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GMC LS3 specs: 402 ci (6.6L) 8-cylinder, 4.125 x 3.76 in (104.8 x 95.5 mm) bore and stroke, 8.5:1 compression. Built 1972. Used in the Chevrolet Chevelle.
The GMC Big Block Chevy (LS3) is oversquare — the 4.125" bore exceeds the 3.76" stroke by 9.7% — a layout that favors airflow and RPM over leverage at the crank. The 3.76" stroke puts mean piston speed at 4,000 ft/min around 6,400 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 50.3 ci (823 cc).
8.5:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 11 engines in the Big Block Chevy family, sitting between the 396 ci and 502 ci members.
| Displacement | 402 ci / 6.6 L |
|---|---|
| Bore | 4.125" |
| Stroke | 3.76" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 8.5:1 |
| Bore/stroke ratio | 1.097 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Firing order | 1-8-4-3-6-5-7-2 |
| Years | 1972 |
| Bore/stroke character | 1.097 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,760 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 50.3 ci |
| Static compression | 8.5:1 — low by modern standards, generally happy on regular pump fuel |
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9 factory vehicles, 1970–1972. Each year opens that model's engine table.