Can someone explain displacement vs HP to me?

Google horsepower vs torque. The bigger displacement engine will make more torque then the smaller displacement engine and the more torque you have the easier it is to do the work.

 

What Roberts450 said plus if it’s new it will gain a lot of power around 80 to 100,000 miles out once its broken in..

 

Displacement is the size or capacity of the engine. Calculated by measuring the bore and stroke of each cylinder and multiplying by the the number of cylinders. This is the same for all piston powered engines, although it can be measured and recorded in different units.

HP is the measurement of power the engine is producing. Which as you figured out can vary by engine by chaging the amount of air and fuel delivered.

There is a trend in the industry to downsize engine displacement. This lowers emissions and rids a unit of unneeded horsepower for the application. Basically wider torque bands and at lower RPM to decrease fuel consumption.
Smart fleets will lower operating cost by lowering fuel consumption and puting more money in everyone's pocket.

Its possible your 13 has different settings than the 15. Even though the 13 is HP rated the same it could likely have less torque than the 15 as @Roberts450 stated.

 

I like the part " more money in EVERYONE's pockets "

 

Yea, I guess your right. I should have said puts money in the owners pocket.

Plain wishful thinking it would be passed down in most cases.

 

Everything else being equal, a larger engine of the same design will have more power than a smaller engine. However, it isn't just about peak HP numbers. So even though your DD13 has the same HP rating as the DD15, torque is what is really important.

In simple terms torque is the power to move things and horsepower is how quickly that work can be done. How, where and for how long that torque is made is what you're feeling.

The larger displacement engine is typically going to make more torque over a longer RPM range and that torque curve is going to be flatter--less drop off--than the smaller displacement engine. The larger displacement engine can do more work over a greater RPM range.

So, although both engines have the same peak HP rating, the bigger engine can do the same work more easily.

 

Displacement doesn't equal horsepower.

Classic example is uprating an electronic engine. Plug in a laptop, upload a new flash file and your C15 just went from 475 to 550.

 

HP is equal top speed in rpm as both the 13 L and the 15 L have the ability to pull the tractor the same speed. But, there is another factor, Torque is equal to power in ft lbs at a range of RPM.

A 15 L can pull harder because it has more Torque at the same RPM, look at the Engine Spec to see the difference.A DD 13 is rated for economy and uses less fuel, the end result, it will cost less to operate and truck divers must understand, fuel cost is a controllable expense.

On flat roads trucks with a 13L will match the same speed.
On Hills you will be downshifting more often.

 

As the old saying goes, "There's no replacement for displacement".