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target boost reference?


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What does the closed loop boost controll refering to? MAP or MGP? I saw in the setup guide that the older firmware had a option to choise Target mode.

Why I'm asking: I made some turbo compressor ratio calculation and realised that there will be big pressure ratio differences if the closed loop referce to MAP. That means you will have more boost on higher altitude (less baro pressure). 

turbo compressor ratio is calculated as follow: 


compressor outlet pressure / compressor inlet pressure

as a example

at sealevel: baro=100kpa absolut, boost = 200kpa boost pressure

Pr1 = (100kpa+200kpa) / 100kpa   =  3.0

lets assume we drive on a very high mountain:

case 1 MGP: baro = 70kpa, boost pressure if boost control refers to  MGP = 200kpa

Pr2 = (70kpa+200kpa) / 70kpa  =  3.86

Case 2 MAP: baro=70kpa, boost pressure if boost control refers to MAP = 230kpa

Pr3 = (70kpa+230kpa) / 70kpa  =  4.29


As you can see from the calculation, we have more boost with a MAP referenced system and the pressure ratio differ alot more than a MGP system. The big increase in pressure ratio, means that you can fall well out of your compressor efficiency map. So in my eyes MGP referred system would make more sense. Otherwise there should be a baro compensation map. 

If I'm missing something, please let me know. 


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Generally it is considered desirable to have a constant manifold pressure.  A constant manifold gauge pressure would suggest less cylinder filling pressure at higher altitudes.  Whereas a constant absolute pressure implies there is a constant cylinder filling pressure as altitude increases. 

If boost target was based on MGP:  If you have 200 kPa gauge pressure boost, then at low altitude (100kPa baro) you would have 300kPa cylinder filling pressure.  If you had the same boost at 70 kPa baro then you would only have 270kPa cylinder filling pressure.  This is overly simplistic and ignores all other dynamic effect, but in general would cause significantly reduced power at high altitude.

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