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Oldsmobile 403 specs: 403 ci (6.6L) 8-cylinder, 4.125 x 3.75 in (104.8 x 95.3 mm) bore and stroke, 8:1 compression. Rated 185 hp. Rod ratio and piston speed.
The Oldsmobile Rocket (403) is oversquare — the 4.1252" bore exceeds the 3.75" stroke by 10.0% — a layout that favors airflow and RPM over leverage at the crank. The 3.75" 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 15 engines in the Rocket family, sitting between the 260 ci and 455 ci members. Factory fitments include the Oldsmobile Cutlass, Oldsmobile Delta 88, Pontiac Trans Am and 6 others.
| Displacement | 403 ci / 6.6 L |
|---|---|
| Bore | 4.1252" |
| Stroke | 3.75" |
| 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 | 3,750 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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