Cm870 engine brake

[Lonestar10]

My dad and I were playing a game of cat-and-mouse today up and down the hills near Mount Rushmore his truck would run about 15 miles an hour faster up the hills and then I blow his doors off going down hills because his engine break is absolute garbage so he has to go slow down the hills where I can be at 65 miles an hour and hold it there going down. Do the 870s have crappy engine brakes or is something wrong I know he's tried to get it working better he's replaced the harnesses and everything else under the sun.

He has a 550 horse Cummins tune from 3 years ago I'm going to guess 2050 torque

And mine's from here 550/1950