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GMC 307 specs: 307 ci (5.0L) 8-cylinder, 3.875 x 3.25 in (98.4 x 82.6 mm) bore and stroke, 9:1 compression. Rated 200 hp. OHV. Rod ratio and piston speed.
The GMC Small Block Chevy (307) is oversquare — the 3.8748" bore exceeds the 3.25" stroke by 19.2% — a layout that favors airflow and RPM over leverage at the crank. The 3.25" stroke does not reach 4,000 ft/min of mean piston speed until about 7,400 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 8 cylinders displaces 38.4 ci (629 cc).
9:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 23 engines in the Small Block Chevy family, sitting between the 265 ci and 400 ci members. Factory fitments include the GMC C/K Series, GMC Suburban, GMC Sprint.
| Displacement | 307 ci / 5 L |
|---|---|
| Bore | 3.8748" |
| Stroke | 3.25" |
| Cylinders | 8 |
| Compression ratio | 9:1 |
| Bore/stroke ratio | 1.192 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 200 hp |
| Bore/stroke character | 1.192 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,250 ft/min — conservative for a street engine |
| Displacement per cylinder | 38.4 ci |
| Static compression | 9:1 — low by modern standards, generally happy on regular pump fuel |
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