The Honda CBX 6 cylinder motorcycle engine of the late 70's and early 80's, has to be included in this debate. The engine is a true masterpiece, being designed by Irimijari, who also designed Honda's 250 / 350, 6 cylinder racers of the 60's. The CBx was the first production road engine to achieve 100hp and have double overhead cams and 4 valves per cylinders. A pretty high tech engine, with 24 valves and quite a complex machine for the day, when the average 1000cc production bike had 2 valve cylinders and 4 cylinders. The CBX commanded a premium of around $1000 more than their 4 cylinder competition, but those that weren't put off by the price and the complexity, were rewarded with a fast, smooth, reliable engine. Admittedly the running gear had a bit to be desired, but the engine was a technological marvel and made a statement! I am biased, having had CBX's for about 32 years. They have some weaknesses and take a lot of time and money to maintain, but they really are special. Not for everyone, but with someone in the know, fettling them, there is nothing better in my opinion! Over the last 20 years I have also been building and racing CBX's in historic racing and when set up to the period rules, they are not perfect, but pretty good machines that could surprise a lot of riders on newer machines. There is nothing like the CBX engine and I doubt if there will ever be anything close. Silky smooth with heaps of torque and the unique howl from the racing Pipemasters 6 into 6 exhausts!