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Porsche 40 991 GT3 specs: 4.0L (244 ci) 6-cylinder, 102 x 81.5 mm (4.0157 x 3.2087 in) bore and stroke, 12.9:1 compression. Rated 500 hp. DOHC.
The Porsche 40 991 GT3 is oversquare — the 4.0157" bore exceeds the 3.2087" stroke by 25.2% — a layout that favors airflow and RPM over leverage at the crank. The 3.2087" stroke does not reach 4,000 ft/min of mean piston speed until about 7,500 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 6 cylinders displaces 40.7 ci (666 cc).
12.9:1 on aluminum heads is high for a production gasoline engine; running a ratio like that on pump fuel depends on chamber design and fuel delivery rather than the number alone.
It is one of 4 engines in the 9A1 family, the smallest at 244 ci. Factory fitments include the 911 GT3 (991.2), 911 GT3 RS, 911 GT3 RS (991.2).
| Displacement | 244 ci / 4 L |
|---|---|
| Bore | 4.0157" |
| Stroke | 3.2087" |
| Cylinders | 6 |
| Compression ratio | 12.9:1 |
| Bore/stroke ratio | 1.252 |
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | DOHC |
| Fuel type | gas |
| Advertised HP | 500 hp |
| Bore/stroke character | 1.252 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,209 ft/min — conservative for a street engine |
| Displacement per cylinder | 40.7 ci |
| Static compression | 12.9:1 — high enough that pump premium or better is normally required |
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