Hannibal wrote:
4x4ord wrote:
Hannibal wrote:
4x4ord wrote:
Hannibal wrote:
shepstone wrote:
I don't quite understand why max hp and max tq are both achieved at different rpm ? and with that which of the 2 rpm's should I be using to get the most out of an engine when climbing or accelerating with a heavy load?
Max horsepower will put the most torque to the rear wheels.
It kind of depends how you look at this question. Say you start climbing a hill with a big trailer. As the grade increases you floor it and your transmission drops two gears and the engine is running at near the rpm where it makes maximum power. The hill is long and getting steeper, you keep your foot to the floor but the engine continues to loose rpm; as it slows it continues to increase the torque to the rear wheels until it either meets the demand of the hill or the rpm drops to where the e.engine is making maximum torque. If the hill still demands more torque another downsjit is required and the engine increases its rpm again so it can start the cycle over again. Anyway you will pull the hill the fastest in the gear that keeps your engine revved close to the rpm where maximum HP is achieved but in each gear the rpm where the engine makes the most torque is always the rpm where the engine will pull the hardest in that gear.
True. However, if I'm rolling along at 65mph in my '03 Cummins powered Ram with it's flat torque curve in O/D at 2k rpm towing our 5th wheel and along comes a 3% hill that's too steep to maintain speed in O/D, if I let it downshift to direct to run 2800rpm, I can increase torque to the rear axle from 1301ft/lbs to 1886ft/lbs and maintain speed. Same 460ft/lbs of flywheel torque but I went from 175hp to 245hp.
With my 5.4L F250, downshifts are common. With it's less flat torque curve, it might make less flywheel torque at 3k rpm or 5k rpm than at it's 3800rpm peak but through gear reduction/torque multiplication, I'll be putting much more torque to the rear axle at 4500rpm than at 3800rpm running the same road speed. More HP means more torque to the rear wheels.
I basically agree with what you're saying. Like you described for the Ram, here is a hypothetical scenario using some real numbers for a Ford. Obviously there are details ommitted; like driveline inefficiencies and wind redistance to keep it simple.
Say a Ford 1 ton hits a steep hill pulling a trailer at 64 mph. The weight of the truck/trailer and slope of the hill determine that a torque of 3431 lb ft is necessary on the rear axle to pull the load up the hill. You push your foot to the floor and the transmission downshifts two gears. At this point the engine is revving 2900 rpm, your in 4th gear and still traveling 64 mph. At 2900 rpm the Powerstroke makes its maximum power of 400 horsepower. So it is making 400 x 5252/2900 = 724 lb ft of torque at the crankshaft. Multiplying this through the 4th gear ratio of 1.15:1 and the final drive ratio of 3.73 we can calculate the torque on the rear axle to be 3107 lb ft. The truck starts to slow down. Now if you chose to manually select 4th gear so the transmission could not downshift, the engine would continue to slow down all the way to 1700 rpm where the Powetstroke makes its maximum torque value of 800 lb ft. At that point the required rear axle torque to get the trailer up the hill of 3431 is just met. So you would be able to pull the load up the hill in 4th gear at 1700 engine rpm (which is the engines maximum torque rpm) at a speed of 37 mph.
So now back up to where the truck begins slowing down from 64 mph. As soon as the engine rpm drops to 2194 rpm you manually shift the transmission to 3rd gear. The engine revs to 2900 rpm and you are going 48 mph. At this engine rpm the crankshaft torque is 724 lb ft. Running that torque through the third gear ratio of 1.52 and the rear axle ratio of 3.73 the engine and transmission are able to deliver 4100 lb ft of torque. The truck would start to accelerate up the hill in 3rd gear until the crankshaft torque drops to 605 lb ft of torque. We can only guess the rpm the engine could rev to.... maybe 3200 rpm and 53 mph.
So running in 4th gear at max torque you could climb the hill at 37 mph and in 3rd gear revving the engine high you could pull the hill at about 53 mph.
Which goes back to higher HP puts more torque to the drive wheels and pulls the grade faster. I saw this many times with my Cummins powered Rams. I like this video of a 6.7L PSD towing 13k? lbs up a long grade. Far more realistic than so many claims made here. I saw 2900rpm a lot with my last two Cummins powered Rams. Peak horsepower numbers do matter when you find yourself maxed out on a grade.
https://www.youtube.com/watch?v=-_1ZefREs0Y
Kind of..... Saying "higher HP puts more torque to the drive wheels and pulls the grade faster." is not really accurate because HP needs to be geared to do what your asking of it. If a 500 HP tractor is capable of putting 600,000 lb ft of torque to the rear axle in low gear but has a top speed of 20 mph it obviously isn't going to be much of a match against a 396 HP Duramax towing a 10,000 lb RV up a 6% grade.
Or if you are comparing two fairly similar pickups to each other, such as was the case when Pickuptrucks.com faced off the Ford against the Duramax and Ram. The lower horsepower and lower torque Duramax pulled its trailer up the Eisenhower pass and sat and waited for nearly 2 minutes for the F350 to finally catch up. there was all sorts of speculation as to why...things like Ford's turbo couldn't spin fast enough at the high altitude of the test (and that could have played a role) but GM conducted the test; The goal was to exploit Ford's weaknesses so if high altitude was going to exploit the turbo let's pick the Eisenhower pass. Then the GM engineers looked at the gear ratios on that new Ford 6r140 and noticed how it is programed to be kind of lazy. The 6r140 will not downshift from 4th to 3rd until the engine rpm drops below 1900 rpm. They knew the slope of the hill they would choose, all they had to do was select a weight for the trucks to hook on to that would pull that poor little Ford down to just above 1900 rpm and it would struggle up the hill in 4th gear where that mighty new 400 HP Powerstroke was only producing 280 HP.
Anyway if you want a simple statement to make regarding horsepower you might be able to get away with saying something like; "Given the right gears Horsepower trumps Torque every time" But going by the actual definition of the words Horsepower and Torque are measuring entirely different things.
