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      02-24-2011, 05:07 PM   #55

Drives: e90
Join Date: Aug 2010
Location: Spokane, WA

iTrader: (4)

Originally Posted by tscdennab View Post
In fact, it's the other way around. Subarus have predefined maps which means they make the same power even when the conditions vary (to a certain degree, of course). N54 is more dynamic and calculates many of the values "on the fly", whereas other ECUs have predefined values. That's why some days you feel the car a bit slow, some days you feel it is a beast.
Subarus have tables set up for ideal conditions. IE 68* F, sea-level. They then have a number of compensations tables to account for variances in atmospheric conditions. They do NOT make the same power with varying conditions and they are not designed to. They are designed to make safe maximum power for the conditions they are operating in.

For example, on the stock ROM for my 2006 LGT, the map calls for 13.54 psi manifold pressure at 95.4% and above throttle plate opening at 3,600 RPM. The ECU then applies a compensation to this based on altitude, which applies a multiplier of .05208 to the current observed atmospheric pressure and then adds an offset of .25 to that product. This is then used as a multiplier to the original requested boost target to compensate for barometric pressure. Take two examples:

14.5 psi atmospheric pressure:
14.5psi*.05208 = .75116 +.25 = 1.00516
Requested boost of 13.54 psi * 1.00516 = 13.609 psi requested boost corrected for atmosphere.

12.5 psi atmospheric pressure:
12.5psi*.05208 = .651 + .25 = .901
Requested boost of 13.54 psi * .901 = 12.199 psi requested boost corrected for atmospheric pressure.

My understanding of the N54 logic, which is still very limited, is that it would try to maintain that same 13.54 psi of manifold pressure, despite a drop in ambient pressure of 2 psi. Remember that the 2 psi drop in atmospheric pressure at the same manifold pressure will increase pressure ratio at the turbo. This makes me question how viable the OEM logic is for tuning the platform well beyond the factory power levels.