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Subaru Turbocharger Explained Part 2

Subaru Turbocharger Explained Part 2:

Service Procedures

Here are some service procedures, including steps to properly remove turbocharger components, and tests and inspections you can perform to check component operation.

Subaru Turbocharger Explained: Here are some service procedures, including steps to properly remove turbocharger components, and tests and inspections you can perform to check component operation.

Intercooler Removal

You may need to remove the intercooler to work on other components beneath it. Removal of the intercooler must be performed carefully so that no damage occurs.

1.) Disconnect battery. Remove the two bolts that attach the bypass valve, then the valve.

2.) Remove the bolts from each end of the intercooler and disconnect the crankcase ventilation hoses from the intercooler.

3.) Loosen the clamps at the throttle body and outlet of the turbocharger.

4.) Gently move the intercooler side to side until the tension of the hoses at the turbocharger and throttle body loosen.

5.) Remove the intercooler from the engine compartment and cover the open areas with tape to prevent foreign material from entering, which could cause damage to the engine or turbocharger after re-installation.

Turbocharger Removal

1.) After removing the intercooler, remove the intercooler mounting bracket.

2.) Remove the eight bolts that secure the protective heat shield around the turbo.

3.) Raise the vehicle and disconnect the rear oxygen sensor harness, then remove the front exhaust pipe mounting bolt. Position the pipe so there is some movement.

4.) Lower the vehicle and disconnect the wastegate hose to the vacuum hose leading to the wastegate control solenoid.

5.) Remove the coolant hose from the reservoir that connects to the turbocharger.

Subaru Turbocharger Explained Part 1

Subaru Turbocharger Explained:

Turbochargers are fairly simple in concept, but adapting the system to modern vehicles can be quite complex. This primer for those new to servicing turbos and review for veterans lays out the function and operation of turbocharging in Subaru vehicles.

Subaru Turbocharger: Starting with 2004 models, the WRX STi incorporates a
water spray system to help cool the intercooler, thereby
further cooling the intake air.

The return of turbocharging in the 2002 Impreza WRX marked an absence of nearly a decade for Subaru vehicles. While the new generation has been around for half a decade, not everyone understands the function and operation of Subaru turbocharging systems.

Naturally, everyone knows these blowers are designed to get the maximum power out of engines by packing more air and fuel into the cylinders to get the biggest bang possible. Just how that is accomplished, however, may be a bit of a mystery to you. Here’s a primer on turbocharging and how it applies to Subaru vehicles.

Subaru Turbocharger Explained:

A Brief History of Turbochargers

Turbochargers were originally invented to increase the volume of air pushed into the cylinders of internal combustion engines, and, along with increased fuel, raise the level of energy produced by the combustion process

Historical references indicate that Swiss engineer Alfred J. Buchi adapted the turbines from steam engines to diesel engines as a method to improve air induction, and, therefore, smoother operation in internal combustion engines. In 1905, Buchi’s idea of powering the forced air induction by exhaust flow was granted a patent. Good idea or not, the fairly crude engines of the day could not sustain even or adequate boost pressures. Buchi worked another ten years before he could produce a working model of a turbocharged diesel engine. By that time, other companies had also produced turbocharging systems

The massive building boom of internal combustion engines to supply ships, trucks and airplanes for World War I saw technologies take a giant leap forward. The first turbocharged diesel engines for ships and locomotives appeared around 1920. Shortly thereafter, European car manufacturers began incorporating them into factory race cars and a few sporty luxury models.

The next milestone for turbocharging came with the military build-up for World War II, when turbo systems were fitted to fighter planes and bombers to allow them to fly at higher altitudes where the thinner air could be compacted into the engines to provide sufficient combustion. However, direct-driven superchargers quickly proved more reliable, efficient and more easily controlled, leaving turbochargers by the wayside.

It wasn’t until the mid-1950s when turbochargers started appearing on diesel trucks that modern turbos began to make a dent in the automotive market. Today, the vast majority of truck engines are turbodiesels.

When turbocharged vehicles began to dominate the international racing scene in the 1960s, car manufacturers began to use them in sporty models to appeal to performance-oriented drivers. By the 1980s, turbochargers for cars were a bona fide success, particularly in Subaru vehicles, due to improved metallurgy, intercooling and efficient boost controls.

The main components of a Subaru turbocharger system are a water-cooled turbocharger, an air-cooled intercooler, a wastegate control solenoid valve, sensors and a controller. Let’s review the individual components and the role they play in the system.

Subaru WRX/STI turbo charger basics

Subaru WRX/STI turbo charger basics:

The primary purpose of an aftermarket turbo is to increase the performance over the stock unit. This can be accomplished by choosing a turbo with better spool, more flow, or a combination suited to the end user’s needs.
Subaru WRX/STI turbo charger basics: TD04 turbo and a VF39 turbo side by side


Common Terms:
Boost threshold- the lowest RPM at which a turbo will generate positive manifold pressure at maximum engine load.
Turbo lag- the time between hitting the throttle and the turbo providing full boost.

Recommended Reading:
Maximum Boost by Corky Bell is considered by many to be THE publication for turbocharger information.

What is the best turbo? There is no best turbo. Generally speaking, aftermarket turbos fall into these generic categories:
a. Turbos with a little more top end power
b. Turbos with a lot more top end power
c. Quicker spooling turbos

What do all the names and numbers of turbos mean?This link sorts many of them out nicely.

What supporting upgrades are required for aftermarket turbos? At a minimum, aftermarket turbos require a fuel pump, injectors, and engine management for safe operation.

What is my stock turbo?

2002-2008 WRX TD04-13T
2004-2005 STI VF-39
2006-2007 STI VF-43
2008 STI VF-48

Is there a turbo upgrade that does not require other upgrades? Yes. A ported and polished (P&P) stock turbo is an easy upgrade over the stock unit. Though there are many turbos that may be used for short periods of time with a boost controller, it is generally unwise to bolt on an aftermarket turbo with a boost controller.

What is the best turbo with a little more top end? The most widely used turbos meeting this criteria are the VF30/VF34 and the 16G.

What is the best turbo with a lot more top end? The most widely used turbos meeting this criteria are the VF22, 18G, 20G, FP Green and it’s clones.

What is the best turbo with quicker spool? The most widely used turbo meeting this criteria is a P&P stock turbo.

What makes a good autocross type event turbo? The big thing to look for in a good performer for autocross use would be quick spool and more than stock flow. The TD04, TD05-16G, VF34, VF22, VF39, 16G, and 18G can all be considered good autocross turbos, but their particular suitability depends on the type of events where the car is generally run.

During the consultation with your Vendor, discuss in depth the course length, speeds seen, gears used, and other local venue particulars to assist in determining what best suits your needs. A word of caution….before modifying or changing your turbo, be aware that this will have an effect on what class your vehicle can legally run.

VF39 STi turbocharger onto your WRX/Forester

VF39 STi turbocharger onto your WRX/Forester:

Swapping a STi VF39 turbocharger onto your WRX/XT is really easy. Find out here the step by step process on swapping a VF39 onto your Subaru WRX/XT.

You can buy a brand new VF39 Turbocharger here:

Subaru 14411aa5729L Turbocharger

A lot of people seem to be getting hung up on the oil return lines and coolant lines. The coolant lines and banjo bolts from the TD04 ARE interchangeable if your vf39 doesn’t have them (mine didn’t). The oil return line is not and must be modified in order to fit the vf39, (mine had it). Or get a new one from the dealer. I will explain in the pictures when I get to these items.

First remove the intercooler, pretty self explanatory there are 2 bolts that it mounts to, 2 on the BPV and the hoses. Done Deal. Next the heat shield. You should be somewhere around here:

VF39 STi turbocharger onto your WRX/Forester: Start removing the head shield and downpipe.


Remove the downpipe:

VF39 STi turbocharger onto your WRX/Forester: Start removing the downpipe.

Then remove the bolt for the oil inlet (top of the turbo), and start removing the coolant lines (2 lines coming off the side of the turbo 1 goes up 1 goes down). Be careful as you will lose coolant during this step be prepared to either catch it under the car or plug the line:

VF39 STi turbocharger onto your WRX/Forester: Remove coolant lines and oil lines.

Then remove the 3 bolts that hold the turbo to the uppipe. Loosen from the intake remove vacuum lines and anything else attached to the turbo and you should be ready to take it off. The hose that the oil return line fits into is directly under the turbo and you should probably just lift straight out. Try to leave the hose where it is and lift the line out.

Now that the turbo is off you can start swapping out the parts you need from the td04. The coolant lines from the TD04 will fit the VF39 however they are attached and too close together to fit directly on the VF39 see pic:

VF39 STi turbocharger onto your WRX/Forester: changing out oil lines.