Project GD STI, Improving the Powerband with Precision Turbo and Injector Dynamics

Previously we had dyno tested and tuned Project STI at Church Automotive testing. Although our car made a pretty impressive 501 whp and 487 lb/ft of torque our power peaked at 6000 rpm and fell very rapidly after 6500 rpm.  The torque peaked at 4800 rpm and dropped like a rock afterward.  Since our engine is built to rev as high as 9000 rpm and our headwork and camshafts are optimized for 7500-8000 rpm, the turbo is the limiting factor.  Our drop-in replacement Precision Turbo is performing excellently, making more power than its rating but we want a wider powerband and a little more peak power to take advantage of our headwork and cams.  From the data, we found that our intake air temperatures started to skyrocket right as our power started to drop.  This indicated that we were pushing our compressor off the map to where it was very inefficient.  We did not want to switch to a rotated mount turbo to gain our power, we wanted some semblance of OEM under the hood and we did not want a thermal management project to make the car track driveable with a rotated mount system so we needed another stock position drop in turbo that fit our needs. Fortunately Precision has just come out with another larger turbo with the exact same footprint as our old turbo that is rated to 630 hp, or about 100 more than our current turbo

You can see the annoying rapid decline in torque and power in our dyno chart.  We don’t want a bunch more power, just to extend the powerband out without the rapid fall-off in torque to gain area under the curve.  You can see that our engine makes less power on the top end at 25 lbs of boost than at 17 psi.  Another indication of the turbo compressor dropping and running off the map.

Our new Precision turbo comes with an improved CNC machined forged 2618 alloy billet compressor wheel with a 58mm inducer vs the old Precision turbo’s 55mm inducer.  2618 is a tough ductile alloy and the billet wheel is stronger than a typical cast wheel which allows for a slimmer hub that increases the wheel’s flow capacity.  The compressor inlet is a full 3″ so you will need to get a bigger diameter inlet tube.  Killer B and Perrin make good 3″ intake tubes.  Our car has a 3″ Perrin inlet tube so we will continue to use this.

The exhaust side of the turbo is unchanged from our prior setup but is much larger than stock. It uses an internal wastegate and is compatible with the OEM up and downpipe for a true drop-in fit. The exhaust housing is stainless steel, this is good because stainless has about half the thermal conductivity of cast iron and this allows for more heat energy to efficiently drive the turbine.  One thing we like is that the Precision turbo uses housings that are designed for the wheels vs carving out stock Subaru parts to work with bigger wheels like many drop-in upgrades on the market.  This makes for a turbo with better flow and better efficiency.