What I attempted to say in my car analogy is that calculating horsepower and calculating torque are two ways to measure Power in an automobile engine....albeit at different points in the RPM range. Horsepower = (TorquexRPM)/5,252.
My inference was that in a STREET car (not a RACE car), torque is the more important number to keep in mind when building an engine for several reasons, one of which is that you do most of your driving (your desired sweet spot, if you will) below 5,252 RPM. A race motor is a completely different animal that is designed to run at much higher RPMs....with various trade-offs.
When running an engine on a dyno, a graph is frequently created that shows both HP and TQ at various RPMs. The lines always intersect at 5,252 RPM. Above that point, a motor will make more HP than TQ. Below it, more TQ than HP. Obviously, the more RPMs you can generate....the more HP you will make. BUT....since most of your driving in a street car occurs below 5,252 RPM, people have found that designing a great/fun street motor calls for a broad, flat, useable torque curve. Why? B/c you feel Torque in the seat of your pants as your car accelerates. Torque pulls you through a curve and accelerates you out if it, etc.
An engine's horsepower rating is really "just" an HP rating at a given point in the RPM range. Typically, it's the peak number on the HP curve.
Why the car analogy to begin with? I was trying to make the point of needing to look at "Power at Speed" versus the mere creation of power. (ie - useable, real world power.) Someone appropriately noted that I also needed to observe "Power Up Grade". Duely noted. (There's no reason why this forum can't serve as an educational source for some of us, is there?")
So, I must stand corrected. (I prefer to use the term "informed".) The NKP 765 has a sweet spot of 45-60 MPH that she can't obtain going up the Curve. The 2-10-4s used by Pennsy and C&O had a different sweet spot (significantly more power down low) to better accomodate this type of heavy lifting.
Let it be known.......
Originally Posted by OGR Webmaster:
Originally Posted by Berkshire President:
...a lot of people make the mistake of measuring a car's performance potential by only looking at the horsepower of a car. In reality, it's the torque that you feel in the seat of your pants when your car accelerates. When driving below 5,252 RPM, torque is actually moving you, not horsepower.
I'm sorry, but what you have stated there is not correct. Horsepower is a combination of torque and RPM. The "5252" number comes from the formula for calculating horsepower. That formula is:
TORQUE X RPM divided by 5252
500 foot-pounds of torque at 5,000 RPM = 476 HP - a typical "crate engine" spec. The number 5252 is simply a mathematical constant used in the formula and has nothing to do with the RPM of the engine involved.
Horsepower is a measure of how FAST a given amount of work can be done. Torque is simply the twisting force on a shaft. Thousands of foot-pounds of torque mean nothing if you can't spin the shaft fast enough to get work done. This holds true whether you are talking about cars, diesel trucks or steam locomotives.
The 765 is capable of about 4,500 hp at the coupler, however that power level is not achieved until around 45 mph. At 30 mph she is well down on her power curve. I don't know how many HP the 765 can develop at 30 mph, but it is nowhere near 4,500.
Both of the locomotives you mentioned are more powerful than the 765. If I recall, the C&O T1 2-10-4 was capable of nearly 6,000 HP and the Allegheny developed 7,498 drawbar HP in testing on the C&O. Clearly they are much more powerful than the 765.
The 765 is no slouch, but she is a mid-sized Lima Super Power freight locomotive that was designed to pull 4,000 ton freight trains on level track at 70 mph, not a 1,500 ton passenger train at 30 mph on a 1.7% grade. By that I mean she is not a "drag" engine designed to make maximum HP at slow speeds. Her HP peak is reached at around 45 mph and extends to about 60 mph where the HP starts to fall off slightly.
