See other engines in this family
Cummins 6.7L ISB specs: 408 ci (6.7L) 6-cylinder, 4.213 x 4.882 in (107 x 124 mm) bore and stroke, 16.2:1 compression. Rated 370-420 hp. OHV.
The Cummins 6.7L ISB is undersquare — the 4.8819" stroke exceeds the 4.2126" bore by 13.7% — a layout that trades peak RPM for leverage and torque low in the range. With a 4.8819" stroke, mean piston speed reaches 4,000 ft/min at only 4,900 RPM — the practical ceiling for a production bottom end — which is why a stroke this long makes its power low and runs out of safe revs early. Each of the 6 cylinders displaces 68.0 ci (1,114 cc).
At 16.2:1 this is a compression-ignition engine: the ratio is the heat source that lights the fuel, not a knock limit to be managed.
It is one of 2 engines in the 6.7L ISB family, all of them 408 ci — the differences between them are in compression, heads and rating, not displacement. 2 factory variants share these dimensions, rated from 370 to 420 hp. Factory fitments include the Ram 3500, Ram 2500, Ram 4500 and 1 other.
| Displacement | 408 ci / 6.7 L |
|---|---|
| Bore | 4.2126" |
| Stroke | 4.8819" |
| Cylinders | 6 |
| Compression ratio | 16.2:1 |
| Bore/stroke ratio | 0.863 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | diesel |
| Advertised HP | 370–420 hp |
| Bore/stroke character | 0.863 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 4,882 ft/min — above the 4,500 ft/min mark where most production bottom ends are considered at their limit |
| Displacement per cylinder | 68.0 ci |
| Static compression | 16.2:1 — high enough that pump premium or better is normally required |
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 |
|---|---|---|
| — | 370 hp | ISB 6.7 |
| — | 420 hp | ISB 6.7 HO |