The WRX/STI/LGT/Forester XT Power Mod Path, Part 6
All right, so you’ve got the full exhaust, you may or may not have an intake, and you’ve got some engine management that we mentioned in the previous blog entry, so what’s next? Before we actually dive into talking about replacing that snail under the hood, it’s important to mention the pipe that leads to the turbo itself from the headers. This is the uppipe, and for the 02-05 WRX, it is home to the third catalytic converter in the exhaust system. Due to this cat being there, it is important to replace this pipe whether or not you decide to go with an aftermarket turbo or not. There’s been more than a few cases of the high temperatures of the turbo heating up the material that makes up the catalytic converter, which then breaks apart and has nowhere else to go but into your turbo. Since the uppipe replacement is probably the most difficult and time-intensive exhaust part to replace, it’s wise to either do this or have it done when you replace your downpipe, and of course when you replace your turbo. Given the difficulty and time needed to do this replacement, we don’t normally recommend replacing an already catless uppipe in the STI or the 06+ WRX for example since the smoother exhaust flow of an aftermarket uppipe is usually negated by the cost and time required for replacement.
So now that you’re not putting your turbo in danger, let’s talk about your stock turbo itself. I’m going to try my best not to get hung up on names and specs, so please bear with me. The 02-07 WRX and Forester XT come stock from the factory with a Mitsubishi TD04L-13T turbo. For the 08 WRX, they changed this turbo to a Mitsubishi TD04-13T to match the new style intercooler from the Legacy GT, but then Subaru decided to change it again to the IHI VF52 for the 2009 and up since the 08 WRX was critiqued as not having enough punch. The 04-06 STI use an IHI VF39 turbo, while the 07 STI uses an IHI VF43 turbo, which is essentially a slightly modified VF39. By contrast, the newer 08+ STI uses an IHI VF48 turbo that is completely different. The 05-07 Legacy GT comes stock with an IHI VF40 turbo, but that has since changed to the IHI VF46 turbo for 08 and up. It’s most likely that the newer 08 and up Forester XT also uses this same turbo, but we haven’t had any at our shop yet to verify.
Now, I know I’ve mentioned a plethora of letters and numbers that probably don’t mean anything to you at this point, but the main point out of all this is the pure and simple fact that choosing an aftermarket turbo for your car is no easy, quick and dry process. The reason why Subaru has gone to different lengths to use such different turbos in each model and model year is to provide a driving experience that is responsive, quick, and satisfying. If Subaru just wanted to go balls out with the power, they could’ve easily slapped on a larger turbo for more high-end kick, at the expense of low-end response. Mitsubishi did this with their FQ-series Evos and while they were fast on the top end, just about every automotive journalist could make a sandwich by the time the car hit boost. In choosing the right turbo for your needs, being able to make a sandwich before boost is probably something you’ll want to avoid for the vast majority of the people out there. Thus, it’s important to set goals for yourself, do your research, and if possible, go for a ride in a fellow Subaru owner’s car so you can actually feel how a certain turbo affects the car’s drivability.
We’re lucky at this point in time because there’s a wide variety of turbo upgrades available for Subarus to match just about every person’s needs, from upgrades for daily drivers all the way to full-on drag racing turbos. This wasn’t always the case however, and there was a time early in the age of the 02-03 WRXs that it was almost unfathomable that a WRX could run a quarter-mile in the 10s, much less the 12s. Back in 2002, just about every turbo upgrade consisted of a take-off turbo from a Japanese-model STI (since there was no STI in the US until 2004). These turbos were all IHI turbos, and ranged anywhere from the VF29 from the Version 6 STI to the VF34 from the Version 7 STI Spec-C. While these turbos were definitely larger than the stock WRX turbo and provided a very good and responsive upgrade, they lacked top-end power. Larger and older IHI turbos such as the VF23 and VF22 added some extra oomph, but even the modified versions of these such as the Power Enterprises PE1818 and PE1820 (respectively) would push a WRX into the 310 whp range with an aggressive tune. However, since these turbos came off of different models of WRX or STI in Japan, they were a direct bolt-on fitment, which still to this day is a big plus. Thus, combined with the limited engine management options at the time, the IHI VF-series turbos and their modified variants were pretty much the only upgrades available in the early years of the WRX in the United States.
Soon though, turbo manufacturers and modifiers started to experiment with different housings and brands. Both HKS and Greddy offered some of their own turbo upgrades, but they tended to be cost-prohibitive at the time, thus they were a rare find. Nonetheless, Greddy’s bolt-on turbo upgrade was their version of a Mitsubishi TD05-18G, and it’s possible that their use of this turbo style drew the attention of US turbocharger companies such as Forced Performance. In late 2003 and early 2004, Forced Performance (FP) began experimenting with taking Mitsubishi turbos and modifying them with housings so that they could easily bolt on to the WRX’s intake and exhaust paths. They would use different housings, different turbine compressor sizes, different sized compressor wheels, clipping the fins to supposedly help with spool, and even polishing and porting to help with airflow. After many experiments, FP ended up with a TD05-16G and a TD05-18G that bolted right into place, yet carried equal or greater punch than the IHI VF30 or VF34, which were the go-to turbo upgrades at the time. FP touted its 16G as being able to give as much power as a VF34, yet spool faster and in the end be cheaper to purchase. Their 18G consequently touted as the new big turbo upgrade for a WRX, capable of producing numbers in the 320 whp range with the right combination of parts, build options, and race fuel. The first models came out as being oil-cooled only, so turbo timers had to be used for proper cool-down after driving, but eventually FP added the stock water lines as well, making them just about as good a turbo swap as the IHI turbos.
While the new FP turbos gained popularity, the advent of the 2004 STI brought a new IHI turbo into the mix: the VF39. Similar in size and power potential to the VF29, the VF39 was often seen as a good upgrade for the pure and simple fact that you could buy one used for quite cheap. However, where would the new STI owners go for an upgraded turbo? The 18G was a possibility, but it wasn’t a huge leap in power over the stock VF39 thanks to the extra 1/2 liter of displacement. FP started to produce a few 20G variants, both with a TD05 or a larger TD06 housing, but STI owners wanted more. During the time that FP was experimenting with WRX upgrade options, they took one of their DSM upgrades, the FP Green, and modified it to fit a WRX. The FP Green is essentially a TD06-20G turbo with a larger compressor wheel, but this large size ended up being just a little too big for the 2.0L WRX and produced quite a bit of turbo lag. On the other hand, the FP Green seemed to be a perfect match for the new 2004 STI, and soon the Green became the go-to turbo for the STI. The 2004 World One STI used an FP Green with the then prototype Element Tuning Hydra EMS, which propelled the car to 440 whp on race gas and resulted in a 2nd Place Class Finish at the first Primedia Time Attack in November 2004. The FP Green was a good upgrade since there wasn’t much turbo lag and it had a decent amount of punch.
Nevertheless, STI owners still wanted more power. FP released their Red turbo, was had an even larger compressor wheel than the Green, but required a 3-inch turbo inlet tube for installation. Not satisfied with the stock-location bolt-on turbos, the demand grew for larger turbos that required the fabrication of special piping, since these turbos were simply too large to directly bolt on to the stock location and oil/water lines. Since turbos were going to be bigger, many companies started to release pre-fabricated kits using Garrett GT-series ball-bearing turbos. The theory behind the ball bearings was that they would help the turbo spool faster, especially given the much larger size compared to stock. These new “rotated mount” turbo kits typically utilized a Garrett GT30R or a GT35R, with the GT35R capable of well over 450 whp with the proper supporting mods. For those going for real big power, the Garrett GT40R and GT42R turbos were available for use with these rotated mount kits, sacrificing spool for big top end power and sub-10 second quarter mile times. Other companies such as Element Tuning also released rotated mount turbos to compete in this arena with faster spool but equal power numbers to the GT35R, but by in large the Garrett turbos were still definitely the most widely used. Although these new rotated mount turbo kits produced great power numbers, they did have a few drawbacks. First and foremost was the cost of these kits. Since new piping had to be purchased which often consisted of a new intake, downpipe and uppipe, along with an external wastegate, everything just added up to becoming quite expensive. Moreover, since the turbo was now rotated, a front-mount intercooler was an absolute requirement, so those people who wanted to retain the top-mount intercooler in exchange for a few less horses were completely out of luck.
Of course, the whole problem with the stock location turbos at the time was the simple fact that they couldn’t be large enough to produce the same power that the rotated mount turbos did, even if they spooled faster. Element Tuning helped solve this problem with their GT52 turbo, which provided the power of a rotated mount GT30R, but also spooled faster. On top of this, since it’s a direct bolt-on turbo, no extra piping needs to be purchased. This differed from the FP Red which produced similar power and spool, but required the 3-inch turbo inlet tube. Eventually, more turbo manufacturers followed suit to compete. Element Tuning then released the direct bolt-on version of the GT65 turbo, which is able to produce GT35R power in a bolt-on configuration, although requiring a 3-inch turbo inlet like the FP Red. ATP soon came out with their own direct bolt-on GT35R turbo, and for awhile, turbo technologies remained a bit stagnant.
Earlier in 2009, Forced Performance came back into the fray with their new HTA-series of turbochargers. Through their R&D processes, they were able to improve both the spool speed and also the power potential of their already venerable 16G, Green, and GT35R. For example, the HTA 68 kept the fast spool of the 16G, but put out power levels equal or greater than that of the 18G. Here at our shop, we’ve already seen great success with the HTA GT35R, as seen in our customer Luke’s 649 whp 05 STI.
So there you have it: the history of turbo upgrades for your Subaru. So which of these is going to work best for you? The most important thing to keep in mind is that generally speaking, the bigger you go, the slower the turbo will spool, but the more top end power you’ll have. It’s also important to remember that the same boost level for one turbo is going to put out a different amount of power than another turbo. By this, I mean that 18 psi on an STI’s stock VF39 is going to produce less power than 18 psi on a FP Green on the same vehicle. Remember, boost is all about the amount of air going into your motor, so with a bigger turbo, you thus are forcing in more air, and if you have more air at the same amount of boost, the pressure is much stronger, which of course leads to more power. To sum it all up, here’s a general listing of some of the turbos I’ve discussed here and their potential power levels (given the proper supporting mods and fuel):
For 2.0L WRX
- Mitsubishi TD04-16G: 210-230 whp
- IHI VF29 and VF39: 250-270 whp
- IHI VF30 and VF34: 260-280 whp
- TD05-18G and Forced Performance HTA 68: 280-320 whp
- TD05-20G: 290-330 whp
- Forced Performance Green and Element Tuning GT49: 300-350 whp
- Forced Performance Red, Forced Performance HTA Green, Element Tuning GT52, and Garrett GT30R (rotated and bolt-on): 350-400 whp
For 2.5L WRX, STI, LGT, and FXT
- IHI VF39: 280-300 whp
- TD05-18G: 300-340 whp
- TD06-20G: 330-370 whp
- Forced Performance Green and Element Tuning GT49: 350-470 whp
- Forced Performance Red, Forced Performance HTA Green, Element Tuning GT52, and Garrett GT30R (rotated and bolt-on): 375-500 whp
- Garrett GT35R (rotated and bolt-on), and Element Tuning GT65: 400-530 whp
- Forced Performance HTA GT35R: 550-700 whp
- Garrett GT40 and GT42R: 700+ whp
That should do it for this blog. In the next blog, we’ll talk about the supporting mods necessary to support the turbos I’ve talked about above, including having to build the motor to support these power levels. This will ultimately end the series, since we’ll no longer be talking about basics. As always, if you have any questions at this point, please don’t hesitate to contact us!