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GMC 400 specs: 400 ci (6.6L) 8-cylinder, 4.00 x 3.975 in (101.6 x 101 mm) bore and stroke, 10.5:1 compression. Rated 345 hp. OHV. Rod ratio and piston speed.
The GMC Oldsmobile V8 (400) is close to square: 4" bore against 3.975" stroke, within 0.6%, which splits the difference between a high-revving layout and a torque layout. The 3.975" stroke puts mean piston speed at 4,000 ft/min around 6,050 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 50.0 ci (819 cc).
10.5:1 on iron heads is in the range that usually runs on pump premium; iron holds heat, so treat it as the less forgiving side of that range.
It is one of 7 engines in the Oldsmobile V8 family, sitting between the 307 ci and 425 ci members. Factory fitments include the Oldsmobile 442, Oldsmobile Cutlass.
| Displacement | 400 ci / 6.6 L |
|---|---|
| Bore | 4" |
| Stroke | 3.975" |
| Cylinders | 8 |
| Compression ratio | 10.5:1 |
| Bore/stroke ratio | 1.006 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 345 hp |
| Bore/stroke character | 1.006 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 3,975 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 50.0 ci |
| Static compression | 10.5:1 — in the range that usually runs on pump premium |
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