I removed my FCA today and performed the modification to the piston as noted on Relentless Diesels website. I did not do the orifice mods to the back of the pump.

First off, if you want more power and thats all you are looking for, it certainly accomplishes that, at least down low in the powerband.

When I first started it up it rattled like crazy, rail pressure @ 15k idling in neutral, sounds similar to a 12V.

Drivability appeared ok at first, I cannot take off from a stop without leaving a puff of smoke. My shifts are harsher, (I believe it is not de-fueling as much between gears). Once in third lockup or fourth lockup, it will not shift down unless I smash the pedal to the floor. If I roll into it, even hard, no downshift and tons of black smoke, I cant believe this would work out towing my trailer, as my EGT's also went over 1500.

Smarty on sw#5 mild tq, TST 5/0, EZ 2. I pulled my rail pressure down to 12k from about 1900rpm to 2600rpm until it started to recover, and never got over 20k until I backed out of it, and then it spiked to 26k. I turned my TST off and very little difference.

This would probably be ok on a dyno for some cheap Hp gains, but as a daily driver I dont recommend it. I have ordered a new FCA from Cummins, (#4932457)$90.00.

Just wanted to share my experience! Jess