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Mercury 428 Cobra JET V8 specs: 428 ci (7.0L) 8-cylinder, 4.132 x 3.984 in (105 x 101.2 mm) bore and stroke, 10.6:1 compression. Rated 335 hp. OHV.
The Mercury 428 Cobra JET V8 is close to square: 4.132" bore against 3.984" stroke, within 3.7%, which splits the difference between a high-revving layout and a torque layout. The 3.984" stroke puts mean piston speed at 4,000 ft/min around 6,000 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 53.5 ci (877 cc).
10.6:1 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 34 engines in the FE family, the largest at 428 ci. Factory fitments include the Mercury Cougar, Mercury Cyclone, Mercury Cougar Eliminator and 2 others.
| Displacement | 428 ci / 7 L |
|---|---|
| Bore | 4.132" |
| Stroke | 3.984" |
| Cylinders | 8 |
| Compression ratio | 10.6:1 |
| Bore/stroke ratio | 1.037 |
| Piston deck clearance | 0.008" |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 335 hp |
| Bore/stroke character | 1.037 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 3,984 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 53.5 ci |
| Static compression | 10.6:1 — in the range that usually runs on pump premium |
| Intake valve diameter | 2.097" |
|---|---|
| Exhaust valve diameter | 1.66" |
| Chamber volume | 68 cc |
| Rocker ratio | 1.73:1 |
| Lifter type | hydraulic |
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