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GMC 305 specs: 305 ci (5.0L) 8-cylinder, 3.75 x 3.48 in (95.3 x 88.4 mm) bore and stroke, 8.5:1 compression. Rated 130 hp. OHV. Rod ratio and piston speed.
The GMC Small Block Chevy (305) is oversquare — the 3.75" bore exceeds the 3.4799" stroke by 7.8% — a layout that favors airflow and RPM over leverage at the crank. The 3.4799" stroke puts mean piston speed at 4,000 ft/min around 6,900 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 38.1 ci (625 cc).
8.5:1 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 Jimmy, GMC Safari and 1 other.
| Displacement | 305 ci / 5 L |
|---|---|
| Bore | 3.75" |
| Stroke | 3.4799" |
| Cylinders | 8 |
| Compression ratio | 8.5:1 |
| Bore/stroke ratio | 1.078 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 130 hp |
| Bore/stroke character | 1.078 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,480 ft/min — conservative for a street engine |
| Displacement per cylinder | 38.1 ci |
| Static compression | 8.5:1 — low by modern standards, generally happy on regular pump fuel |
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