What is is that makes say, a Suzuki DRZ400 single push out 40 hp while the Bullet does <20. What are the mechanical elements that lead to that big of a difference?
This is one of those questions that sounds simple enough on the surface, but has a huge complex answer.
Basically, the simple answer is that Royal Enfield doesn't even make a half-hearted effort to make horsepower. For whatever reasons, they simply don't want to make any hp with their bikes. The Japanese bikes, on the other hand, use power numbers to sell their bikes, and so they put great effort into making hp.
To start with, there is a basic equation for horsepower, which is:
hp = torque x rpm/5252
This is a true function for any engine. You take the torque reading at some rpm, and that gives you the hp output at that certain rpm.
So, we can see that rpm plays a big role, and the higher you can rev, the more hp you can make, for a given engine size.
The Japanese bikes all have short strokes, and can rev high and make high hp, because their short strokes give lower piston speeds that allow the mechanical parts to not reach their breaking limits until the rpms get very high.
On the other hand, the RE bikes have very long strokes, which give high piston speeds at lower rpms, and cannot rev as high before their mechanical parts reach breaking limits.
So, why do they have these different kinds of engines, and for what?
Well, the Japanese engines make plenty of hp, but they have to rev up to high rpms to do it. Consequently, they have poor hp output at lower rpms, because engine designs can only stretch out a powerband just so far. If you make a lot of power at high rpms, you aren't going to make much power at lower rpms. It's that simple.
So, the Japanese bikes are going to make a lot of power while they are buzzing around at the top of their rpm range, like buzzing little bees.
But, that is not really a cruiser style behavior for relaxed riding. The RE gives a long stroke with low rpm limits, and this allows it to begin the meaty part of its power band much lower in the rpms, so that you can start off from a light with some decent power and ride at moderately lower rpms with good torque and hp without revving the bike up high. It's a more relaxed riding powerband that is typical of cruiser bikes or standards.
However, since it doesn't rev up high, the numerical equation doesn't turn out with a very high number for hp, since the rpms aren't adding a lot of multiplication factor with a low revving engine. We make just as much torque, but we make it at a lower rpm, so our hp number looks lower, and it is. But we can still pull well early in the rev range with our torque production, which is something that the Japanese bikes really don't do very well.
So, in the end, it's all about what you want out of the bike, and how you like it to behave for your purposes.
The Bullet can produce 40hp, and do it within the rev limits of our long stroke engine, but we have to modify it to do that, with mods similar to the Fireball. RE simply isn't interested in making power. It can be done, but they clearly don't want to do it.