Measured 928S4 Compression Ratio
 

Before I set forth on turning up my boost screws on the twin turbo S4, I thought it would be a good idea to know the exact CR of the engine. Rarely are the factory specs for CR right on the money, usually the specs are high as compared to the actual CR.

I ccd the head and piston and ran the math on the rest and I came up with a figure of 9.52:1. So, the S4 is not a full 10:1 on CR as stated in the documentation. No surprise, the 80-82 US cars measure 8.1:1 and I've done one Euro 4.7 which measured 9.8:1 (the closest to spec I have had in the bunch).

The CR certainly limits the amount of manifold pressure which can be run. It's a balancing act between CR, fuel octane and ignition advance. The tune must be good to run the double digit boost numbers on this CR. Ignition timing is the real key to pulling this all off without scattering the engine.