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Old 01-27-2013, 07:01 AM   #5
sendler OP
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Joined: Apr 2012
Location: Syracuse, NY USA
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Originally Posted by btcn View Post
Related, but not the same. I have a Dodge Ram 3500 Cummins diesel. A BMW sportscar will smoke it in a race. It has more HP. But lets see it pull 25,000 pounds of round hay bales at 70 MPH up a steep grade! My diesel has a redline of just 3,375 RPM or so. It makes all it's power from only 1,600 RPM and up. It only has 230 or so HP, but it has gobs or torque, something like 460 ft/lbs@1600.

Thanks for the reply. I wondered if there were any gear heads here that would want to talk about this. Yours is the common view based on intuition but it isn't quite right. This would actually make an amusing episode of Top Gear. Car vs truck: TOWING!
I found an interesting dyno test where the Ford 6.7 Powerstroke engine made 700 foot pounds and 350 hp.
The truck will be better to tow with since it has greater torque and power down low. So when you take off from a light, it is already in the power band at the torque converter stall rpm. It is easier to access it's POWER band right off the line. The car is tuned for high power closer to redline. At the expense of not having as much torque or power at lower rpm. So even in first gear it's rpms are to low to have the torque or POWER to yank the trailer to get started.
But, if you were to race two trucks and two trailers at the drag strip, the driver who makes his shifts on either side of the engines power peak would beat the other driver who shifted on either side of the torque peak. Power is doing the work of moving the trailer. And the rear wheel torque of the winning truck will avreage out to be HIGHER than the earlier shifting truck who used his engine at the range of greater torque. The slower truck sent more torque to the trans but the faster truck stayed in a lower gear more often and used a greater POWER band to get greater rear wheel torque, hence power, at any given speed.
Here is the crazy part.
Once it finally got moving so it could use it's gears at the power peak, a 400 hp car will pull the trailer up the hill at a faster speed than a 350 hp truck. Even though the car doesn't make as much torque as the truck anywhere. Power is doing the work. A lesser engine torque can be multiplied with gears but the power at the wheels is always the same as the engine is putting out.. The trans is trading higher wheel torque for lower wheel rpm. The power at the wheel is the same as the engine at any gear. And higher power will do more work than lower power. Most towing vehicles have giant engines which are tuned for low rpm so they have tons of torque, and so, power, down low to yank with, and plenty of peak power to keep moving up the hill from the large displacement. But it is always the amount of power that is doing the work.
This is interesting. The first google search I made came up with test data on a dyno which states that the rear wheel power was the same in any gear. If anything, the power showed to be slightly higher in 5th than in 3rd. Must be the dyno is not really that linear at different speed ranges because that is the complete opposite from what most people would think where they would guess that the lower gear would give the most rear wheel power. And in reality the power would be the same discounting different losses in the trans at different gears.
"We strapped down the 2011 Ram 3500 dualie to the rollers first. The guys at ATS made three dyno pulls with the new engine, one in Third, one in Forth, and one in Fifth gear. The results were surprising. Regardless of which transmission gear we tested the trucks in, the power rankings were all the same"
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