Project STi Part 4 Engine Tuning & Suspension - Turbo Magazine

When we last left Project Sti we had done the first round of engine mods and reached the limits of the stock fuel system. Project STi was running on the verge of dangerous leanness with the air/fuel ratio fluctuating and the power levels inconsistent from pull to pull. If one was concerned about engine life, this condition was barely acceptable for street use and defiantly suicidal on a road course.

Eric Hsu of XS Engineering informed us that typically the stock STi fuel system is marginal even with our bolt-on engine mods. Eric said that the stock fuel pump and injectors are too small for anything other than basic bolt-ons. To resolve this issue we used a high volume fuel pump kit from Powertech Imports. The kit consists of a 255-liter-per-hour Walburo fuel pump with Subaru compatible pump bibs and pick up. The kit installed in minutes, practically dropping right into the stock fuel tank. The high volume pump is slightly noisier than stock, but the difference is hardly noticeable. The pump should provide enough fuel to reach all of our immediate power goals for Project STi.

We also upgraded the stock injectors at this time. The stock STi injectors flow 550cc/min. XS Engineering modifies the stock injectors to flow 780cc/min for a nominal fee. XS checks the injectors' balance at this time as well. In order to run the larger injectors, XS recommends a reflash of the stock ECU to XS' specifications. We'll get more into the details of that later.

Another issue we were facing was the poor performance of the car's stock top-mount intercooler. When we drove the car on the track, we could feel the power drop like a rock over the course of three laps. The power loss felt like it was in the 30 hp range.

XS tested the stock intercooler on their dyno with the hood closed so the hood scoop would function, while huge fans were directed straight into the hood scoop. Eric from XS shared test data on the stock intercoolers effectiveness, and we've listed it below.

As you can see, after just 4 pulls the stock intercooler is completely ineffective. During track conditions the cooling airflow might be a little better at times than even a very powerful fan but at low speeds it will be worse. No wonder we were feeling such a large power drop. Although it would be difficult to tune the ECU more aggressively with such a widely possible variation in the intake temperature, something had to be done to address this issue.

At one time we were told by Subaru public relations that the top mount intercooler evolved from Subaru's WRC rally experience. The intercooler was top mounted to make it less prone to damage. However, if you look at any of the WRC Subarus now, you will note that they all have front mount intercoolers. XS Engineering stepped up to help us solve this problem with their Power IC front mount intercooler system made specifically for the STi.

The Power IC has a huge tube and fin core. In testing, the tube and fin heat exchanger performed better than a bar and plate element in regards to both pressure drop and cooling efficiency. The tube and fin element also proved to be much lighter. The Power IC came with lightweight polished aluminum intake tubes treated with a protective anti-corrosion coating. The tubes fit well, making the installation easy. However, we did have one issue. The Power IC's compressor discharge pipe went through the same hole in the chassis as our AEM cold air intake did, which made for a rush trip back to XS in the middle of the night to pick up one of their Powermax cold air intake systems as the AEM part no longer fit.

According to XS Engineering, the Powermax cold air intake has been dyno-tuned to be compatible with the intake tract length changes caused by the front mount intercooler's long pipes; and has been found to be the most powerful cold air intake on the market when coupled with a front mount intercooler. The Powermax intake fits into a chassis hole further forward of the hole that the AEM intake used, assuring compatibility with most front mount intercooler kits. This is good because most of the other front mount compatible air intakes are simply short ram types that gulp hot underhood air. The Powermax intake had a chrome finish that matched the intercooler piping, and set off the engine compartment nicely.

To demonstrate the front mounts' effectiveness, Eric shared some more data he had collected in prior testing of the Power IC. Under nearly the same test conditions on the same car, the following data was collected:

This is an amazing temperature increase of only 5 degrees! The additional surface area of the long intake pipes out of the engine's heat-soak zone probably resulted in lower temperature to the intercooler as well. Keeping the intake charge up to 160 degrees cooler allows for more boost, more spark advance and a leaner mixture with greater safety.

After the installation of the intercooler, we were now ready for Eric to work his tuning magic on the stock ECU. Based on his prior experience with the STi, Eric reflashed the ECU using ECUTEK's software package with XS Engineering's basic performance program. He strapped the car to the dyno with a wide band A/F ratio meter to see if more tuning would help our car. The reflash consisted of changes to the injector-sizing constant to compensate for the larger injectors, it also changes the drive by wire throttle response. By adding more spark advance and using a slightly leaner mixture, we can take advantage of the much cooler, more stable intake air temperatures than our larger front mount would give. Since intake air temps are much cooler and stable, XS was able to safely increase the boost pressure from the stock 14 psi to 20 psi.

Project STi responded to the reflash with a vengeance, and too well for the stock clutch that immediately went up in smoke. Eric aborted the testing and reduced the boost to 18 psi, at which point the stock clutch held. Project STi was pumping out 313 hp and 314 lb-ft of torque at this point. This was a gain of 28 whp over our last installment. The thing that numbers alone don't show is that the power gain is across the board from idle all the way to the fuel cut with a peak gain of 50 hp at 5000 rpm. The power is broad and the dyno chart flat with the power level hovering over 300 hp from 5000 rpm to 7700 rpm. The engine makes over 200 whp from 3500 rpm on up - this is broad, useable, maximum under-the-curve power.

Many people say that a front mount intercooler with its larger core, larger end tanks and long ducting contribute to turbo lag on a WRX/STi. XS proves this is not an issue. With proper pipe sizes and by not going overboard in core dimensions, the XS set-up actually has less lag and more bottom end power than the stock top mount intercooler. If our clutch had not begun to slip we probably would have been able to extract another 12-20 hp from the stock turbo with ease. We will revisit this issue when we upgrade our clutch and add a light flywheel later.

Our car feels like it got a new lease on life - the engine is much more snappy and the throttle response is crisp with no drive-by-wire throttle lag. The car feels eager and jumpy. There is hardly any discernable turbo lag. We can say that there seems to be no negatives associated with the front mount intercooler and reflash, a very rare thing for an aftermarket performance modification. The reflash has probably been the biggest single improvement to the car to date.

To continue our upgrade path we once again turned to the suspension. We were planning to attend a NASA track day at Willow Springs and wanted to have a more track ready suspension - with damn near a stock ride feel. In previous segments, we had installed Whiteline's basic handling package and Group 4 coilover suspension kit. Although we were pleased with the results, we wanted to up the ante and take the car to the next level, the next level being near-racecar levels of grip and response. We also wanted to take care of a few lingering faults that the car had in the suspension department as well.

The first step was to give the car some more roll stiffness. Although the car had little body roll as is, we knew that we would get a lot more when we bolted on a set of soft R-Compound tires. The tires' additional stick loads the chassis harder. To counter this we installed a set of Whiteline's racing XRD bars. The XRD bars are two-way adjustable in the front and three-way adjustable in the rear. We believe that bar adjustability is important in a track car as it enables you to quickly adjust the car for changes from track to track or changing from high-speed events such as track days to lower-speed tighter work such as autocross.

The stout XRD bars are a whopping 27 mm in diameter and over 160 percent stiffer than the handling package's 22 mm bars. They are also an amazing 280 percent stiffer than the stock 20 mm bars. We also installed Whiteline's optional solid spherical bearing end links and heavy-duty rear mount kit. The spherical bearings have length adjustment so they can be installed without preload, which can affect handling balance from side to side. Without the cushion of bushings, they assure that any body roll will be fully against the sway bars' full torsional stiffness. This greatly improves response. To protect the bearings, Whiteline's end links have a neat feature: urethane dust boots. To prevent the stiff rear bar from tearing off the chassis, we opted for the optional heavy-duty rear mount. This is much stronger than the stock rear mount and should prevent the bar from damaging the rear suspension. We set our bars at full soft in front and the middle of the three holes in the rear.

When shifting briskly at or near wide-open throttle, we also noticed a disturbing banging noise coming from the back and underneath the car. Sometimes when accelerating out of a tight corner we got shuddering wheelhop from the rear wheels. To alleviate this we installed Whiteline's diff carrier lock and transmission mounts. The stock diff carrier and transmission mounts are soft rubber and they allow these parts to bang around quite a bit. The Whiteline parts are made of harder 70 and 80 durometer polyurethane, and stop the unwanted motion with no detectable increase in vibration or harshness. We feel that these parts can probably go a long way to prevent drivetrain breakage as the wheelhop and banging sure felt like something was going to break.

Since we were planning to so some serious track work, we also replaced our street anti-lift kit with the Whiteline's race spec kit. The main difference between the race kit and the street kit is a change in durometer in the bushing from 70 durometer to 90. We noticed a slight increase in harshness, but felt it was barely discernible. We also noticed a slight improvement in steering response.

Finally, following Whiteline's recommendation, we installed their rear crossmember lock. When Whiteline was developing the WRX, they noted that after a few miles the torque load would skew the rear diff carrier to one side, which messed up the alignment settings. The problem was worsened when using sticky tires. They determined that the diff carrier was actually shifting 2-4 mm under load throwing the alignment off. This is a considerable amount, and it made the WRX impossible to set up properly. Because of this issue, the locks are mandatory for any WRX/STi that is going to be driven seriously.

Whiteline developed a special bolt with a large shoulder that threaded into an unused set of weld nuts in the chassis. Conveniently enough, it lined up with a hole in the crossmember in such a way it's almost as if Subaru intended to have this sort of lock in place as standard equipment. The bolt solidly locates and centers the entire rear diff carrier in place. This allows for precision alignments, but doesn't allow shifting of the rear wheels' location - no matter how hard the side loads. On our car this also eliminated the last bit of nibble from the rear of the car under acceleration.

As a last step we brought project STi to Westend Alignment in Gardena, California, for a more aggressive track alignment. We set up Project STi as follows:

If you were to drive this set-up daily, it would probably have significant negative effect on front tire wear - in which case we would probably recommend zero front toe and 1.5 to 2 degrees negative front camber.

At the track we set our tire pressures to 35 psi hot, and had a go at it. We were amazed. Our car was the best handling STi we had ever driven. Steering response was razor-like with excellent turn-in. The car refused to understeer, and would simply go into a nice easily controllable four-wheel drift. Lifting the throttle or trail braking would cause the tail to step out nicely, which would help get the car to pivot and rocket out of the turns. We felt no need to fiddle with damping or sway-bar adjustment, as we had hit the nail on the head. Tire wear was excellent under track conditions. There was no chunking, and the tires wore absolutely flat and even across the tread.

Even though I have driven and fallen in love with several modified EVOs in various track events, I was surprised to find myself liking Project STi better. The car was a killer with sharp turn-in, predictable on-throttle antics and wide powerband. Even with a young inexperienced driver, the Project STi turned racecar-like lap times and beat nearly every car in NASA's advanced lapping class. Our little car was delivering near exotic performance on a budget.

In Project STi's near future we will bring you some improvements to its clutch, cooling system, brakes, chassis stiffness and, after that, maybe some more power. After this, it will be time to bring on Project EVO. Stay tuned...

Stock IC (all temps in F)Ambient test temperature 70.6 deg FTemperatures were measured at the surface of the core 0.25" away from the end tank

XS Power IC (all temps in F)Ambient test temperature 77.4 deg FTemperatures were measured at the surface of the core 0.25" away from the end tank