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According to Fere, 87.5mm and 86.7mm cylinders were Biral. He says that 87.5mm was the maximum Biral cylinder diameter the factory could use with enough reliability.
For the 2380cc engine, 87.5mm bore Biral cylinders were used with the 66mm stroke crankshaft. For 2494cc, 86.7mm with a 70.4mm stroke crankshaft was used.
Porsche wanted to use the short stroke 66mm crankshaft for better revving and less vibration so they used the Mahle Nickasil technology already tried on the 917. This was the 89mm bore cylinder with the 66mm stroke crankshaft.
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1971 911S, 2.7RS spec MFI engine, suspension mods, lightened
Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
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