Project GD Subaru STI Part Three- Making More Power with Greddy and Cobb Tuning


Time to tune.  After doing some research we decided that we wanted to retain the OEM ECU to take advantage of its simplicity and OEM reliability, especially important with our car’s drive by wire throttle.  We considered an open source solution but decided we like the Cobb AccessPort due to its ability to store multiple maps and its real time emulation.  This means that you can edit maps while tuning on the fly without having to reflash each iteration of a map.  The ECU is only reflashed at the end once the program is developed.  This greatly speeds tuning.
We ran our car on the dyno with no tuning to see what the Greddy RS exhaust would do by itself and were amazed to see that it produced average gains of 29 whp from 244 whp to 273 whp!  The gains were solid from 3700 rpm on up.  This exhaust really makes a lot of power!  The engine gained an amazing 23 lb/ft of torque in a solid fat wide band from 259 lb/ft to 282 lb/ft.  Everyone who witnessed this was pretty impressed that just a cat back would do this.
Next up it was time for some tuning.  Mitch Mckee initialized our AccessPort and fired up his laptop to wring more power safely from our STI.  On the EJ motor tuning is very critical as the engine has inherently weak ring lands and cannot take detonation.  Tunes need to be done carefully and conservatively.  Mitch has plenty of experience.
After a dyno pull, Mitch checks out the AccessPort’s data logs and decides what changes he wants to make.
Mitch edits the boost table.  He brings boost in sooner and spikes it 3 psi higher than stock to 16 psi tapering it to 12.5 psi by redline.  about 1.5 psi more than stock.  Mitch keeps it conservative to keep our ring lands on the pistons.
The Subaru is tuned sort of dangerously from the factory.  The factory keeps the car in closed loop, holding a lean air fuel ratio of 14.7 to 1 even when the engine is making a lot of boost at the lower rpm.  Since our low rpm boost was raised Mitch richened things up in this zone from 14.7:1 to 12.8:1 making the engine more tolerant of the extra boost.  Our engine was pig rich above 4500 rpm running at 9.5:1.  This is misfire territory and so rich that it is not contributing to detonation protection.  Mixtures this rich can often promote detonation!  Mitch leaned things out to a more conservative 10.8:1  which should also give us more power.

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