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Triumph FM specs: 91 ci 4-cylinder, 73.7 x 87.5 mm (2.9000 x 3.4450 in) bore and stroke. Rated 53 hp, 1974–1980. Used in the Triumph Spitfire.
The Triumph 91ci (FM) is undersquare — the 3.445" stroke exceeds the 2.9" bore by 15.8% — a layout that trades peak RPM for leverage and torque low in the range. The 3.445" stroke puts mean piston speed at 4,000 ft/min around 6,950 RPM, which is where a production bottom end is generally considered at its limit. Each of the 4 cylinders displaces 22.8 ci (373 cc).
5 factory variants share these dimensions, rated at 53 hp across 1974–1980.
| Displacement | 91 ci |
|---|---|
| Bore | 2.9" |
| Stroke | 3.445" |
| Cylinders | 4 |
| Configuration | I4 |
| Compression ratio | 7.5–9.1:1 (published figures vary) |
| Bore/stroke ratio | 0.842 |
| Fuel type | gas |
|---|---|
| Advertised HP | 53 hp |
| Years | 1974–1980 |
| Bore/stroke character | 0.842 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 3,445 ft/min — conservative for a street engine |
| Displacement per cylinder | 22.8 ci |
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 |
|---|---|---|
| 1974 | — | FM |
| 1975 | 53 hp | FM |
| 1976 | — | FM |
| 1979 | — | FM |
| 1980 | — | FM |
1 factory vehicle, 1974–1980. Each year opens that model's engine table.