Bore, stroke, displacement and compression for all 3 Porsche 91ci engines, compared side by side. Plus rod ratio and head specs where published.
| Displacement | 91 ci |
|---|---|
| Cylinders | 4 |
| Bore | 3.150–3.346" |
| Stroke | 2.598–2.913" |
| Compression | 6.8–9.0:1 |
| Advertised power | 55–100 hp |
| Years | 1952–1957 |
| Engine | Displacement | Bore × Stroke | CR | HP |
|---|---|---|---|---|
| 546; 527 early 1952 | 91 ci | 3.1496" × 2.9134" | 6.8:1 | 55 |
| 528 | 91 ci | 3.1496" × 2.9134" | 8.2:1 | 70 |
| 547/1 | 91 ci | 3.3465" × 2.5984" | 9:1 | 100 |
Both bore and stroke vary across the family, so these are separate castings rather than variations on one block.
Compression spans 6.8:1 to 9:1, so fuel requirements differ meaningfully between members — worth checking before assuming parts interchange.
All are oversquare — bore larger than stroke, which favours revs over low-end torque.
Each engine page carries rod ratio, mean piston speed, bore/stroke character and cylinder head specs where published, and links into the calculators with its dimensions already filled in.