The time has come for the final blog post in this series. In the last blog entry, we talked about turbo upgrades, and this blog will discuss the proper supporting modifications needed for a turbo upgrade, along with building up the motor. This of course is where all the basics end.
So you’ve got your exhaust, engine management, and you know what turbo you’d like to upgrade to, but you can’t just slap that bigger turbo on and call it a day. The reason why it’s not quite that simple is because the turbo adds much more air to the motor, thus you need to balance it out with additional fuel, cooling, air flow, and of course, a tune to balance everything out, since the stock ECU will not be able to compensate properly for the amount of air the turbo is going to need. Thus, after you’ve figured out what turbo you’d like to upgrade to, you also have to plan on upgrading the following (if you haven’t already):
- Fuel pump
- Fuel injectors
- Air intake
- Intercooler
- Engine Management
If you’ve been following our discussed mod path thus far, chances are you already have some sort of engine management, but probably not a larger intercooler or air intake. With an upgraded turbo, you’re going to need all of these, along with fuel system upgrades. We’ll go ahead and start with the fuel system upgrades first.
For most small to moderate-sized turbo upgrades, a simple Walbro 255lph fuel pump is more than capable of pumping out enough fuel for your needs. Walbro fuel pumps are pretty much a staple when it comes to fuel pump upgrades, and will be more than enough up to about 500 whp. If you decide to go with a turbo that’s going to put out more than 500 whp, then you’re going to need a fuel pump system capable of supporting your turbo’s thirst. There are a few options out there, such as dual Walbro pump, Bosch high flow pumps, and Aeromotive fuel pumps. Each of these is capable of doing the job properly, but we’ve had our best success with the Bosch and Aeromotive fuel pump upgrades. The thing to keep in mind however is that all of these options aren’t a direct replacement for your stock pump like the standard single Walbro pump, as you’re going to need additional lines, fittings, etc. As you can see, we’re definitely getting out of the basics now.
Of course, an upgraded fuel pump is no good if they don’t have the proper injectors to actually put the fuel into the motor. Stock WRX fuel injectors are 440cc, whereas stock STI fuel injectors are 560cc. However, this is further complicated because the WRX and the 07+ STI uses a top feed style injector, whereas the 04-06 STI uses a side feed injector. The jury is out on why Subaru decided to change this and then change it again, but it’s just another complication that you need to keep in mind. Thus, if you’re upgrading your WRX to use a VF39 off an STI, you can’t just swap in the stock STI fuel injectors unless you get the 07+ specific ones. They simply will not fit.
The size of your fuel injectors is going to depend on the size of the turbo they’ll be supporting, but generally it’s better to be capable of more fuel than to run out of fuel. On that same token, you don’t want to go too overboard either. For example, if you’re running a VF39 on your WRX, there’s absolutely no need to go with 850cc fuel injectors. This causes problems not just in the fact that there’s too much fuel available, but it’ll ultimately frustrate your tuner when they try to scale the injectors down properly. Generally, here’s what you’d be shooting for in terms of fuel injectors:
For 2.0L WRX
- 16G to VF34: 560cc – 740cc injectors
- 18G or larger: 740cc – 850cc injectors
For 2.5L WRX, STI, LGT, and FXT
- 18G to 20G: 740cc – 850cc injectors
- FP Green to GT35R (pump fuel, low boost): 850cc injectors
- GT35R (race fuel, high boost) and larger: 1000cc or larger injectors
Again, the above is all generalizations, since the amount of fuel you’ll need is also dependent on the amount of boost (and thus forced air) you’re going to be running. It’s also important to keep in mind that 850cc is pretty much the limit that the stock fuel pressure regulator will be able to handles, so if your car requires 1000cc injectors or larger, you’re going to have to replace your fuel pressure regulator with an aftermarket one, such as a Turbosmart or Aeromotive. On top of this, not every company makes direct swap-in 1000cc fuel injectors, so it’ll be important to make note if there’s anything you need to splice or add in order for the fuel injectors to function and install properly.
All right, so now you figured out your turbo, your fuel upgrades, and probably your intercooler and intake too. Of course with most rotated turbo setups, the intercooler piping and intake are typically included with the kit, otherwise you’re off to do something custom on your own. Once all of your new parts are installed, all you need now is a good tune for your choice of engine management and you’re pretty much off to the races! But wait, what about building up the motor? Are you even going to need it? In our experience, building the motor is typically dependent on a few things: tuning, size of the turbo, amount of boost you’ll be running, and your choice of fuel. For 99% of the street builds we’ve done, which are all at or below 450 whp, there really is no need to build the motor at all with proper tuning. Even at this power level in a racing situation, a proper tune has shown to keep a motor happy and healthy. 
For example, throughout the entire 2007 Time Attack Race Season, Phil from Element Tuning competed and won races across the country with his 2006 STI with a bone stock motor. His car was consistently powered between 450-500 whp on race gas in one of the most punishing types of racing possible, yet it still held up great and he never had a problem. Since he decided to add more power to the car in the 2008 season, he eventually built up the motor, but for all of 2007, everything was great. This shows that proper tuning is key to making sure your motor stays healthy, especially since no matter what you decide to build a motor with, a bad tune will still make it pop. Like pretty much all shops, we have our preference in terms of cost, availability, and reliability, but as long as you follow the formula below and stick with a trusted brand, everything should work great. Here’s what we recommend if you’re sure you’ll be above the 450whp level, based on our experience:
- Forged Pistons – The stock pistons tend to be the first thing to go given enough boost, since they are cast. Forged pistons are typically the first and foremost part that we recommend replacing.
- Forged Connecting Rods – These are also a smart thing to replace if you’re going to be replacing the pistons anyway. Forged rods will stand up to more stress and thus keep your motor healthy in the long run.
- High Performance Main and Rod Bearings – The bearings are always smart to replace if you’re going to have everything taken apart to begin with. We always replace the bearings when we have the motor apart.
- High Performance Camshafts – Building up the bottom end was the “easy” part, but building up the heads is a completely different story. The one thing to keep in mind about the cams is that all of them will shift your powerband toward the top end, thus we usually don’t recommend cams for someone who wants low-end/street power.
- High Performance Intake and Exhaust Valves – Valves will definitely help cams breathe, but aren’t always a necessary item, as it all depends on how big of a build you’re doing.
- Titanium Valve Springs and Retainers – If you’re going to replace the valves, you might as replace the valve springs and retainers too. This will help the valves perform properly and stand up to more power you throw at the motor.
- Oil Pump and Oil Pan – If there’s one thing that a build motor and big turbo needs, it’s proper lubrication of its parts. A high volume oil pump helps keep the oil flowing when needed, and most aftermarket oil pans not only hold more oil, but also act as a heatsink to help keep the oil cool. In our experience, external oil coolers actually tend to inhibit the flow of oil and since Subarus already come with an oil cooler from the factory, we usually don’t recommend any sort of external oil cooler.
- Headstuds – You want to make sure there’s a good seal when you put the motor back together. Thus, getting proper headstuds that can take the pressure are very important.
And with that, we pretty much have an awesome motor that can handle almost anything you can throw at it. Sure, you can go with a sleeved block and increase displacement, but if that’s really your goal, you probably wouldn’t stopped reading this a long time ago, since that’s WAY beyond the basics. In any case we’ve built cars that are capable of well over 650 whp with all of the above build motor mods, which is going to be more than enough to satisfy just about everyone. I hope you’ve enjoyed this series of blog entries, and I hope it’ll remain as a good reference to all Subaru enthusiasts out there. Happy modding!
Starting with the 2008 model year, Subaru changed to a new 5-door hatchback chassis for its flagship Impreza STI line. Along with the new chassis design included a completely revamped suspension that promised to not only deliver better handling, but also a step above in ride quality over the previous model Imprezas. The step up in ride quality cannot be questioned, however feedback from many new STI owners was that the car did not feel as responsive or crisp as the previous version STI. With this in mind, EPIC Engineering sought to solve these issues, while still maintaining the crucial aspect of ride quality.
