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Old 11-21-2009, 10:27 PM   #1
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Default 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
BSFC: .55
Manifold temperature: 130 *F
VE at redline: .95
VE at max torque: .98
Redline: 7000
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!
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Old 11-21-2009, 10:36 PM   #2
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Try plotting it on this calculator and see if your computations end up with similar results. If they are the same or close then you will know the numbers are correct. The 2560 is one of the listed choices.
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Old 11-21-2009, 10:45 PM   #3
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Using the examples on Garrett's sight, under turbo basics 103, my calculations were right on with their examples for the 2.0 liter. I used a spreadsheet and just changed the values for what I thought was right for the miata's 1.6

I just used that website and plotted my basic values in and plotted it onto the gt2560 map and was WAY above the top speed line ... so now I'm really confused =(

edit: did it again and their plotted values for the pressure ratio are a little bit lower than mine, 2.22 vs my 2.38.
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