 * | — |
| 2010 | LMM net | 250 @ 3200 * | 460 lb-ft @ 1600 * | — |
| 2010 | LMM net | 365 @ 3200 * | 660 lb-ft @ 1600 * | — |
Factory specification sheet. * Not yet cross-verified.
| Bore/stroke character | 1.040 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 3,898 ft/min — in the range typical of performance street engines |
| Rod ratio | 1.646 — moderate — the range most production V8s fall in |
| Displacement per cylinder | 50.4 ci |
Each link opens the calculator with this engine's specs already filled in.
These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| 2007 | 365 hp | LMM |
| 2008 | 250 hp | LMM |
| 2008 | 365 hp | LMM |
| 2009 | 365 hp | LMM |
| 2010 | 250 hp | LMM |
| 2010 | 365 hp | LMM |
2 factory vehicles, 2007–2010. Each year opens that model's engine table.