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.


  1. Great artilce, but it would be helpful fo provide id’sfor the 3 different curves for the two dyno runs. The legend on the left side of the graph was clipped of in the window. Based on text, the left side was @ 25 psi, and 17 on the right ?

    Good Garret article on A/R’s, with differences shown on track performance.

    1. The traces on the dyno chart are 25 psi, 17 psi, and a full bolt-on VA STI. The point was that the power falls as the IAT climbs drastically. Conversely, on our time attack cars, we have been going to smaller turbos and A/R and turning faster lap times even with sometimes hundreds less HP. For instance, the ENEOS 86 went from around 1000 whp to 750 and we went many seconds a lap faster due to better driveability and not being so solid on the traction control. Most of the time in Time Attack mid range helps quite a bit for reducing lap times.

      1. Thanks for the reply Mike. Your faster TimeAttack laps with the smaller upgraded turbo & A/R are similar to how a smaller A/R turbo was better for circuits with more corners, and the larger A/R was better at fast tracks, as discussed in the Garret link I provided.

        But my question about the Dyno runs was related to the 3 pulls for each boost pressure. I figure the segmented green curve is stock, the dark maroon is 1st upgrade, and the light maroon the bigger Precision Turbo upgrade.

        1. “The traces on the dyno chart are 25 psi, 17 psi, and a full bolt-on VA STI.”

          I get it. I should have looked at the graphs more carefully.

  2. Can’t wait to see the results! Thanks for being so transparent with all the teething problems. I can’t imagine the dyno bills being too pleasant on your wallet, but even if money is no object, Church must have a looong wait list as well.

    1. The results we currently have are decent, we are just looking to optimize things and make them efficient as possible.

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