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.

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. Most of the models listed below are for the older models, so you will have to refer to the manufacturer/retailer for fitment, or use this link as a general guide.

Which TMIC construction method is best? TMICs have two main construction methods:
1. tube and fin
2. bar and plate
There is much debate as to which construction method is best. There are many pros and cons with each design type, but no real hard data. Bar and plate designs are consistently reported as more damage resistant which gives them the edge with regard to appearance longevity. In the end, you are best advised to chose a TMIC based on other qualities rather than concentrate on construction design.

What about the STI or JDM STI intercoolers? The JDM ver 7 & ver 8 intercooler are 15% larger than the USDM STI intercooler. The Spec C/S202 intercoolers are the same size as their ver 7/8 cousins, but have different end tank designs. All are tube and fin design. This is comparative information of the STI intercooler types only and not meant to sway or dissuade anyone from purchasing these units.

Which TMIC has the best gains? There is no irrefutable evidence that any TMIC has better gains than another. The consensus, if there is one, is they are all within 5HP or less, gain wise, of each other.

What is the best size TMIC to buy for my turbo? This topic is highly debated. The two general trains of thought are:
a. Bigger is better
b. A TMIC should be purchased based on need
For those willing to do their homework on what particular size would show the most benefit with the least amount of drawbacks, this article outlines the steps necessary to calculate the "best" size.

Are there any downsides to TMICs? There have not been significant amounts of problems with TMICs. Care should be taken by the end users to ensure through the Vendor that any special fitment requirements are discussed and understood prior to purchasing. Even "bolt on" intercoolers maybe have slight issues that you need to be aware of prior to purchasing. STi owners need to specifically ask about fitment as the majority of TMICs on the market were designed for the WRX. Some TMICs also have fitment issues with aftermarket BOVs or the hood scoop splitter on the STi. There is also the general consensus that TMICs are prone to heat soak when used in situations of stop and go traffic.

How can I combat heat soak? Heat protections options for TMICs fall under two categories:
1. Heat shielding. The primary purpose of heat shielding is to decrease heat soaking of the TMIC and other engine components. Examples of heat shielding are the stock/modified heat shield, Speedmonkee Performance Chimney Heat Shield, Cobb Tuning Heat Shield, Gruppe-S Heat Shield, MadDad Heat Shield, and the Cusco Heat Shield.
2. Turbo insulation. The primary purpose of turbo insulation is to keep the heat trapped inside the turbo for better turbo efficiency. The side effect of this is to decrease heat soaking of the TMIC and other engine components. Examples of turbo insulation are ceramic coating, Thermo Tec's wrap, or DEI's wrap.
These may be used in conjunction to provide the ultimate level of thermal protection.

What are the hidden costs with TMICs? When switching to a larger TMIC, ensure you understand whether it comes with the intercooler piping or not. Some TMICs come bundled with it, some do not.

Once I have a larger TMIC, what steps are helpful to increase it's efficiency?
1. Increase the size of your stock hood scoop to accommodate the increased TMIC surface area. Maximum efficiency of a TMIC depends on full air flow to the core. This generally means you should obtain a STi or aftermarket larger hood scoop.
2. Proper ducting/shrouding plays a large role as well. OEM and aftermarket options exist for intercooler ducting/shrouding. This step may require custom trimming to ensure a perfect seal or the addition of an alternate substance to make up the difference if needed. Many people have custom trimmed garage door weather stripping/sealer available from Home Depot or Lowes to fill up any gap between the ducting/shrouding and their TMIC. The end goal of ducting/shrouding is a complete seal between the TMIC and hood scoop. This ensures that 100% of the air entering the hood scoop passes through the TMIC core assembly.
3. Additionally, there are OEM and aftermarket options that exist for air splitters. Air splitters divide and route the air coming into your hood scoop over the entire core of the intercooler.

This thread will help you figure out what scoop/splitter to use.

What are the advanced efficiency options?
1. Waterspray. There are aftermarket and OEM options that provide increased cooling efficiency and combat against heat soak issues with TMICs.
2. NO2 and CO2 Sprayers. There are aftermarket kits that spray CO2 or NO2 to cool TMICs down and combat against heat soak issues with TMICs.

This article highlights how proper airflow and waterspray can increase your efficiency:
Article

Where do I buy a TMIC? Every Subaru/Import performance store sells TMICs. For purchasing, support your local economy or the NASIOC Vendors.

How hard is it to install a TMIC? Allow around one hour for install time. Professional installation, depending on your area, is around $75. This is one vehicle modification that is very simple and can be successfully accomplished by even the greenest shade tree mechanic.

How do I install a TMIC? Refer to the TMIC manufacturer's instructions. For TMICs without instructions, below are links to some of the better known installation instructions:
scoobymods.com instructions (for eBay models)
scoobymods.com instructions
scoobymods.com instructions
APS instructions
Perrin instructions
Perrin 08+ instructions
USDM STi TMIC installation
USDM STi TMIC installation
JDM Ver. 7 TMIC installation
JDM Ver. 7 TMIC installation on a 2006 WRX
08 STI installation on a 2002 WRX

Should I reset my ECU after this mod? It is never a bad idea to perform an ECU reset after a mod. The traditional route is to disconnect the negative battery terminal, press the brake pedal for a few seconds to bleed the system of charge, and reattach. Some use the more advanced Vishnu Reset.

What is another good sources of TMIC information?

ARE Cooling's tech article

Which is better a FMIC or a TMIC? There is no definite answer for this. Choose your upgraded intercooler based on the pros and cons of each and your personal goals. Perfect testing of intercoolers is not possible and though not specifically catered towards intercoolers, this Car and Driver magazine article will help point out the reasons why we will never know which is "better".

Comparative Data

Manufacturers of TMICs

APS
ARC
AVO
BPM
Buschur Racing
CSS
Cartech
Daddy's SCP
GP Moto
Helix
Hyperflow
M2
MRT
PWR
Perrin
Process West
Spearco
Speed Factor
TurboXS (utilizes a Hyperflow core)
Vishnu (utilizes a Spearco core)
Worx
XO2 Racing

2002-2007 WRX
2008-2009 WRX
STI
JDM STI (all versions)

Pressure Drop

Pressure drop can change based on manufacturer's testing levels. Generally speaking, the higher the turbo PSI, the higher the pressure drop. Manufacturers often quote the lowest testing levels and often times do not quote their test data. This means you cannot compare intercooler A VS. intercooler B.

APS: .3 PSI
ARC: Unknown PSI
AVO: .5 PSI
BPM: 1 PSI
Buschur Racing: Unknown PSI
CSS: less than 1 PSI at 14.5 PSI
Cartech: Unknown PSI
Daddy's SCP: Unknown PSI
GP Moto: Unknown PSI
Helix: .2 PSI PSI
Hyperflow: .27 PSI
M2: Unknown PSI
MRT: Unknown PSI
PWR: .8 PSI
Perrin: Unknown PSI
Process West: Unknown PSI
Spearco: Unknown PSI
Speed Factor: .81 PSI
TurboXS: Unknown PSI
Vishnu: Unknown PSI
Worx: .5 PSI
XO2 Racing: 0.2 PSI at 15 PSI

2002-2007 WRX: 2-3 PSI
2008-2009 WRX: Unknown PSI
STI (all versions): Unknown PSI
JDM STI (all versions): Unknown PSI

Flow Rate

APS: Unknown CFM
ARC: Unknown CFM
AVO: 60 lbs/min
BPM: Unknown CFM
Buschur Racing: Unknown CFM
CSS: 800 CFM
Cartech: Unknown CFM
Daddy's SCP: Unknown CFM
GP Moto: Unknown CFM
Helix: Unknown CFM
Hyperflow: Unknown CFM
M2: Unknown CFM
MRT: 650 CFM
PWR: 975 CFM
Perrin: Unknown CFM
Process West: Unknown CFM
Spearco: 1000 CFM
Speed Factor: Unknown CFM
TurboXS: 635 CFM
Vishnu: Unknown CFM
Worx: 678 CFM
XO2 Racing: Unknown CFM

2002-2007 WRX: 390 CFM
2008-2009 WRX: Unknown CFM
STI (all versions): Unknown CFM
JDM STI (all versions): Unknown CFM

Core Construction

APS: bar and plate
ARC: tube and fin
AVO: bar and plate
BPM: bar and plate
Buschur Racing: bar and plate
Cartech: bar and plate
CSS: bar and plate
Daddy's SCP: bar and plate
GP Moto: bar and plate
Helix: bar and plate
Hyperflow: bar and plate
M2: bar and plate
MRT: bar and plate
PWR: tube and fin
Perrin: bar and plate
Process West: bar and plate
Spearco: bar and plate
Speed Factor: bar and plate
TurboXS: bar and plate
Vishnu: tube and fin
Worx: bar and plate
XO2 Racing: bar and plate

WRX/STI/JDM STI (all versions): tube and fin

Physical size

APS: 20" x 7.5" x 4.5"
ARC: 18.5" X 7.5" X 2.75"
AVO: 18.3" x 7.23" x 3.5"
BPM: Unknown 20.125” x 7.625” x 4.5”
Buschur Racing: 6.5" x 18.5" x 4.5"
CSS: 20" x 11.5" x 4.5"
Cartech: 23" x 6" x 3"
Daddy's SCP: 19 7/8 " x 8" x 4.5"
GP Moto: 20.125” x 7.625” x 4.5”
Helix: 19” x 8” x 4”
Hyperflow: 19.1" x 7.3" x 4.5" (based on metric conversion)
M2: Unknown
MRT: 15.25 " x 6 " x 4.5" 19.25" x 7.5" x 4.5"
PWR: 16 x 6 1/4" x 4 3/4"
Perrin: 20" x 7" x 4"
Process West: 19.3" x 7.5" x 3.5"
Spearco: 18" x 10" x 4.5"
Speed Factor: 20" x 7.5" x 4.5"
TurboXS: 19 1/8" x 7 3/8" x 4.5"
Vishnu: 18.5" x 6.5" x 4.5"
Worx: 18.5" x 8" x 3.5"
XO2 Racing: 19 7/8" x 8" x 4.5"

2002-2007 WRX: 16.5" x 6" x 3"
2008-2009 WRX: 11" x 9.5" x 2.5"
2004-2007 STI: 20.5" x 7.3 x 4"
2008 STI: 21 5/8" x 7.5" x 3" or 3.6% more volume than older STI models
JDM STI (ver 7): 15% dimensionally larger core size than the 2004-2007 USDM STI IC

Do I need engine management/tuning with a TMIC? No. Tuning is highly encourage though as you won't see the full benefit without tuning.