James Parker has an interesting comparison of horsepower per liter among several different motorcycles in the March issue of Motorcyclist. It's not a difficult calculation but I seldom think about it, though I may think about it more after reading these numbers.
According to Parker, the 800cc MotoGP bikes are making about 240 horsepower or 300 hp per liter, pretty high output for so little displacement, it reminds me of what the Formula One race cars are doing, small engines with ridiculous power. The B-King tested in the current issue was about 120 per liter, the V-Max about 101. Buells and Ducatis come in around 85 per liter. Sliding down to the low end of the scale, the Royal Enfield 500cc puts out 22 horsepower or 44 per liter. He points out, however, that the Harley Davidson Cross Bones they tested with its 1584cc 70 horsepower engine, also comes in at 44 per liter. The Victory KingPin bests it by a tiny bit at 46. Of course, the Harley is a big engine so it still has a lot more power than the Royal Enfield but from a pure efficiency standpoint, it's in the same spot. Sort of shows that a large, air cooled, pushrod, 2 valve engine has limitations.
Now, before someone starts making remarks about Harleys, just remember, efficiency is interesting and shows us how well the engines use the displacement they have but the actual horsepower and torque are what make the performance difference. Unless you are racing in a displacement defined class, efficiency, in a street bike, isn't that important. Need more power, just go bigger, adjust size until you get what you need.
Comparisons like this are interesting for the tech oriented motorhead, but as soon as you start down this road the bragging rights crowd starts playing the "mine is bigger" game, even though it doesn't mean much on the street. It does get me thinking though, ... we need a chart! I may have to get some numbers together.