I do not wish to put anyone down or start an argument (there is plenty of that going on right now about other much more important issues), but actually, the statement that H-Ds are low on power strictly or even largely, because of the crankshaft arrangement, is not entirely correct.
Here are some facts:
- one horsepower = 746 Watts (same thing by which light bulbs are sized) and the horsepower is defined as the product of torque and RPM (with some fudge factors to work out the units).
- The actual formula (as stated by Ryan, I believe) is: HP = (torque (in foot lbs) x RPM) / 5252
- Torque is determined by the mean effective pressure (MEP) in the cylinder which is a result of the combustion process and the crankpin offset (which is half of the stroke of the piston).
It really is as "simple" as that. So, to get more horsepower you can:
- increase the crankpin offset (or the stroke, if you like) - assuming the MEP doesn't decrease with increased stroke;
- increase the MEP of the cylinder(s) - which means improving the combustion process by using a better head-piston crown design, better intake and exhaust tract design with better valve geometry, better fuelling (carb or EFI), incorporating turbo- or mechanical supercharging to increase the charge density, or by simply increasing the compression ratio;
- increase the RPM at which the power is measured - assuming the torque doesn't decrease with increased RPM, of course.
Now, each of the above solutions to the horsepower issue has its own set of problems:
- increasing the stroke will increase the peak piston speed which generally leads to increased engine wear (especially pistons, rings and crankshaft components) and can make it tough to get the fuel-air mixture in and the exhaust out quickly. It can also increase the level of engine vibration - to one degree or another depending on the arrangement of the cylinders (number of cylinders, bank angle and crank-pin orientation etc.).
- some of the the methods of increasing MEP will also increase peak piston crown, cylinder head and valve temperatures which can cause durability problems and others (such as the improvement of intake and exhaust tract geometry will help, but increasing the valve count and cam lift and duration can add complexity). Some of these tactics are not easy to duplicate in large volume production and for some engine configurations, they aren't always feasible because of space limitations. There can also be noise and emissions problems which make achieving government approvals a challenge.
- increasing the RPM is a good plan - but it can also entail serious durability problems for long-stroke engines (again, rings, pistons, cylinders and valve components due to higher speeds) and depending on the engine geometry (i.e. V8, V-twin, flat four etc.) it can lead to very difficult vibration and rocking couple issues which impact occupant comfort.
So, you see, as in almost every engineering problem, there is no single and simple solution, in effect, there is
no free lunch. Most successful solutions involve a combination of approaches in various measures to arrive at the one that is optimal for the application.
For example, Japanese bikes generally use:
- over-square designs (larger diameter pistons which travel through shorter strokes). This lowers peak piston speeds and allows for higher operating RPM without undue vibration or durability problems.
- more cylinders with more valves per cylinder and overhead camshafts (which enables higher RPMs because the moving parts are lighter and enable a better distribution of intake charge and exhaust pulses). This helps to improve MEP by promoting a more efficient combustion process while maintaining lower vibration, reduced operating temperatures and lower noise levels.
- liquid cooling (which allows for tighter piston-cylinder fits and thus improves MEP, fuel economy and running consistency and durability).
- ...and of course, nowadays, every OEM uses fuel injection, which does not necessarily, in itself, increase horsepower - but it does provide more consistent running and permits closer control of the combustion process over a wider range of engine RPM and load conditions.
So why do Harley engines make less power than other large-bore brands? Well, as Ryan said, it has to do with their traditional engine design which has a tight 45 deg. Vee formation that makes having better intake and exhaust tract designs difficult. They also use an under-square design (smaller bore / longer-stroke engine due to a large crank-pin offset) which does not permit high RPM operation due to high reciprocating forces and higher peak piston speeds, in addition to being a challenge with long slender pushrods.
And finally, some of their bikes are still air-cooled which means that they need to be run a little richer (either by a carb or EFI) than optimum to ensure that they don't burn valves and piston crowns during high temp operation. That is also the reason why the air-cooled models cannot make the latest emission standards and it is also why BMW doesn't sell airhead twins any longer.
....also, Harley riders like the sound and vibration that these big slow-turning engines produce....and that helps to sell bikes to their demographic - which is afterall, the entire idea of being in business in the first place.
Sorry, but the crankshaft configuration is only peripherally involved in all of this. Basically, it affects the peak permissible RPM - but that is its only effect on the horsepower produced by an engine. Afterall, the Suzuki SV650:
- is a vee-twin;
- uses liquid cooling, overhead camshafts and four valves per cylinder (which fit easily because of the 90 degree cylinder angle);
- has a 10,700 RPM redline (ahhhh - remember that HP = torque x RPM x some fudge factors to work out the units);
- produces a peak power of 75 HP;
- ....on a displacement that is only 1/3 that of a 118 cu.in. (that is 1920cc sports fans) Harley Davidson.
Someone raised the issue of two different model bikes that produced differing peak horsepower values on the same displacement. The notion was advanced that the only difference was the crankshaft geometry. I'll bet that is not the only difference. I would bet that the valve opening-closing geometry was different due to different camshafts and that the RPM was lower in the lower HP engine. I'll bet it was tuned for higher torque at a lower engine RPM so that it felt and sounded more like a....Harley Davidson. More torque at a lower RPM will generally (depending on the math...) result in lower peak horsepower.
EDIT: I fixed a typo
(merci beaucoup Pierre) and thought I'd add a few words about why later models of that Honda produce lower power. I suspect that either Honda wanted to tune the bikes for more low/mid-range torque or they could not meet the emissions regs. at the higher power levels so they simply de-tuned the engine. Whatever the reason, either of those goals could be met with re-programming the EFI system and leaving everything else alone on the engine.
Do the math....and you will understand.