HP is a measure of work/time.
Horsepower = (Torque x RPM)/5252
Let's look at this right here. 5252 is that magic number and always an easy way to spot a photoshop dyno because people forget math always wins. On a dyno sheet, torque and hp always cross each other at 5252, because the formula says so.
Since we rarely see that far on the graph on a diesel like this, we can also say torque is double horspower at 2626 rpm.
A very general rule of thumb (there is a mathematical equation behind it that Cold Roller explained earlier) torque is GENERALLY twice your HP #. Guys, don't lay into me that its not true, because I know it is not. I am simply saying a general rule of thumb our engines produce a 1:2 ratio of HP to Torque. My first and latest dyno run was 366HP/607Ft/lbs
That can sometimes be used as a very loose rule of thumb for a diesel, but far more accurate to just say torque is 2x hp @ 2626rpm.
This is exactly why a 6.4 and 6.0 have different torque numbers. A 600hp 6.0 and a 600 hp 6.4 will have different torque values. The 600hp 6.0 probably doesn't make that power until 3500 rpms, and the 600hp 6.4 makes it lower in the rpm range, right around that 2500 rpm mark. So the 600hp 6.0 is probably only making 450-500hp at 2626, which means 900 ft lbs. When the 6.4 makes 600 at 2626, it's making 1200 ft/lbs. The increase in engine displacement helps, but the quick spool high pressure charger helps the most. A single turbo 6.4 will probably show dyno graphs very close to that of a 6.0... given similar power outputs.
When we think of OTR trucks, we always think of high torque, low speed... Gearing is the biggest helper, with so many close gears, the operator can keep the truck running at an almost sustained RPM and keep the power right where the peak torque is all the time.