12.7 Detroit need a new turbo (upgrade?), hauling light and best mpg?

Good move. IF, and this is a big IF, a 515hp program with a 171702 with a 115 housing was going coast to coast, it would have less back pressure than the K31 and egt will be less. But, if you are local/Regional, the K31 is perfect for a 12.7 up to 515hp, but no more. The K31 is at its limit with 515hp.
The 115 housing on the 171702 is good to ~550hp.

That’s why the 145 sucks so bad on a mostly stock 12.7.

 

the 171702 is not a bad turbo at all. it all depends in the setup and how much you are willing to spend. i actually have my 171702 up for sale if anyone is interested.

 

A K31 isn't even close to its limits at 515hp.

 

Is that your opinion or fact? The K31 on a 12.7 with a 575 program will gain ~100*f for every ~100rpm past 1700rpm. There is a reason for that little thing.

 

I'm looking at the dyno results for a K31 on a BK 12.7 right now so I'd say fact. 600hp@1800rpm and the EGTs are only 950 and the drive pressure is just starting to pass the boost pressure. At 515hp its nowhere near it's limits.

 

That’s my point. 950* on a dyno pull that lasts 45 seconds is fine. That number will rise on a long pull, and more rpm will certainly raise temps even more.

I am surprised that drive pressure is anywhere near equal to boost, but I defer to your testing. Mine is only watching egt’s vs rpm.

 

If it is possible, and not proprietary, posting the dyno chart would be most helpful...

 

950 on a sustained pull won't hurt anything. That's still in the safe zone.

 

More rpm=more airflow=less egt