I wonder whether an engine with say 9.5:1 CR and no turbo can be as efficient as the same engine with a 8.5:1 CR (achieved purely by different pistons) and a turbo.
I ask this because this has been claimed. I don't see it can be true on a turbonormalised engine but it could be true if the output pressure of the turbo is above normal atmospheric.
The TB20 does about 10% more MPG than the TB21, in reality, but there are also different operating practices.