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I looked in "Internal Combustion Engine Fundamentals," by John Heywood (very well-respected professor of Engineering at MIT). I have the book on my desk at work, so it was handy.
Page 182 has a table of fuel conversion efficiency as a function of compression ratio for a fuel-air cycle. This isn't quite the same thing as your issue (your compression ratio isn't changed if you have excessive leaking past the rings, for example), but it would serve as an estimate. For a stoichiometric mixture (i.e. not lean or rich), the fuel conversion efficiency drops approximately 9% for a 30% drop in compression ratio in the range of typical car engines (CR=10 to CR=7).
Zack
1980 245DL M46 282k
1988 745T+ M46 214k
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