ed1 wrote: Whether you look at the physics book or the CAT web site, theory says that HP will determine acceleration and hill climbing capability. I was really confused as to why so many drivers swear by their high torque engines. After some investigation, I think I found the answer. I looked at the engine power curves for my Ford V-10 and a similar HP diesel engine. It was apparent that the difference is that the diesel cruises with the engine speed much closer to the HP peak. (Don't know why it still gets good mileage with much lower gearing than a gas engine). Anyway, the result is that the gas engine has much less HP available without shifting down. So when you press the gas to the floor to climb a hill, the diesel will have more power available, and will climb better. When the engine shifts down, the diesel will need to go down only one gear to hit the HP peak, while the gasser will likely need to go down two, and will be operating at a much higher and noisier RPM than the diesel. I believe the theory is correct that for for equal HP and weight, both will perform the same. But the perception of the driver will certainly be that the higher torque provides better hill climbing performance, because the engine does not appear to be working as hard.
Ed
Very well said.
Others have also mention the shape of the torque curve and that contributes to how the engine "feels". What is important in hill climbing is how much HP is produced at the rpm the engine is operating at and that is determined by the shape of the torwue curve. Each engine has a torque peak at some rpm and the torgue declines at faster engine speeds. Horsepower is torque times rpm so if the rpm increases 10% and the torque only drops 5% the horsepower will increase by 5%, this continues until the decrease in torque occurs faster than the increase in rpm, this transition point is the peak HP speed. The same thing happens in reverse when the engine speed declines from peak horsepower speed to the peak torque speed, the torque rises; and the rate that the torque rises is called "torque rise". The faster the torque rises with decreasing engine speed the flatter the horsepower curve will be and this results in an engine that feels like it has a lot of power and needs less gear shifting to maintain speed on hills.
The C12 Cat used in some MHs from 1998 thru 2003 are a good example. Three versions of this engine all had the same peak torque at the same rpm, 1550 ft-lbs at 1200rpm. The first version produced 425hp at 1800rpm (and the torque at 1800rpm was 1232 ft-lbs; 318 less than the peak torque. The next version of this engine produced 455hp at 1800rpm, and the torque at 1800rpm was 1320 ft-lbs; 230 less than peak torque. The last version of the engine produced 505hp at 1800rpm and the torque at 1800rpm was 1465ft-lbs; only 86 ft-lbs less than peak torque.
The first engine had a 318ft-lb torque rise when the engine speed dropped from 1800 rpm to 1200 rpm, and the engine only lost 69hp. This engine felt like it had a lot of power and didn't need a lot of gear shifting to climb hills. The last engine lost 150 hp between 1800rpm and 1200rpm and felt fast if you could keep it buzzed at 1800 plus rpm, but also felt like the tranny never had enough gears. The reason for these torque and HP values from simular engines is the power of computing, all three engines are computer controlled. The last engine produced 1550 ft-lbs of torque from 1200 rpm to 1500rpm, a very untypical torque curve.
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