Well, either you're looking at the wrong model, full of it, or I'm just lucky because it absolutely works better than the Banks WG or non WG turbos I had previously on that engine, the ATS 088 on my dump truck or the IDIT truck I drove a while back - It spools faster & makes more boost, overall truck pulls better and I have no issues with EGTs. Can't compare it with a S366, but IMHO it's a great match with the 6.9 and bull moose pump which lets me pull 15-16k GCW and keep 60+ mph on pretty much any hill. Even at 20-22k GCW I'm able to hold 45-50 on most any hill. Not sure what the engineering says, but it practice says it works well.
Well Im glad youre so pleased with your setup! Out of curiosity, did the other turbo idis have new fuel system with a bull moose and water to air intercoolers? Its important when comparing turbos that everything else be identical so you really see the different characteristics of the turbochargers, remember fuel is what makes power and towing ability, air flow is just keeping those cylinder temps down and supplying oxygen to burn that extra fuel.
As for my comment, I apologize for just making a statement without any data, heres why I think the hx35 is a downgrade from the stock turbo:
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Without getting to the complications of any calculations, you can see the air flow graphed starts at 10 lbs/min and stops at 52 lbs min on the horizontal axis, but if you look on the vertical axis, you can see it is 3.0 pressure ratio at 52 lbs/in and 1.4 pressure ratio at 10 lbs min. Now lets look at the factory turbo compressor map.
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We can see the airflow is quite similar with the stock turbo flowing a bit more than the hx35, but what is important here is the pressure ratios. Looking at the vertical axis, we can see that it takes 1.2 PR to get 10 lbs/min, and 2.5 PR to get 56 lbs min. This means the turbo needs to be driven much harder in the hx35 to get the same airflow as the stock turbo, which is a contribution of the hx35 being designed for a smaller, lower revving engine. As a note, 3.0 pressure ratio on a 6.9 is around 31 psi, which no 6.9 is capable of handling with head bolts, possibly not even with studs.
You mentioned the s366 so heres the map:
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You can see this turbo is capable of much more airflow than either of these turbos, and is capable of 85+ lbs/min at the same pressure ratio your hx35 is making 52 lbs/min.
Now you can take this info and do as you like with it, obviously youre happy with your truck and ultimately it is what matters, I just didnt want others to follow the same conversion, put in a lot of work and have a weaker truck than what they had before. I actually have a dyno graph of a built 7.3 with an hx35, the truck had a cam, studs, similar pump as yours, and some upgraded injectors, work done by justin anderson.
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All that work was done and run after run the truck finally peaked 200 hp. The following week they changed nothing other than the turbo to an s364 and picked up 45 hp and 80 ft lbs.