Turbocharger: How to choose a Turbocharger

Turbocharger: How to choose a Turbocharger for your Turbo Subaru:

In order to make an informed decision when purchasing an aftermarket turbocharger, the consumer needs to avail themselves of the different types of turbochargers first. To this end, we will discuss the various types of turbos on the market. These are just the basics of turbo information though. Please do not confuse this as the main source for turbo information as there are many other factors to an informed turbo choice such as compressor maps, matching the turbo to your displacement, etc. For the best advice, please consult an experienced turbo vendor and/or your tuner.

A regular turbo is, in essence, a pump that forces air into your intake system. The end result is a denser air charge that will produce more power vs. naturally aspired vehicles. The only downside is that more power produces more heat, and the engine’s internal components must be properly suited towards turbo charging. Upgrading this unit to a larger one is the easiest route in terms of time, trouble, and expense. Common upgrades for all turbocharged Subaru models include the VF-30/34/22 and 16/18/20G.

A twin scroll turbo is designed to be used on an equal length exhaust set-up. By internal turbo design and having all the exhaust gases enter the turbo at the same time, this allows the turbo to spool faster vs. an equally sized regular turbo. This is a very important point as many people are confused by the marketing hype of twin scroll set-ups. When comparing a twin scroll turbo that will flow say 500 cfm vs. a normal 500 cfm turbo, the twin scroll should see full boost sooner. So if there are two suitably sized turbochargers, with one being twin scroll and one regular, the twin scroll unit may be your best choice if you do not mind the extra exhaust expense and prefer faster spooling.

This type of turbocharger requires more expense than a simple upgrade though. The biggest concern is the use of an equal length header, proper uppipe design, and the possible use of a different oil pan to accommodate the new twin scroll exhaust piping. Quirt Crawford of Crawford Performance recently did some testing on a GT32 twin scroll turbo Legacy to test the theory about the importance of exhaust flow to a twin scroll unit. When he switched from the correct equal length header and uppipe to a traditional unequal length header and normal uppipe, he saw degradation in turbo response by 750 RPM. This should be word to the wise to anyone who thinks they can avoid the expense of the correct exhaust components and still see the quicker spooling benefits of a twin scroll turbo.

Another consideration is the change in exhaust tone. An equal length header required in a twin scroll set-up sounds entirely different than an OEM or aftermarket unequal length header. To fans of the familiar boxer rumble, equal length headers are just not an option. It may sound silly, but for many, this reason alone is enough to keep them from buying a twin scroll turbo.

A rotated mount turbo is any turbo that’s physical size prevents it from fitting in the stock location and must be mounted at a slightly different angle. Most of the turbos that fall into this category are of the larger variety. Many require custom piping, a front mount intercooler, external wastegates, custom tuning, tumble generator valve deletes, and other technical or expensive upgrades to support it. Most would consider this type of turbo to be outside the scope of the average do it yourself person and should be farmed out to a professional or at least utilize one of the kits supplied by various manufacturers.

As well, many feel that when going this route, strong consideration should be given to fully built motor, or at the very least, forged pistons. Rotated mount turbos produce large amounts of power and though there is no magic horsepower number for switching to forged internals, the larger rotated mount set-ups seem to be commonly used on built motors.

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Header Aftermarket Turbo Subaru FAQ

Header Aftermarket Turbo Subaru FAQ

The primary purpose of an aftermarket header on a turbocharged Subaru is to remove or replace the stock exhaust manifold with a better flowing unit.

An Agency Power Stainless Steel Header for a EJ257
An Agency Power Stainless Steel Header for a EJ257.

HP gain is 15HP and 20TQ. This figure is highly debated as different manufacturers use different dynos with different cars with different levels of mods. It also varies because some headers incorporate an uppipe into their design. This makes it nearly impossible to compare the gains of a header without an uppipe vs. a header with an uppipe.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent “bad” headers on the market. Obviously, there may have been bad pipes sold, but not enough to report as “bad” overall.

Where can I find headers?

Subaru WRX STI Header Prosport Unequal Length Stainless Steel Header

Perrin Subaru STi /WRX Header Equal Length Big Tube

AP WRX/ STI Stainless Steel Unequal Length Header w/o Uppipe

Invidia HS05SW1HDR Racing Header for Subaru WRX STI/Legacy GT

Agency Power (AP-GDA-175) Unequal Length Header, Stainless Steel

Perrin PSP-EXT-050 04-08 STi /LGT/FXT/06-08 WRX (06+ WRX requires modification to oil cooler) Header (02-05 WRX Requires STi Oil Pan)

TOMEI 193082 Headers

What differences are there with headers? The main difference is exhaust piping length. Headers are made to be unequal length or equal length. Unequal means the piping on the driver’s side of the engine will be longer than the passenger’s side. Equal means the piping length is equidistant from the engine outlet to the header outlet for all piping.

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Downpipe Turbo Subaru FAQ

Downpipe Turbo Subaru FAQ

Buying a aftermarket downpipe is a typical modification which removes or replaces the stock catalytic converter with a better flowing unit. It also increases the exhaust diameter for better flow.

HP gain is 15-20HP. This figure is highly debated as different manufacturers use different dynos with different cars with different levels of mods. It also varies because some downpipes use one high flow cat, while others are catless and actually extend far enough back to eliminate the 3rd cat as well.

I have an 06/07 WRX, is the downpipe the same? No. The 06/07 WRX has a unique exhaust in that it’s downpipe is both the downpipe and catpipe sections of the “older” exhaust. This means you must use a “long” downpipe to bolt up to the rest of your exhaust system. A full TBE will fit fine, but when replacing just the downpipe, you must use a long downpipe.

I have an 08/09 WRX, is the downpipe the same? No. The 08/09 WRX has a unique exhaust in that it’s downpipe is the same, fitment wise, as the Legacy GT. So if you have one of these models, you must ensure you state your model/year to your vendor or specifically request the “Legacy GT downpipe” for your car in order to assure proper fitment.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent “bad” downpipes on the market. Obviously, there may have been bad pipes sold, but not enough to report as “bad” overall.

What downpipe metal material is best? Downpipes are made from mild steel and stainless steel (304 & 321). There is no irrefutable evidence that one material is better than the other. Obviously, corrosion levels are higher with mild steel (coated or otherwise). Article on exhaust materials.

Which downpipe construction method is best? Downpipes have 4 main construction methods:
1. Blank plate: Identical to stock construction with the wastegate portion completely covered.
2. Bellmouth: Completely open design.
3. Split bellmouth: Similar to bellmouth only with a divider inserted to separate the wastegate.
4. Divorced or Twin Dump: Separate exhaust and wastegate piping that connect further downstream.

There is no irrefutable evidence that one design is better than the other. The thought process is that the greater the separation there is between the wastegate gases and exhaust gases, the smoother the overall exhaust flow.

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BOV: Blow off Valve Subaru FAQ

BOV: Blow off Valve Subaru FAQ

Term usage: “Blow off valves” go by several names, among them are compressor bypass valve (CBV), air by-pass valve, bypass valve (BPV), blow off valve (BOV), Diverter valve, and possibly a few others. BOV is the common and incorrect term that lumps true blow off valves and bypass valves under the same term. For the sake of correctness, this post will refer to either aftermarket BOV, aftermarket BPV or OEM BPV as these are the most correct terms.

What is the function of a blow off valve (BOV)? To release pressure from the intake tract of a turbo car when the throttle closes. It is a vacuum-actuated valve designed to releases the air to the atmosphere.

What is the function of a bypass valve (BPV)? To release pressure from the intake tract of a turbo car when the throttle closes. It is a vacuum-actuated valve designed to recirculate the air back into the intake before the turbo inlet, but after the airflow sensor.

A stock Subaru BPV (not a BOV).
A stock Subaru BPV (not a BOV).

What is the purpose of a BOV/BPV? When the throttle closes and the intake system is under pressure, the high-pressure air entering the motor will bump into the closed throttle plate, and in the absence of a BOV/BPV, a pressure wave will travel back to the turbocharger. The result is that the compressor wheel will stall (a phenomenon known as “compressor surge”) and slow down very quickly. This is hard on the bearings and decreases the turbo’s lifespan, but it also means the turbo will take longer to spin up the next time the throttle is opened.

Are aftermarket BOVs necessary with Subaru turbos? No. The OEM BPV is perfectly fine up to 20psi of boost. For applications using higher boost levels, an aftermarket BOV/BPV should be considered.

Can I mod my stock BPV to hold higher boost? Yes. I’ll be creating a post giving detailed instructions on how to do so.

Is the STi BPV better than the WRX BPV? No. They are the same. However, the JDM STi BPV will hold more boost as it is physically different than both the USDM STi BPV and the WRX BPV. The specific PSI rating of the JDM STi BPV is unknown, but users have reported it is good up to 25 PSI.

Is an aftermarket BPV better than the stock BPV? No. Unless you are considering an aftermarket BPV solely for the purposes of holding higher boost levels. An aftermarket unit should sound just like the OEM unit.

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Intake pipe turbo subaru FAQ

Intake Turbo Subaru

The primary purpose of an aftermarket intake is to increase the amount of air flow the engine receives.

HP gain is 0-15HP. These HP figures are a range as different intakes have different dyno results.

What is an aftermarket intake? Aftermarket intakes can be broken down into two types:
1. Short ram: AKA shorty or ram pod.
2. Cold air intake: AKA CAI.

What does the stock intake look like? To gain a perspective on how aftermarket intakes function, the stock intake features will be shown and discussed for further clarity.

This is the stock intake as viewed from the driver’s side.
The “U” shaped assembly and the tank that connects to it on the left is commonly referred to as the snorkel,  silencer, or resonator. It is actually hidden in the fender.

Stock Subaru turbo intake piping.
Stock Subaru turbo intake piping.

This is the stock intake as viewed from the passenger’s side.

This is the stock intake as viewed from the passenger's side.
This is the stock intake as viewed from the passenger’s side.

This is the stock intake as viewed from above.
1. Stock “ram intake” funnel.
2. Stock air filter box.
3. MAF sensor.
4. MAF sensor piping.

This is the stock intake as viewed from above. 1. Stock "ram intake" funnel. 2. Stock air filter box. 3. MAF sensor. 4. MAF sensor piping.
This is the stock intake as viewed from above.
1. Stock “ram intake” funnel.
2. Stock air filter box.
3. MAF sensor.
4. MAF sensor piping.
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Intercooler Hose FAQ for Subaru STi/WRX

The primary purpose of an aftermarket intercooler hose is to replace the stock intercooler hose with a better flowing unit. Airflow testing indicates 14.2% better flowing capacity than the OEM piece. It can also decrease intake temperatures, increase reliability, and dress up the engine compartment.

HP gain is 5HP. This figure is highly debated as different manufacturers use different dynos with different cars with different levels of mods.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent “bad” intercooler hoses on the market. Obviously, there may have been bad hoses sold, but not enough to report as “bad” overall. The current manufacturers of intercooler hoses that fit the OEM intercooler are:

a. APS: “Y” hose only
b. Perrin: 3 hose set
c. Prodrive: “Y” hose only
d. Samco: 3 hose set
e. Vishnu: 3 hose set
f. GP Moto: 3 hose set

Prodrice silicone intercooler hose
Prodrice silicone intercooler hose

What intercooler hose material is best? Intercooler hoses are made from silicone and cast alloy. There is no irrefutable evidence that one material is better than the other. Obviously, heat soak levels are higher with the cast alloy model.

Which intercooler hose construction method is best?
There is no irrefutable evidence that one design is better than the other. With regard to the number of silicone plys, here is a breakdown:

Perrin= 5 ply
Prodrive= unknown ply
Samco= 3 ply
Vishnu= 3 ply
GP Moto= unknown ply

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TMIC basics on a Subaru WRX/STi

TMIC basics on a Subaru WRX/STi:

The primary purpose of a top mount intercooler is to reduce post turbo air temperature prior to entering the combustion chamber via the throttle body.

HP gain is around 15HP. This figure can vary as many TMICs replace the restrictive OEM piping and results can be further enhanced with post installation tuning. This is one modification that is extremely difficult to put a traditional HP figure on as results truly vary from car to car based on tuning and turbo output in terms of CFM.

How much HP can my stock TMIC hold? 300WHP on the WRX and 400WHP on the STi are attainable. That does not mean those power levels are 100% efficient, but that those power levels are attainable with their OEM TMICs. As discussed below though, TMICs are not meant to be HP rated, but rather CFM related, but this is an FAQ so the HP figures are given as a good “bad” answer.

TMIC basics on a Subaru WRX/STi: 04-07 STi oem TMIC
TMIC basics on a Subaru WRX/STi: 04-07 STi OEM TMIC

Which manufacturer is best? This topic is highly debated. There have been no reported consistent “bad” TMICs on the market. Obviously, there may have been bad TMICs sold, but not enough to report as “bad” overall.

I have a 2008/2009 model, any differences? Yes. Your engine bay has an entirely new layout vs. the 2002-2007 models.

TMIC basics on a Subaru WRX/STi: A 2008 STi stock TMIC
TMIC basics on a Subaru WRX/STi: A 2008 STi stock TMIC
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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.
TD04 turbo and a VF39 turbo
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.

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