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).
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.
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.
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:
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:
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
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
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.
The primary purpose of an aftermarket exhaust is to remove or replace the stock catalytic converter(s) with a better flowing unit. It also increases the exhaust diameter for better flow. The term “aftermarket exhaust” can be broken down into the three main types:
1. Turbo back exhaust (TBE) covers items 1-4 as described below.
2. Cat back exhaust (CBE) covers items 3&4 as described below.
3. Axle back exhaust (ABE) covers item 4 as described below.
Keep in mind that the above information is catered to the 2002-2005 WRX and is likewise applicable to all years of the STi, though there are slight variations. For example, the STi (all years) has a catless uppipe and the 2006+ WRX has a one piece downpipe/midpipe.
HP gain is dependent on the type of exhaust chosen. TBE gains are 20-30HP. CBE gains are 5-15HP. ABE gains are 5-10HP. These figures are highly debated as different manufacturers use different dynos with different cars with different levels of mods. It also varies because exhausts use different amounts of catalytic converters or are catless.
What about overall fitment between the WRX and STi or different model years? All OEM or aftermarket exhausts designed for 02-07 WRX/STi will fit either the 02+ WRX sedan/wagon or 04+ STi. Occasionally, you will have a slight fitment issue with some models (moreso if your car has the optional rear differential cover) that usually get be fixed via hanger adjustment, longer exhaust hangers etc. 06+ models have fitment issues with some exhausts (mainly the muffler portion) due to the new rear diffuser.
I have an 06+ WRX, is the downpipe the same? No. The 2006+ WRX has a unique exhaust in that it’s downpipe is both the downpipe and midpipe 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. Check with your Vendor for other possible fitment issues prior to ordering.
You can easily install the upgraded STI top-mount intercooler in your garage using basic tools.
1. Disconnect your battery and make sure you have all the necessary tools. You need a flat bladed screwdriver, a 10mm wrench, and a 12mm wrench. Consider the picture before you in the engine compartment. The parts you’re looking for are centered on top of the engine. You need the new intercooler, an STI Y-pipe and inlet elbow and a new gasket for the bypass or blow-off valve. You also need the STi air conditioning line bracket for the firewall.
This is the stock WRX engine with the stock intercooler on top.
2. Two 12mm bolts that hold the intercooler to its mounting bracket on either side of the cooler. On the passenger side, note that there’s an open hole in the bracket—this hole is there to accommodate the STI intercooler. Three 10mm bolts hold a metal tube to the front of the intercooler, and two 12mm bolts that attach the bypass valve. Underneath the intercooler on the passenger side, there’s a hose clamp that holds the Y-pipe to the turbo, and there’s another hose clamp in front that holds the intercooler to the throttle body.
Note the extra bolt-hole Subaru thoughtfully placed in just the right spot for an STI intercooler.You need to undo all these bolts along the front of the intercooler.
3. Loosen the hose clamps first (and hopefully the one under the intercooler is oriented to make this easy) and then undo all the bolts. The intercooler should lift out and take the Y-pipe with it. Leave the bypass valve on the intake manifold—you don’t need to totally disconnect it. You might need to wiggle it loose, but don’t pull too hard.
Carefully lift out the intercooler and the hoses should come free if you’ve loosened the clamps.
Remove the glow in the dark release handle. Once this is done you can get to the lock. Simply twist the orange thing around, and pop the metal rod. Take a screw driver (you also might need a hammer as well) and pry that black clip off. Its really tight on there so you will need to use some force (this clip is exactly like the clip securing the brake lines).
Remove latch from trunk (optional as it may not be needed)
Remove the 2 bolts, twist yellow thing around and pop out that metal rod. You will also need to pop out the cable from that glow in the dark handle.
Remove trunk
Remove the 4 12mm bolts that secure the trunk to the trunk arms. Beware that the STi trunk weighs a TON (something like 50+ lbs). You will also have to unplug the 3rd brake light and clip all the wire retainers off the STi trunk arm to get the trunk off.
We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. By clicking “Accept”, you consent to the use of ALL the cookies.
This website uses cookies to improve your experience while you navigate through the website. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. We also use third-party cookies that help us analyze and understand how you use this website. These cookies will be stored in your browser only with your consent. You also have the option to opt-out of these cookies. But opting out of some of these cookies may affect your browsing experience.
Necessary cookies are absolutely essential for the website to function properly. These cookies ensure basic functionalities and security features of the website, anonymously.
Cookie
Duration
Description
cookielawinfo-checkbox-analytics
11 months
This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Analytics".
cookielawinfo-checkbox-functional
11 months
The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional".
cookielawinfo-checkbox-necessary
11 months
This cookie is set by GDPR Cookie Consent plugin. The cookies is used to store the user consent for the cookies in the category "Necessary".
cookielawinfo-checkbox-others
11 months
This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Other.
cookielawinfo-checkbox-performance
11 months
This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Performance".
viewed_cookie_policy
11 months
The cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. It does not store any personal data.
Functional cookies help to perform certain functionalities like sharing the content of the website on social media platforms, collect feedbacks, and other third-party features.
Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors.
Analytical cookies are used to understand how visitors interact with the website. These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc.
Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. These cookies track visitors across websites and collect information to provide customized ads.
