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Porsche 506 specs: 1.3L (78 ci) 4-cylinder, 80 x 64 mm (3.1496 x 2.5197 in) bore and stroke, 6.5:1 compression. Rated 44 hp, 1951–1957.
The Porsche 78ci (506) is oversquare — the 3.1496" bore exceeds the 2.5197" stroke by 25.0% — a layout that favors airflow and RPM over leverage at the crank. The 2.5197" stroke does not reach 4,000 ft/min of mean piston speed until about 9,500 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 4 cylinders displaces 19.5 ci (320 cc).
6.5:1 is low by modern standards and generally happy on regular pump fuel.
It is one of 2 engines in the Porsche 78ci family, all of them 78 ci — the differences between them are in compression, heads and rating, not displacement. 7 factory variants share these dimensions, rated at 44 hp across 1951–1957.
| Displacement | 78 ci / 1.3 L |
|---|---|
| Bore | 3.1496" |
| Stroke | 2.5197" |
| Cylinders | 4 |
| Compression ratio | 6.5:1 * |
| Bore/stroke ratio | 1.25 |
* Compression ratio from a single source; not yet cross-verified.
| Fuel type | gas |
|---|---|
| Advertised HP | 44 hp |
| Years | 1951–1957 |
| Bore/stroke character | 1.250 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 2,520 ft/min — conservative for a street engine |
| Displacement per cylinder | 19.5 ci |
| Static compression | 6.5:1 — low by modern standards, generally happy on regular pump fuel |
Each link opens the calculator with this engine's specs already filled in.
These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| 1951 | 44 hp | 506 |
| 1952 | 44 hp | 506 |
| 1953 | 44 hp | 506 |
| 1954 | 44 hp | 506 |
| 1955 | 44 hp | 506 |
| 1956 | 44 hp | 506 |
| 1957 | 44 hp | 506 |
1 factory vehicle, 1951–1957. Each year opens that model's engine table.