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Shelby 260ci specs: 4.3L (260 ci) 8-cylinder, 96.5 x 72.9 mm (3.8000 x 2.8700 in) bore and stroke, 12:1 compression. Rated 260 hp, 1963.
The Shelby 260ci is oversquare — the 3.8" bore exceeds the 2.87" stroke by 32.4% — a layout that favors airflow and RPM over leverage at the crank. The 2.87" stroke does not reach 4,000 ft/min of mean piston speed until about 8,350 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. A rod ratio of 1.796 is long by production standards — less side-load on the cylinder wall and gentler piston acceleration around TDC, the geometry that tolerates high RPM. Each of the 8 cylinders displaces 32.5 ci (533 cc).
12:1 is high for a production gasoline engine; running a ratio like that on pump fuel depends on chamber design and fuel delivery rather than the number alone.
It is one of 3 engines in the Shelby 260ci family, all of them 260 ci — the differences between them are in compression, heads and rating, not displacement.
| Displacement | 260 ci / 4.3 L |
|---|---|
| Bore | 3.8" |
| Stroke | 2.87" |
| Cylinders | 8 |
| Compression ratio | 12:1 |
| Rod length | 5.155" |
| Rod ratio | 1.796 |
| Bore/stroke ratio | 1.324 |
| Piston deck clearance | 0" |
| Induction | 4-2 |
|---|---|
| Advertised HP | 260 hp |
| Years | 1963 |
| Bore/stroke character | 1.324 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 2,870 ft/min — conservative for a street engine |
| Rod ratio | 1.796 — long — less side loading on the cylinder wall, favours high RPM |
| Displacement per cylinder | 32.5 ci |
| Static compression | 12:1 — high enough that pump premium or better is normally required |
| Intake valve diameter | 1.843" |
|---|---|
| Exhaust valve diameter | 1.625" |
| Chamber volume | 49 cc |
| Rocker ratio | 1.6:1 |
| Lifter type | mechanical |
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