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Old 07-30-2008, 01:13 PM   #5 (permalink)
EcoModding Lurker
Join Date: Jul 2008
Location: Idaho
Posts: 45

RockMagnet - '04 Subaru Impreza wagon WRX
90 day: 26.16 mpg (US)
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The problem with turbo gas motors is they have low compression(unlike a TDI). The subarus have about 8.5:1 compression. That means when the engine is pulling vacuum, all that expensive high octane you put in it isn't being utilized.

Max torque is somewhere above 3000rpm.

Turbos help overcome pumping losses by scavenging exhaust heat. A little boost makes up for the low compression and intake restrictions. Too much boost and the ecu goes into open loop mode and dumps extra fuel into the engine(10.5:1 in some cases). The subarus go into open loop mode at 60% throttle, ~4200rpm, or certain load ranges. It varies a bit depending on model. There's also no knock detection below ~2000 rpm so don't lug the engine too much.

The current internet theory is low rpm, highest gear, and slight boost, slight throttle. No one has tested to see what psi works best.

A retune might help you out as well. Subarus run really rich from the factory and have some odd blips in the timing map. It can be a pretty cheap upgrade, though I don't know if the romraider guys have cracked the 08 ecu yet. You could even set the close loop air/fuel target to burn leaner than stock.
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