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Jaguar XK 4.2 EFI specs: 259 ci 6-cylinder, 92.1 x 106 mm (3.6250 x 4.1730 in) bore and stroke, 8.1:1 compression. Built 1980–1987. Used in the Jaguar XJ6.
The Jaguar XK 4.2 EFI is undersquare — the 4.173" stroke exceeds the 3.625" bore by 13.1% — a layout that trades peak RPM for leverage and torque low in the range. The 4.173" stroke puts mean piston speed at 4,000 ft/min around 5,750 RPM, which is where a production bottom end is generally considered at its limit. Each of the 6 cylinders displaces 43.2 ci (707 cc).
8.1:1 is low by modern standards and generally happy on regular pump fuel.
It is one of 2 engines in the Jaguar 259ci family, all of them 259 ci — the differences between them are in compression, heads and rating, not displacement. 6 factory variants share these dimensions across 1980–1987.
| Displacement | 259 ci |
|---|---|
| Bore | 3.625" |
| Stroke | 4.173" |
| Cylinders | 6 |
| Configuration | I6 |
| Compression ratio | 8.1:1 |
| Bore/stroke ratio | 0.869 |
| Fuel type | gas |
|---|---|
| Years | 1980–1987 |
| Bore/stroke character | 0.869 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 4,173 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 43.2 ci |
| Static compression | 8.1:1 — 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 |
|---|---|---|
| 1980 | — | XK 4.2 EFI |
| 1982 | — | XK 4.2 EFI |
| 1984 | — | XK 4.2 EFI |
| 1985 | — | XK 4.2 EFI |
| 1986 | — | XK 4.2 EFI |
| 1987 | — | XK 4.2 EFI |
1 factory vehicle, 1980–1987. Each year opens that model's engine table.