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GMC 250ci (1967–1978) specs: 250 ci (4.1L) 6-cylinder, 3.875 x 3.53 in (98.4 x 89.7 mm) bore and stroke. Rated 105-155 hp, 1967–1978.
The GMC 250ci (1967–1978) is oversquare — the 3.875" bore exceeds the 3.53" stroke by 9.8% — a layout that favors airflow and RPM over leverage at the crank. The 3.53" stroke puts mean piston speed at 4,000 ft/min around 6,800 RPM, which is where a production bottom end is generally considered at its limit. A rod ratio of 1.615 is in the range most production engines fall in. Each of the 6 cylinders displaces 41.7 ci (683 cc).
It is one of 2 engines in the GMC 250ci family, all of them 250 ci — the differences between them are in compression, heads and rating, not displacement. 11 factory variants share these dimensions, rated from 105 to 155 hp across 1967–1978.
| Displacement | 250 ci / 4.1 L |
|---|---|
| Bore | 3.875" |
| Stroke | 3.53" |
| Cylinders | 6 |
| Configuration | I6 |
| Compression ratio | 8–8.5:1 (published figures vary) |
| Rod length | 5.7" |
| Rod ratio | 1.615 |
| Bore/stroke ratio | 1.098 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Firing order | 1-5-3-6-2-4 |
| Advertised HP | 105–155 hp |
| Years | 1967–1978 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. | Duration int / exh | Valve lift int / exh | LSA |
|---|---|---|---|---|---|---|---|
| 1967 | L22 (Chevelle option) gross | 155 @ 4200 | 235 lb-ft @ 1600 | 8.5:1 | 336° | 0.388" | 105.25° |
| 1968 | gross | 155 @ 4200 | 235 lb-ft @ 1600 | 8.5:1 | 336° | 0.388" | 105.25° |
| 1969 | gross | 155 @ 4200 | 235 lb-ft @ 1600 | 8.5:1 | 336° | 0.388" | 105.25° |
| 1970 | gross | 155 @ 4200 | 235 lb-ft @ 1600 | 8.5:1 | 244° | 0.388" | 105.25° |
| 1976 | net | 105 @ 3800 | 185 lb-ft @ 1200 | 8.25:1 | 244° / 294° | 0.388" / 0.405" | 101.5° |
| 1978 | net | 115 @ 3800 | 195 lb-ft @ 1800 | 8:1 | 244° | — | 105.25° |
Factory specification sheet. Durations are the factory's advertised figures at the basis printed on the sheet, not duration at .050" lift; lobe separation is calculated from the printed valve timing.
| Bore/stroke character | 1.098 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,530 ft/min — in the range typical of performance street engines |
| Rod ratio | 1.615 — moderate — the range most production V8s fall in |
| Displacement per cylinder | 41.7 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 |
|---|---|---|
| 1967 | 155 hp | — |
| 1968 | 155 hp | — |
| 1969 | 155 hp | — |
| 1970 | 155 hp | — |
| 1971 | — | — |
| 1972 | — | — |
| 1973 | — | — |
| 1974 | — | — |
| 1976 | 105 hp | — |
| 1977 | — | — |
| 1978 | 115 hp | — |
1 factory vehicle, 1972–1976. Each year opens that model's engine table.