How long, generally, is a break-in period?

It’s not going to hurt anything other than fuel economy.

 

My Detroit is the same way. I had a egr Isx it was a slug unless it was at 16 or 1700. The Detroit is better but still needs to be over 1450

 

Figure 70mph at 1400 would be 3.36 gears.

Cummins like 1500 and above rpms for longer life.

You should be getting at least 30 psi of boost under full throttle.

Run it and see if it gets better.

 

It's under warranty. Have you spoke to the dealer? Tell them you think the fuel pressure is low causing it to lag in power at the low end and limiting you boost at the upper end. What kind of turbo boost are you getting?
Check your, Air-to-Air isn't leaking too. Do you hear a whooshing noise when you stomp on the accelerator?

 

Thats how u got to run these cummins. I run at 1400-1500 all day long . Pull hills at 1700... just keep er up in that range. Especially with a flatbed hauling 80k all the time.ur gonna want ur rpms up higher and she will power thru the hills.

 

I don’t know about Cummins, but Detroit usually is fully broke in around 70k.