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2.5L conversion


Vehicle Information: Model: 911; Year: 1970; Body Type: Targa; Engine (type, size, modifications): 911, 2.7L, race motor; Total Mileage: race car--speedo broken;

Because of a change in my group classification in SVRA I would like to turn my 2.7 into a 2.5. It appears I can do this by changing P&C's to 86.5mm and keeping the 70.4 mm crank or by changing to a 66mm crank and rods. The latter approach is more appealling and straightforward. In Bruce Anderson's book he briefly mentions this approach and notes that changing to a 66 mm crank reduces the cr approximately 1.5 points. I am not sure I understand why this is since the 66 mm crank has larger main journals and cylinder heights are the same for 84mm and 90mm cylinders. It would seem that with a 66 mm crank, the piston would just sit higher in the cylinder and thus would not have any effect on the compression. Any insight you can provide would be helpful.

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