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

By Mike Kojima

When we last left off, we had turned our STI into a G machine without giving up much daily driving comfort.  Now it was time for us to turn our attention to the engine.  The GD STI really responds to very basic tuning.

Just uncorking the car and spending a bit of time on the dyno can produce surprisingly big results for just a little bit of cash.  Let us show you how.

Doing the basics mean that at first it is a good idea to uncork the engine.  We did just that by choosing a Greddy RS exhaust.  We chose the Greddy because of its all stainless steel construction and large 80mm piping.  The Greddy uses 304 stainless which is corrosion resistant and polishes well.  The exhaust is laid out to have minimal bends which can cause a reduction of power.  The RS has a main muffler and a sub muffler.
The Greddy RS has this optional butt plug which goes in the main muffler if you want your car to be really quiet.  The butt plug probably has a loss of power so we left it off.  It takes about 3 minutes to remove or install.
Both the main muffler and the sub muffler are absorption type with a smooth perforated core.  These mufflers have essentially no backpressure and do not hurt performance.  The sound passes through the perforations and is absorbed by stainless steel wool packing backed by ceramic felt cubes.  This packing resists blowing out like typical fiberglass.
The RS uses these lightweight stamped flanges and thin wall tubing.  The RS saves over 20 lbs from the stock exhaust and is significantly lighter that most aftermarket systems without having to go to the expense of something exotic like titanium.

 

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Comments
Wes Dumalski
Wes Dumalskilink
Monday, August 20, 2012 5:50 AM
Good stuff! Funny how Mitch tuned our GR project at AMS and is now tuning the GD in So Cal. Good stuff for sure!

The GD is making more HP peak than the GR but less torque. I will be curious to see what it looks like with a down pipe!

As usual good stuff from Greddy and Cobb.
Fly'n_Z
Fly'n_Zlink
Monday, August 20, 2012 5:59 AM
If you guys ever get your cars to the same part of the country simultaneously it'll be time for a GD/GR showdown.

Sorry Wes but my money is on Mike :)

These three mods: intake, exhaust and tuning really seem to wake up the EJ25 motor.
Hexer
Hexerlink
Monday, August 20, 2012 1:33 PM
I think 10.8 is still a lot rich ... ~11.50 would be an idea range on full boost, otherwise you are changing the oil way to often.

Dough the exhaust got a lot of hp's...
Der Bruce
Der Brucelink
Monday, August 20, 2012 5:12 PM
This just proves the ability of boosted motors from the factory and their capability with just the right people working on them! Great stuff Mike, I'm looking forward to what a few more bolt-ons and tuning will do :-)
spdracerut
spdracerutlink
Monday, August 20, 2012 5:24 PM
WRXs/STIs are known to either get too hot or go lean on cylinder #4 causing busted pistons. So running a little extra rich is not a bad idea.
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