Project Subaru STI Part 6

In part 4 of Project STi we got some pretty good results with some tuning by XS Engineering that nicely complemented our bolt-ons. However, we were stymied in our efforts to extract more power by a slipping clutch. Also, we noted that our rather high mileage motor was starting to burn a little oil, probably from the wear of over 100k miles that was put on it as a daily driver. Many of these miles also happened to be grueling track ones. No doubt that an engine rebuild was going to be needed in our plans soon, but we still wanted to pursue more power with the stock engine first.

Our first problem to fix was the clutch. What we wanted in a clutch was a tall order. We wanted a clutch that could withstand over 400 whp in an all-wheel-drive vehicle, a punishing application to begin with. We also wanted good streetability, since this car sees daily use. No juddering on/off switch metal discs and super stiff pressure plates were going to work here.

For a cure to our clutch woes we consulted with Daryl Sampson of Advanced Clutch Technology. ACT supplies a wide range of clutches for most popular sport compacts, from heavy duty to race applications. Knowing our penchant for road racing, Daryl suggested a heavy duty disc and pressure plate coupled with ACT's super light ProLite flywheel.

The heavy-duty pressure plate features a clamping force of 35 percent stiffer than stock while maintaining a reasonable pedal effort and release travel. This means that your left leg won't become overly buff causing you to walk in circles, and you won't need knee surgery if you have to drive in bumper-to-bumper traffic.

The clutch disc features a copper-woven, organic friction material for higher burst strength and improved heat dissipation, with a marcel spring and a sprung center hub for smooth engagement characteristics. The clutch is conservatively rated for a maximum of 411 lb-ft of torque, an amazingly high number for its daily driver smoothness.

We bolted the clutch to a ProLite Flywheel. The ProLite is forged from high strength chromoly steel. Forging is a superior way to make flywheels and results in a flywheel with superior burst and wear characteristics. Its one-piece construction is probably stronger than your typical two-piece aluminum light flywheel, and it can be resurfaced - unlike some very thin steel flywheels.

The ProLite has a series of lightening holes machined around its periphery between the friction surface and the starter ring, where it makes the biggest difference in inertial weight. Because of this, a ProLite feels like a lighter flywheel than its 11.1 pounds suggest.

We were impressed with this combo's smoothness; and the light flywheel makes the car much more responsive to throttle input and reduces the time for the turbo to spool. Our car felt springy, light and eager to go with this combination. We were also pleasantly surprised how easy it was to drive, even in bumper-to-bumper traffic. Of course, instead of going back to the dyno, we decided to increase the power.

Our next upgrade was a simple one: we installed a set of Unorthodox Racing's underdrive pulleys. The Unorthodox pulleys got rid of several pounds of rotating weight off of the engine and reduced parasitic drag by slowing the drive speed of the power steering, alternator, water pump and air conditioning. Usually this equates to 3 to 6 wheel hp and improved throttle response.

The Unorthodox pulleys are machined from 6061 T6 aluminum billet and weigh in at a feathery 3 pounds, saving over 6 pounds of rotating weight. Unfortunately, we installed them at the same time as the rest of the parts covered here, so we didn't get an individual dyno result from them.

When looking at the stock turbo, we knew there would be quite a bit of power to be found by upgrading it. The tiny stock IHI VF39 turbo was too small for the 2.5 liters of displacement behind it. Our goals were also rather difficult here.

Since at this point Project STi is more to show easy ways for the average guy to get power rather than showcase the ultimate in performance, we wanted to stick to strictly bolt-on turbos. No huge turbo with a fancy angle-mount and an external wastegate - at least not yet. We wanted to show the potential for a simple bolt-on that could bolt to our DC uppipe or the stock system with a no-fuss internal wastegate, again compatible with both stock and typical aftermarket parts.

After much research we turned to Forced Performance for one of their Green Turbos. The FP Green turbo features a huge Mitsubishi TDO6 compressor cover that houses a custom machined compressor wheel with an inducer of 2.123 inches and an exducer diameter of 3.00 inches. This wheel flows a whopping 730 cubic feet per minute (CFM) over the stock IHI's paltry 470 CFM. The compressor housing has a 3-inch radiused inlet and a 2-inch outlet.

On the exhaust side, a big Mitsubishi turbine wheel with an exducer diameter of 2.315 inches and an inducer diameter of 2.650 inches is used. A Mitsubishi turbine housing with an inlet area of 7cm squared is used, up from the stock 6cm squared. This is the Mitsubishi way of specing out turbine housing size, compared to the A/R figure that is quoted for most other turbos. A bigger area of exhaust housing inlet will flow more and make more power, but tend to have more turbo lag.