25-08-2017, 10:46 AM
What's your basis for 160HP = 16lb/min? I assume this is a petrol engine calculator which is spitting that number out - your AFR is going to be significantly leaner than they would suggest, something like 17.5:1 AFR will be about your limit for stopping excessive EGTs. I'd not say that an XUD9TE is hitting 88%VE @ 4000rpm - closer to probably 85%, maybe lower frankly! With those numbers and an optimistic 0.42BSFC - the numbers are 25.2psi boost and 19.6lb/min airflow for 160HP @ 4000rpm. That makes the compressor much better suited for the numbers you're suggesting. 
You're still running pretty close to the surge line in honesty, but I suspect the turbine won't drive it hard enough to actually produce that pressure, you might just cross into surge on transients but I suspect your BSFC is actually higher, so you'll need more airflow again, which isn't so bad as it'll keep you away from the surge line.
Your comments about the compressor not being suitable past ~4400rpm are quite right, however, the VE% is dropping significantly at that RPM, so you just end up going down the map for the same boost pressure, which keeps you away from the danger area of overspeeds. Also you're not attempting to burn fuel up there - this is normally where the ECU would start tapering boost pressure down to avoid compressor overspeeding.
HTH.
You're still running pretty close to the surge line in honesty, but I suspect the turbine won't drive it hard enough to actually produce that pressure, you might just cross into surge on transients but I suspect your BSFC is actually higher, so you'll need more airflow again, which isn't so bad as it'll keep you away from the surge line.
Your comments about the compressor not being suitable past ~4400rpm are quite right, however, the VE% is dropping significantly at that RPM, so you just end up going down the map for the same boost pressure, which keeps you away from the danger area of overspeeds. Also you're not attempting to burn fuel up there - this is normally where the ECU would start tapering boost pressure down to avoid compressor overspeeding.
HTH.
