I'm in the process of hotting up the engine on my Fatboy, looking around at builds etc. Then I came to realize that I don't really know about HP and torque and what they really mean. From what I understand torque is how hard the bike will accelerate, and HP is more about top speed, and maybe not that important on a cruiser. Am I correct?

 

http://www.nrhsperformance.com/tech_power.shtml

 

In simple terms Horsepower makes you go fast.

torque is what gets you there.

Torque is always better than H.P imho.

 

Torque is your power in lower RPMs but after 5252RPMs HP is where it's at. TQ and HP are always equall at 5252 RPMs by this time the TQ has already peaked and is going down and HP is still rising. Most of us on HD's spend our time in the 2000-4000 ranges. Obviously when passing RPM's go up and that is where your HP comes in handy.

I hope this is a good enough explanation.

 

For all but the highest performance drag bikes and cars, torque is desirable. And if you dial in a cam to fit your riding style, it can even increase your mileage.

 

So torque is the driving force below 5252rpm, and HP is the driving force above 5252rpm?

 

There you go. You learn quickly grasshopper.

 

It takes force to accelerate a motorcycle. A little force will accelerate a motorcycle slowly; a lot will do it faster. Force on a wheel is best measured by torque. With the right gearing, the little motor that opens and closes the plastic tray on your computer's CD-ROM drive can apply as much torque as you want ... it just can't do it very fast. That's where power comes in: how fast the motor can do the work of accelerating the motorcycle.
Torque all by itself is not terribly useful as a number for an engine's ability. There's this gearbox and final drive that changes the torque to whatever range is needed. In first gear, any given engine speed can produce a lot more toque at the rear wheel than in top gear ... but more slowly. We have gearboxes because piston engines have this annoyingly nonlinear torque curve: they're better at making torque at some RPMs than others, and they don't make any at zero RPM. So as long as the gearbox has the right ratios, I don't really care what the peak torque is. I want the engine to have a more-or-less linear torque curve.
Accelerating and maintaining top speed both require force and power. At speed, there's no more acceleration to be done, but you have to push all that air out of the way. So "Torque gets you moving, Horsepower = MPH" isn't true either.

 

Just a quick question. I saw some graphs from Europe, and here the graphs cross at ~7000rpm (probably because we use NM instead of lb-ft). So how is it over here? Is torque the driving force up to 7000rpm here in Europe, before HP takes over from 7000rpm and up? It's confusing... [&:]

 

With your build, you will want a combonation of parts that will give you very broad torque form around 2K rpm to 5,500 rpm and a touch of HP to get you to 6K rpm!

Basicly a few cubic inches, a point or so of engine compresion and a minimal amount of head work to get you over the top!, and dont for get some cams and some sort of fuel management, SE race tuner or thundermax!