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Triumph FH / FK specs: 79 ci 4-cylinder, 73.7 x 76 mm (2.9000 x 2.9920 in) bore and stroke, 8.5:1 compression. Built 1971–1972. Used in the Triumph Spitfire.
The Triumph FH / FK is close to square: 2.9" bore against 2.992" stroke, within 3.1%, which splits the difference between a high-revving layout and a torque layout. The 2.992" stroke does not reach 4,000 ft/min of mean piston speed until about 8,000 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 4 cylinders displaces 19.8 ci (324 cc).
8.5:1 is low by modern standards and generally happy on regular pump fuel.
It is one of 2 engines in the Triumph 79ci family, all of them 79 ci — the differences between them are in compression, heads and rating, not displacement. 2 factory variants share these dimensions across 1971–1972.
| Displacement | 79 ci |
|---|---|
| Bore | 2.9" |
| Stroke | 2.992" |
| Cylinders | 4 |
| Configuration | I4 |
| Compression ratio | 8.5:1 * |
| Bore/stroke ratio | 0.969 |
* Compression ratio from a single source; not yet cross-verified.
| Fuel type | gas |
|---|---|
| Years | 1971–1972 |
| Bore/stroke character | 0.969 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 2,992 ft/min — conservative for a street engine |
| Displacement per cylinder | 19.8 ci |
| Static compression | 8.5:1 — 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 |
|---|---|---|
| 1971 | — | FH / FK |
| 1972 | — | FH / FK |
1 factory vehicle, 1971–1972. Each year opens that model's engine table.