This calls for some interesting math......
By definition, the measure of the rate of doing work is power. So if we are talking about moving a mass over some distance, work, in a given time, that is power.
If we are going to move an RV up a hill at 60 mph, we are going to talk power, or in our case horse power. Now I know some of you are going to say it takes torque to climb a hill, but bear with me.....we will get there.
Horse power is a measure of the rate of doing work. We all know that diesels make huge torque at low rpm and gas engines make good power at higher rpm. But to state it that way is kind of like looking at the same horse from different ends and getting different descriptions. Whether at either low or high rpm, it is power that matters.
By definition, Horse Power (HP) is equal to torque at some rpm x the rpm and then divided by the constant of 5252.
HP = torque x rpm/5252
If we apply this formula to any torque, engine speed, and mph that will give us a true picture of the comparison of two engines whether gas or diesel.
Since this is my little explanation, I'm going to make some assumptions I know to be true from my experience.
We will take a truck with 3.73 gears, 275/70 - 18 tires, a 68 RFE transmission, a 6.7 Cummins 370/800, and a new 6.4l Hemi with a 8 speed.
For the diesel.... I know that my old dually would pull a 6% grade with an 18,000# Mobile suites 5th wheel at 60 mph in 4th gear on the 68RFE. That is a fact, but the dually had smaller tires.
Dually tires turned 634 revolutions/mile, which gives us an engine speed of 635 x 3.73 x 1 = 2364 rpm. The 1 in the equation is because we are in 4th gear which is 1:1.
Given that this engine was rated at 370 HP @ 2800, we can calculate the torque at 694 to make that HP at 2800. Since the torque curve on this engine is pretty linear, we can assume the torque at 60 mph or 2364 rpm to be about 745 lb-ft of torque. By plugging this number into our original equation, HP = T x RPM/5252, we come up with about 335 HP to pull the load up the hill at 60 mph. The 335 HP is the measure of rate of doing that work.
So lets see how the gas engine holds up in this example.
We know the 6.4 is rated at 429 lb-ft at 4000 rpm. That is 326 HP at 4000 rpm......amazingly close to the 335 needed to get the load up the hill at 60 mph, but what gear will we have to be in?
Going back to the tire size, the tires on my truck are taller, which is no help at all. But let's see what we have. These tires give 608 revs per mile or an engine speed of 2364....not nearly enough to give us the power we need to pull the hill. We need to rev the engine up quite a bit. If we divide the engine speed we need by 2364 in 6th gear, we should get the ideal gear to make the power needed to pull the hill.
4100/2364= 1.8 So lets see if we have a gear that gets us close. We know that because of tire size and gear ratio, we are going to have an engine speed of 2267, which is not enough. Looking at the 8 speed gear ratios I see 3rd is 2.1 and 4th is 1.67. Not ideal, but to pull this hill with this load, we pull it in 3rd gear and ease off the throttle to give us around 4100 rpm. That makes the power needed to get the load up the hill.
Now, not knowing ANYTHING about Ford powertrains, let's put that new 7.3 in a Ram and see how it works.
We know it takes 335 HP to pull the load up the hill. We also know the Ford makes 475 lb-ft at 4000 and 430 HP at 5000 rpm From our equation, we find the Ford makes 361 hp @4000 rpm, and we only need 335. From the torque curve, it appears it makes about 465 lb-ft at 3800 rpm. That gives us 336 HP at that point which is perfect. So let's find a gear that gets us to 3800 rpm.
Divide 3800/2364 and we get 1.67. Ding ding ding......looks like we have a winner here. 4th gear at 1.67 puts our engine speed at 3800 rpm @60 mph. 3800 rpm gives us the HP needed to pull the hill at 60 mph.
So what does all this mean?
The Cummins produces enough power at 2364 rpm to pull the hill at 60 mph....no sweat.
The 6.4 makes enough power at 4100 rpm to pull the hill at 60 mph in 3rd gear on the 8 speed.
If we put the new Ford 7,3 in my truck, it makes enough power at 3800 rpm to pull the hill at 60 mph in 4th gear.
The exercise highlights the difference in the way gas and dlesel....and even different gas engines make the power needed to do work.
I would also like to hope it sheds some light on the age old question of whether it takes power or torque to pull a hill. It takes power, but to get more power at very low rpm or any given rpm takes torque.
I hope that answers some questions, but I fear it will also fan the flames!
