Quote:

[.... what is/are some instances where efficieny is improved but HP does not go up?



The one special case where
efficiency goes up, but
peak torque and
max HP goes down
is the "Atkinson Cycle"

http://www.animatedengines.com/atkinson.html

http://en.wikipedia.org/wiki/Atkinson_cycle

I guess to be fair to the original Mr James Atkinson
his original idea had a very different crankshaft mechanism
that gave a bigger expansion ratio
than compression ratio,
so it is when modern manufacturers twiddle with conventional intake valve timing,
either closing it extra early,
or leaving it open extra late so that "reversion" happens on purpose,
that there is a penalty on peak HP.