Engineering Explained: Torque = how far you move the wall?

Non-repair car talk
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kevm14
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Joined: Wed Oct 23, 2013 10:28 pm

Engineering Explained: Torque = how far you move the wall?

Post by kevm14 »

No. No no no. He explains: https://www.youtube.com/watch?v=7eZyXp1i4GU

He does a pretty good job as usual. The basic error that clouds people's understanding of what torque and horsepower are is that torque has become conflated with "low RPM torque" (which actually means low RPM power) and horsepower has become conflated with "high RPM." Of course when you are talking PEAK horsepower, that always occurs at an RPM higher than peak torque - that's just how physics works. But it is the horsepower at EVERY RPM that actually dictates vehicle performance at any RPM.

He makes a good comparison with the S2000 vs Ford F250 diesel. The S2000 has way more power/weight empty and thus out accelerates the F250, despite the F250's huge torque to weight advantage. But the F250 towing 5,000 lbs would vastly outperform the S2000. Is it because of torque? No. It's because the NEW power/weight ratio would now favor the Ford because 5,000 lbs only adds ~50% to the total combined weight whereas it triples the S2000s total combined weight. It's all about power/weight, assuming the gearing is appropriate for the engine's RPM range and vehicle speeds. Torque to weight was not a predictor of anything. Which is why I like trap speed.

300 hp from a V6, V8 or 8.5L turbo diesel does the exact same amount of work (at the RPM where 300 hp is produced), regardless of the spec sheet that says each engine makes 270 lb-ft, 350 lb-ft or 900 lb-ft of torque. Why do engines with more torque tend to be paired with heavier duty applications? Because it is a byproduct of the decision of optimizing for durability. By the time you beef up a turbo diesel to output 300 hp with a high duty cycle, you end up with super heavy reciprocating components that cannot handle high RPM. So that 300 hp gets produced at like 2,200 rpm. Because the entire curve gets crunched down to the left, the torque naturally goes way up because the amount of torque required to produce 300 hp @ 2,200 rpm is 3x that required to produce 300 hp @ 6,600 rpm. It also helps that the idle on big engines tends to be a lot closer to its peak power output which means you don't need to rev through several thousand RPMs to make enough power to move a trailer. In fact you may only be 1,500 rpm away from peak power.
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