11-21-2009, 10:27 PM
Join Date: Dec 2004
Total Cats: 1
1.6 miata: compressor map questions
So I've seen some over-simplified explanations of how to read compressor maps ... but using Honeywell/Garrett's equations to figure out the points on the maps aren't working out on why the gt2560's do so well on the 1.6 miata. Maybe you guys can help me out.
Here are my used values:
HP goal: 250
a/f ratio: 11.5:1
Manifold temperature: 130 *F
VE at redline: .95
VE at max torque: .98
RPM at max torque: 5000
Displacement: 97.6 in^3
Pressure loss from compressor to intake manifold: 2psi
Here are my calculated values:
Airflow capacity for 250hp: 26.35 lb/min
Airflow capacity for max torque: 19.42 lb/min
Pressure Ratio: 2.38
Now, onto the compressor map for the 2560, my calculated airflow values vs the pressure ratio, I'm right up near the top speed line of the map, but pretty equally spaced from the surge and choke lines, on the ~70% island.
Why is this turbo ideal? Is it due to the .64 turbine a/r which allows for faster spool over a larger a/r like on the gt2860 (.86 a/r), even though the points on the compressor maps are almost identical 72% vs the gt25's 70% and compressor speeds 140k vs 143k rpm?
Any insight would be appreciated!