Cat vs Powerstroke torque curves
I drew out what I expect the Powerstroke torque curve would look like relative to a Cat C11. I think it is very interesting how these curves reveal the different pulling characteristics of ...
Forum Discussion
ShinerBock wrote:
I don't think he is spot on in the article. To say that it is only horsepower that helps you up hills is false. It is a combination of both. You need torque to overcome the force pulling you back and that force working against you is constantly changing. If you don't have enough torque to do the work, then all the horsepower in the world is not going to help you.
Take a Ram 2500 6.4L Hemi and 6.7L Cummins. They both have the exact same transmission gearing with the rear end gear advantage going to the Hemi(3.73 or 4.10) versus the 3.42 of the Cummins. The Hemi has 40 hp more while the Cummins has 371 lb-ft more torque. Send both up a grade 7 hill with 14k lbs strapped to them and I guarantee the 6.7L Cummins will get to the top quicker even though the Hemi has more horsepower.
You do need torque to overcome the force pulling you back but that torque needs to be applied at the wheel. That is where gearing comes in. It directly multiples engine torque at the wheel. With its higher crankshaft torque, the diesel engine needs much less multiplication in the drivetrain.
The higher HP engine will always get to the top faster if both engines are kept at their peak HP RPM. In the real world, that difficult to do. With the limited gear choices available for the given road speed, they will rarely be outputting peak HP. The diesel gets up the hill faster not because of its higher peak torque, but because it produces more HP at the RPM(s) the engine is operated at when climbing the hill.
Diesel engines can achieve high wheel torque OR good mileage at similar RPM by changing how much fuel is supplied. Little to no downshifting is needed to go from flat ground cruising to pulling a hill.
Gas engines need to spin up (with correspondingly low gears) to achieve high wheel torque but need just the opposite to get good mileage. What we end up with is a terrible compromise, pushed toward mileage. This results in downshifts whenever higher wheel torque is needed. The corresponding change in engine RPM when shifting almost certainly means the engine is not producing its maximum hp.
With more and more gears in the transmission, this wheel torque gap between diesel and gas closes.
