How about water injection through the intake to turn to steam and assist with expansion (hence power) during the power stroke?
It also effectively provides detonation supression so more ignition advance can be used.
IIRC approximately as much water can be injected as is fuel without causing combustion problems.
http://autospeed.com/cms/A_110368/article.html
http://autospeed.com/cms/A_110369/article.html