Use cars as an example. Take, for example, a typical small-block, pushrod engine. You'll usually find them with lots of torque between 2000 and 5000 rpm, depending on the build. By design, they are RPM limited. So with horsepower being time-related,and pushrod engines RPM limited, theyreduce ability to build really high horsepower. Granted, many pushrod engines attain very large RPM numbers, but this limits them to race use, like NASCAR, INDY, and so on.

An overhead cam engine, however, likes to rev and sustains high-rpm well, because of the extremely reduced valvetrain load and instability.

I think that if you made an overhead cam harley engine, and built it well, then you'd get higher into the rpm range and make higher hp numbers. However, you'dthen have the problem of having to lug a 700# bike up to speed.

Now, none of this has anything to do with the exhaust noise level, but it gives another view on the differences between the sport bikes and the cruisers and how they're designed to operate.

As far as sound goes, I believe that the stock mufflers will support all the HP you could possibly make with a big-twin. Pretty darn close anyway. The only thing is that you wouldn't be at the full power potential of more open pipes, but c'mon, what do mufflers get on an otherwise stock bike anyway? Just a couple hp. I'd say they're not HP limited, but they do hold back the full potential a little.

I bet if you bump the compression to 11:1, put in ahuge cam, aftermarket intake, and lots of 110, you'd make some pretty impressive hp numbers through stock exhaust. But really, how practical would that be to stick on a cruiser?

it's like a nascar engine in your work car.

 

Regarding noise, this reminds me of when I once bought a BMW car. It came with a video about how they design and make their cars. BMW has a whole system where they adjust the exterior noise and road feel that a driver experiences depending on the make and model of the car. Sportier models are purposefully less quiet than non-sporty models. The sound engineer they interviewed said it's pretty easy to seal out exterior noise for the car's occupants, however the more aggressive drivers like more feedback, both in sound and feel through the steering wheel, so they carefully add those elements back. So, the point is that sound and feel are important to drivers and riders and manufacturers are keenly aware of this.

 

There are plenty of ways to make high horsepower without loud noise. It doesn't matter the size of the engine, it only matters how much air the engine pumps in and out and how much horsepower the engine makes.

That said, I don't care how quiet your 150hp sports bike makes at idle and on the highway - when you get on the throttle 100%, I can hear you coming from 1/2-mile away. So there might be something in the exhaust system that opens up.