Quote:
Originally Posted by olddog
Torque is the rotational equivalent to Force, in linear motion.
Back in the stone age, Uglug was the biggest and strongest man around. A rich man was paying people to carry stones from the river to the top of a mountain, to build a stone house. Uglug pushed on a huge bolder all day, never budging it, while the smaller men carried smaller rocks up the mountain. At the end of the day, the smaller men were paid according to the rocks they moved. Uglug was furious, when the rich man wouldn't pay him. Uglug argued he was twice as big as anyone else and he had worked hard. The rich man explained that he had not accomplished anything, as the bolder was still in the same spot.
If I push on a huge bolder with all of my might, I might be able to put 200 or maybe even 300 pounds of Force on the bolder. If the bold does not move, I did no work, on the bolder. However if I push with much less force on a smaller rock and move it a long distance, I have done work. Work is Force times Distance.
W = F * D
When you factor in the time it took to move the rock that distance, you can calculate the Power require to do the Work. You can generate more Power by moving a light rock, a long distance, in a short time, verses moving a heavy rock, a short distance, over a long time.
Hp is the rotational equivalent to Work per unit of time, in linear motion.
So Torque, like force, accomplishes no work, if there is no rotation. When you pull on a wrench and the bolt does not turn, you have accomplished nothing. On the other hand, if you apply a Torque and the object rotates, you have done work, and when you factor in the time it took to rotate an angular distance, you can calculate the Power expended. Or in the case of an engine, the power generated.
So folks listen carefully here. You do not have to calculate the torque to the wheels in each gear at every rpm and graph it, to figure out which gear will give more acceleration at a given speed. If you know the Hp at every rpm (if you know the torque at every rpm then you know the Hp it is simple math) and you know what rpm you will run, in each gear, at a given speed, it is simple to calculate the Hp the engine will make, at a given speed, in each gear. Whichever gear allows the engine to make the most Hp, at that speed, is the gear that will accelerate the vehicle at the highest rate.
Maximizing Torque does not yield maximum acceleration! Maximizing Hp yields maximum acceleration! I cannot shout that loud enough. Hp is the measurement of work per unit of time. It is the the measurement that everyone is trying to figure out, but they do not understand it and ignore it!
Peak Hp always happens well past peak torque, at considerably less torque. Get your rpm at peak Torque in 4th gear, floor it and feel the acceleration. Now get your rpm at peak Torque in 4th gear again, only this time shift down to 3rd gear and floor it. Much more acceleration, right? Yep, and you accomplish it with the engine making less Torque, but more Hp. There's your proof. You've done it a thousand times.
Torque is just rotational force. If nothing moves, all the force in the world will do no work. Work moves your car from point "A" to point "B". Power is the distance per unit of time. If you want to get from point "A" to point "B" in less time, increase the Power. Hp is rotational Power. Folks they understood this in the 19th century. That is why they figured out how to calculate Hp in the first place. They wanted to compare the work a horse could do, to the work an engine could do. Here we are in the 21 century with every learning aid and google at you fingertips, and most people still do not get it.
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You make some good points, however, just a bit misguided in the real world.
You see, our friend Uglug is much smarter than what we give him credit for in our story meant to substantiate a theory in the village of Internet. You see, he comes from a village called Reality. When he pushes on that big rock (mostly to impress the ladies) he quickly realizes that it isn’t moving and continuing along that path will most certainly result in an embarrassing stall. No, our friend Uglug stops pushing the big rock and picks up a smaller one, however, the smaller one is just too small to make best use of his genetic advantage (torque).
Uglug does some testing and finds the perfect size rock to allow him to bring the greatest tonnage to the top of the hill (peak horsepower), but upon looking at the quarry, he sees that there are very few rocks in his perfect size, he then has to evaluate which rock size range will give him best advantage over the little fast guys. Some are a bit smaller and some a bit bigger than his best size. This spread is what we hot rodders see as the spread between transmission gears. The smaller we keep that spread (close ratio tranny) then the better we will be able to perform work, but this quarry only has a wider spread available (street tranny) so Uglug must find the optimum range, not just the single optimum size.
Back to our reality. Unless running a CVT, we spend almost 100% of our time either above or below peak hp. The torque curve both before and after peak hp. combined with transmission spread tell us the optimal shift point for best acceleration. In real testing, peak hp will be found in every gear, you may hit it just after shifting or maybe just before shifting, or maybe in the middle. It all depends on the torque curve in that area. Does torque fall off quickly after peak? Maybe builds quickly to peak and then holds. Do you hit peak hp 100rpm before the rev limiter? Only the dyno can tell us everything we need to know.
That said, yes, you can get close calculating shift points with a single gear pull and math, but simply knowing peak will never be sufficient, you must know the curve. That said, consider that the single gear/math method completely eliminates factors such as MOI of rotating parts and differing frictional losses in each gear. You are already on the dyno, why would you bother with a math approximation when all you have to do is change gears and see the facts?
EDIT: Here is the factory dyno chart for my motor (MSTR). Note how torque falls off rapidly immediately after peak hp. My speculation is that this curve will require me to shift immediately after hitting peak hp, but with such a flat curve I won't know until I test.
