docj
Well-known member
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- Oct 16, 2010
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taoshum said:
There's a real simple explanation of the relationship between horsepower and torque. HP governs the speed you can maintain going up a mountain. HP is a "power" measurement and power is defined as energy per unit time. The small engine will have to turn faster than the big diesel but 400HP is the same in both.
Torque is a bit of a different story. It's the rotational analog of force in Newton's Second Law. In simple terms it's how quickly you can change the rotation rate of a motor which, in turn, governs how quickly the vehicle powered by the engine can accelerate. The electric motor example is a good one--big diesels (for example, C-12, C-15, ISM, ISX) develop enormous amounts of torque (>1500 lb-ft) at very low rpm's (~1100-1300rpm). That means that full torque is developed as the vehicle starts moving from a stop. The 5.9L or 6.7 ISB has a max torque output of ~800-1,000 lb ft. but can achieve more through gearing but, as with horsepower, it has to spin faster to do that. Starting from a stop it accelerates more slowly than will the big diesel.
With an automatic transmission, torque multiplication occurs in the "torque converter" but the maximum amount of torque multiplication produced by a converter is highly dependent on the size and geometry of the turbine and stator blades, and is generated only when the converter is at or near the stall phase of operation. Typical stall torque multiplication ratios range from 1.8:1 to 2.5:1 for most automotive applications. So the ISB can achieve torque equal to that the base torque output of a larger engine, but that large engine will also experience significant torque multiplication from its torque converter.
For the old-timers here, this is the same situation we saw when imported cars with small, high-revving engines first began to compete against the big American V-8's we grew up with. Those little engines were rated at the same HP as our Ford and Chevy 8's but they weren't able to produce the same wheel-spinning starts as were the cars we grew up with. The difference was that the small engines achieved their max performance at higher rpm's (and usually developed much less torque). Therefore, they couldn't spin their wheels unless you were prepared to pop their clutches at some pretty high rpm's.
Since you can't pop a clutch with an automatic transmission, with a smaller, lower torque engine you will accelerate more slowly until you have enough engine speed to get into your max torque zone. By way of comparison my ~34,000 lb Beaver with a C-12 producing 1,550 lb-ft of torque at ~1200 rpm is virtually in its max torque zone as it moves off the line. Therefore, it accelerates more like a car than a MH.
Before people start reacting, I'm not dissing anyone's MH or engine. I'm simply pointing out that there are real differences between the performance of large-block, high torque diesels and smaller ones with lower torque outputs even when both are rated at the same HP. At(Caution--I'm a retired physics teacher with a Ph.D in physics, so let's not debate the science.)
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