The new Jakes can put out the same retarding horsepower as pulling horsepower, been that way for quite sometime. The Cummins Signature 600 was one of the first huge horsepower / torque motors with a jake that would do that. 600 pulling hp and 600 retarding hp. If I recall correctly, it was a 6 stage jake as well when on cruise control. That motor has to be 10 years old by now.
Of course our pickups don't have a jake, it's simply an exhaust brake which can never produce that kind of retarding power. I've often wondered myself why the gauge reads 230hp, I was all excited to see it up in the 200 range as well. No dice.
I wonder what the braking maximum rpm of the Cummins is? I know the Duramax redlines at 3200, but the computer will downshift and run it up over 4,000 on really steep, heavily loaded descents. Maybe if the Cummins was up in that rpm range it would be in the 200hp area?
Is there a Cummins spec somewhere that somebody could find out of curiosity showing max rpm and max braking hp?