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GMC 403 specs: 403 ci (6.6L) 8-cylinder, 4.126 x 3.752 in (104.8 x 95.3 mm) bore and stroke, 8:1 compression. Rated 185 hp. OHV. Rod ratio and piston speed.
The GMC Oldsmobile V8 (403) is oversquare — the 4.126" bore exceeds the 3.752" stroke by 10.0% — a layout that favors airflow and RPM over leverage at the crank. The 3.752" 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 50.4 ci (825 cc).
8:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 7 engines in the Oldsmobile V8 family, sitting between the 307 ci and 425 ci members. Factory fitments include the Oldsmobile Cutlass, Pontiac Trans Am, Buick Riviera and 1 other.
| Displacement | 403 ci / 6.6 L |
|---|---|
| Bore | 4.126" |
| Stroke | 3.752" |
| Cylinders | 8 |
| Compression ratio | 8:1 |
| Bore/stroke ratio | 1.1 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 185 hp |
| Bore/stroke character | 1.100 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,752 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 50.4 ci |
| Static compression | 8:1 — low by modern standards, generally happy on regular pump fuel |
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