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Cadillac HT4100 (1988–1989) specs: 273 ci (4.5L) 8-cylinder, 3.622 x 3.307 in (92 x 84 mm) bore and stroke, 9.5:1 compression. Rated 155 hp, 1988–1989. OHV.
The Cadillac HT4100 (1988–1989) is oversquare — the 3.622" bore exceeds the 3.3071" stroke by 9.5% — a layout that favors airflow and RPM over leverage at the crank. The 3.3071" stroke does not reach 4,000 ft/min of mean piston speed until about 7,250 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 8 cylinders displaces 34.1 ci (559 cc).
9.5:1 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 8 engines in the HT4100 family, sitting between the 250 ci and 300 ci members. 2 factory variants share these dimensions, rated at 155 hp across 1988–1989.
| Displacement | 273 ci / 4.5 L |
|---|---|
| Bore | 3.622" |
| Stroke | 3.3071" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 9.5:1 * |
| Bore/stroke ratio | 1.095 |
* Compression ratio from a single source; not yet cross-verified.
| Block material | aluminum |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 155 hp |
| Years | 1988–1989 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 1988 | LR8 net | 155 | 240 lb-ft | — |
Factory specification sheet.
| Bore/stroke character | 1.095 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,307 ft/min — conservative for a street engine |
| Displacement per cylinder | 34.1 ci |
| Static compression | 9.5:1 — in the range that usually runs on pump premium |
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 |
|---|---|---|
| 1988 | 155 hp | — |
| 1989 | 155 hp | — |