For any turbocharged car, there are several ways to make more power. The first method is to open up intake and exhaust passages and use larger diameter piping; this reduces restrictions in intake and exhaust flow and allows the engine to operate more efficiently. Another way to increase power is to increase boost pressure which forces more pressurized air into the engine. When combined, these two methods of increasing power can realize serious gains, albeit with proper tuning.
Yet, there will come a point in your quest for power where bigger pipes will not due and your stock turbo has simply run out of breath. To get more go for your chariot, you will need a larger turbo. That is exactly where we found ourselves with our Project 2007 Legacy GT. While we had a pretty solid setup going we were still not a part of the 300hp club. This even though we had a larger-than-stock turbo. The Blouch, TopSpeed 18G, the turbo that we have been running, is a compromise of sorts. It stuffs the guts of a larger turbo into the stock compressor and turbine housings of the VF40 Legacy GT turbo. We were also running a full Perrin turboback exhaust with catalytic converters, Perrin TMIC, and an ECUTek reflash. This was good for a peak of around 290whp and 300lb/ft of torque. Not bad. The problem is that while the Blauch turbo is a great turbo and makes much more power over a stock unit, it is essentially limited by the housings. In short, we can only cram so much intake air and exhaust gas through the stock housings before they choke off. Normally on a true TD05 18G, peak horsepower levels of 320-340whp can be achieved but because this was not a true 18G, and peak power was limited. To be sure, turbo lag is at a minimum with this setup, but it does run out of breath on the top end.
The Legacy also presents another challenge. Since the turbo bolts directly into the intercooler and the intake manifold is quite a bit different than that of an STI, bolting-in an STI style turbo is not an option without swapping over an entire STI intake manifold setup, which is difficult indeed since the two are quite a bit different. However, there are a few companies that make Legacy-specific turbos, like AVO. It should be noted that you can use a STI-style turbo with your stock manifold and a standard Perrin front-mounter intercooler kit, however, this can be a costly option and larger turbos can spell doom for your engine. See our engine build article in this issue.
AVO sells a range of turbos for the Legacy owner wanting more go than the stock snail can provide. The AVO 380 is the baby of the bunch, about the size of a 16G. The AVO 420, the turbo we installed, is a bit bigger, about the size of an 18G. There are a few design elements that make the AVO turbo a very robust unit. First off, they are based of Garrett ball-bearing turbos. Secondly, they use slightly oversized turbine housings which reduce wear on the center cartridge but do hurt spool somewhat.
AVO rates this turbo at 420bhp (flywheel hp). Keep in mind, these hp ratings were achieved on Australian 98-octane wonder gas, not our anemic 92 octane, which is even more anemic when you get it in Washington. The AVO turbo comes with everything you need to install it and fits very well. However, if you have a later Legacy GT with the smog pump, be warned that the turbo oil return tube will have to be slightly modified to clear the smog-pump tubing on the back of the head.
Also note that these turbos will work with the new 2008 Impreza as it shares the same intercooler and turbo setup as the Legacy GT.
Our results with this turbo are a bit mixed. While this turbo does make good power, it falls a bit short of expectations, especially when compared with Mitsubishi turbos of similar size, namely the TD05 18G. Secondly, we found spool to be an issue. Since the AVO 420 uses a larger hot side with a smaller compressor, it takes a while for the turbo to come up to boost. We did not hit peak boost until 4,000rpm.
“The car is a blast to drive, the only problem is with the power being so far up the tach, I usually run out of road before I really get into it,” said Subiesport publisher, Ryan Douthit. “I think it’s a turbo that is best suited for track applications and may do better with higher octane fuels than what we can find on the corner here in the United States.”
While the AVO may not rock the house in the power or spool department, it makes up for this somewhat in ease of installation and longevity. AVO claims that thanks to their larger turbine housing, Garrett ball-bearing center sections and superior grade of construction materials, that this turbo will out-live our car, or at least live as long as we own the car. This is important when considering that some journal bearing turbos may live very short lives indeed of around 30,000 miles or less, depending on how hard you push your turbo.
To have our AVO 420 installed, we turned to IPD/Rallitek in Portland, Ore. To see a short video of this installation complete with helpful installation hints, go to www.drivingsports.com.