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.
