I share info with a G-Force Engine Developments in the USA, and if anyone knows a thing or two about V4 Hondas it's Mike Norman. Now, he's getting high 70's rwhp out of 400cc, and over 80rwhp on big engines (428cc~450cc). The amount of work (and money) to achieve these figures is shocking. Mike has left no stone unturned, and has broken many engines in the search for big numbers.

An 800cc GP bike is making in excess of 200hp, so it should be more than possible to make a 100hp 400cc. Especially given the fact that the parts weigh less so you can (theoretically) spin the bits faster. Honda redline the NC30 at 14,500rpm, which just so happens to be 20m/s, the standard for mean piston speed. Well, with modern materials we can safely go higher than this. Probably up to 16,500~17,000rpm without astronomical expense.

Let's just say that we're making 28ft/lb of torque at 14,000rpm (peak power), this would give us (28*14000)/5252 = 74.6hp. Now if we could keep that 28ft/lb but at say 15,000rpm then we'd get (28*15000)/5252 = 80hp.

Sounds easy when you're just saying it, but if you can get the ports to flow the extra air/fuel mix, and an efficient combustion chamber and ignition, then there's no reason why it can't be done.

A grand prix 250cc stroker can make over 100hp and still be rider friendly, so with good engineering (and non-standard parts) a 400cc boiler would be right behind it.