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Jeep ELF specs: 318 ci (5.2L) 8-cylinder, 3.909 x 3.311 in (99.3 x 84.1 mm) bore and stroke, 9.1:1 compression. Rated 220 hp. OHV. Rod ratio and piston speed.
The Jeep Magnum (ELF) is oversquare — the 3.9094" bore exceeds the 3.311" stroke by 18.1% — a layout that favors airflow and RPM over leverage at the crank. The 3.311" stroke does not reach 4,000 ft/min of mean piston speed until about 7,250 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 8 cylinders displaces 39.8 ci (651 cc).
9.1:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 2 engines in the Magnum family, all of them 318 ci — the differences between them are in compression, heads and rating, not displacement. Factory fitments include the Grand Cherokee ZJ.
| Displacement | 318 ci / 5.2 L |
|---|---|
| Bore | 3.9094" |
| Stroke | 3.311" |
| Cylinders | 8 |
| Compression ratio | 9.1:1 |
| Bore/stroke ratio | 1.181 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 220 hp |
| Bore/stroke character | 1.181 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,311 ft/min — conservative for a street engine |
| Displacement per cylinder | 39.8 ci |
| Static compression | 9.1:1 — low by modern standards, generally happy on regular pump fuel |
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