What size turbine housing you have on that hx35, its stamped on the inlet to the turbine? I have twin hx35's with 10cm housing and is about right in size to have non wastgated housings, with about 6psi back presure high in the rpm with no load. I dont have my gauge panel sorted and dont have a long enough high heat to have my other gauge in the cab, to get you anymore data. For a single, you would not want a 10cm or 12cm turbine but a 14cm might fly with a wastegate. I would think optimal would be a 16cm or 18cm turbine housing, so that a wastegate might not be needed to keep drive pressure down. The turbine housing Im using, dose not seem to have boost till about 1400-1500 and I fine with that since gives increasing boost all the way up to and probably well past 2800rpm. I see probability that I could step down to 8cm hx30 turbines to get boost off idle but dont see need to test that, in turn is why I think 16-18cm would work non wastgated and 14cm might out work with a wastgate. After you have picked a turbine housing size and tested it to see where boost starts and end, you can move that range a few 100s of a rpm by going bigger or smaller with the compressor wheel and matching housing. Im not sure if a single would react the same as my twins, but going from 54mm billet wheels to 50mm billet wheels moved the boost range down ~500rpm.
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