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GMC 305ci (1977–1978) specs: 305 ci (5.0L) 8-cylinder, 3.736 x 3.484 in (94.9 x 88.5 mm) bore and stroke, 8.5:1 compression. Rated 145 hp, 1977–1978. OHV.
The GMC 305ci (1977–1978) is oversquare — the 3.736" bore exceeds the 3.484" stroke by 7.2% — a layout that favors airflow and RPM over leverage at the crank. The 3.484" 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. A rod ratio of 1.636 is in the range most production engines fall in. Each of the 8 cylinders displaces 38.1 ci (625 cc).
8.5:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 6 engines in the GMC 305ci family, all of them 305 ci — the differences between them are in compression, heads and rating, not displacement. 2 factory variants share these dimensions, rated at 145 hp across 1977–1978.
| Displacement | 305 ci / 5 L |
|---|---|
| Bore | 3.736" |
| Stroke | 3.484" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 8.5:1 |
| Rod length | 5.7" |
| Rod ratio | 1.636 |
| Bore/stroke ratio | 1.072 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Firing order | 1-8-4-3-6-5-7-2 |
| Advertised HP | 145 hp |
| Years | 1977–1978 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. | Duration int / exh | Valve lift int / exh | LSA |
|---|---|---|---|---|---|---|---|
| 1977 | net | 145 @ 3800 | 245 lb-ft @ 2400 | 8.5:1 | 280° / 288° | — | 113° |
| 1978 | net | 145 @ 3800 | 245 lb-ft @ 2400 | 8.5:1 | 272° / 288° | — | 111° |
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 | 1.072 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,484 ft/min — conservative for a street engine |
| Rod ratio | 1.636 — moderate — the range most production V8s fall in |
| Displacement per cylinder | 38.1 ci |
| Static compression | 8.5:1 — 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 |
|---|---|---|
| 1977 | 145 hp | — |
| 1978 | 145 hp | — |