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GMC LB4/Z79 specs: 262 ci (4.3L) 6-cylinder, 4.00 x 3.48 in (101.6 x 88.4 mm) bore and stroke. Rated 280 hp, 1992–1993. Used in the GMC Jimmy. OHV.
The GMC LB4/Z79 is oversquare — the 4" bore exceeds the 3.4803" stroke by 14.9% — a layout that favors airflow and RPM over leverage at the crank. The 3.4803" stroke puts mean piston speed at 4,000 ft/min around 6,900 RPM, which is where a production bottom end is generally considered at its limit. Each of the 6 cylinders displaces 43.7 ci (716 cc).
It is one of 7 engines in the Vortec 4300 (4.3L V6) family, all of them 262 ci — the differences between them are in compression, heads and rating, not displacement. 2 factory variants share these dimensions, rated at 280 hp across 1992–1993.
| Displacement | 262 ci / 4.3 L |
|---|---|
| Bore | 4" |
| Stroke | 3.4803" |
| Cylinders | 6 |
| Configuration | V6 |
| Bore/stroke ratio | 1.149 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 280 hp |
| Years | 1992–1993 |
| Bore/stroke character | 1.149 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,480 ft/min — conservative for a street engine |
| Displacement per cylinder | 43.7 ci |
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These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| 1992 | 280 hp | LB4/Z79 |
| 1993 | — | LB4/Z79 |