[mikeytown2]

The best naturally aspirated piston engine is 166 hp per liter. The best naturally aspirated Pistonless rotary engine is 188 hp per liter [1]. The Duke is at 71 hp per liter; it has a long ways to go but it's already better than a naturally aspirated Diesel engine (44 hp per liter).

[1] http://en.wikipedia.org/wiki/List_of_automotive_superlatives...

[bradleyland]

I agree that Duke has done relatively well, considering their limited resources. That is to say, Comparing superlatives isn't entirely fair to Duke.

BMW makes for good comparison here because they make a lot of 3.0L inline-6 engines. Their N52B30 engine [1] was (mass) produced in a variety of specific outputs, the nadir of which was the 200kW (272 HP) version used in two of their sporty SUVs. This engine makes/made 90 HP/L. The most common variant (used in the mid-tier 3-series) was the 190 kW version, making 86 HP/L. Both of these engines have a better specific output than the Duke engine, but BMW has considerably greater engineering resources, as well as the benefit of nearly 100 years of development.

1: http://en.wikipedia.org/wiki/BMW_N52#N52B30

[aunty_helen]

I always find it interesting that when hp/l stats are quoted they often neglect to include motorcycle engines... which have been in the 180-210 hp/l realm for about 5 years now.

And a high hp/l isn't necessarily the best piston engine as it's often at the cost of massive valve train losses resulting in poor fuel efficiency.

[bradleyland]

It's difficult to compare the specific output of engines of dramatically different displacement. As a general rule, displacement and specific output are inversely correlated. That's not to say that motorcycle engines aren't an absolute marvel of engineering and technology, but it's not possible to compare them directly to engines with twice the displacement because you can't simply scale up the motorcycle engine and get the same results at the larger displacement.