When choosing an intercooler the things that need to be considered are, piping length and sizing, air flow, intercooler efficiently, heat soak, ease of installation/maintenance and compliance for a road registered vehicle.
Top Mountplazaman cooler
Let’s consider pipe length and sizing first. Pipe length is an easy one. The top mount will have shorter pipe length. The pipe sizing will only be limited by available space. In many instances pipe size is not a major issue with some intercoolers having a lager pipe diameter than required for the given application. The pipe length and sizing will have an affect on throttle response and drivability. We are not talking a major affect but every little big counts. The pipe length will not change the maximum power produced on a dyno run.  A top mount with shorter pipe length and correct pipe sizing will win the throttle response battle every time.
Top Mount Piping
Front Mount piping
Air flow and heat soak
The FMIC is an easy victor when stationary. Hot air rises, so the TMIC sitting above the engine will absorb some of this heat. The heat soak is most noticeable sitting in traffic, staging or on the grid. When in motion and when both options have well designed air ducting the difference is not as measurable. The TMIC will take some additional time for the absorbed heat to be radiated away once in motion. On a track day under full throttle, high load conditions the lower airflow through the TMIC will allow gradual buildup of the heat. FMIC is the winner for track and racing.
Having mentioned all that the vertical top mount or verticooler needs to get a mention. Sitting in the position of the top mount and sitting vertical as opposed to horizontal offering better airflow and somewhat reduced heat soak does provide a good compromise.
Intercooler efficiently
The efficiency is more of a function of the core design than the placement. FMIC will often allow greater room for a larger core which will allow greater efficiency especially at higher boost levels. Looking broadly at efficiency a larger TMIC will restrict room for a larger turbo charger (Subaru applications). A higher quality core will always win.

Installation and maintenance
This is not so much a straight forward question as it may seem. A TMIC does involve less piping, however the area of fitment is more confined, and the FMIC has more complex piping and has the need to occasional cut holes and trim bumpersand trims. The installation and maintenance is more vehicle specific than installation location and could go either way depending on vehicle and intercooler kit.

Vehicle compliance
Vehicle compliance was straight forward, once upon a time. No cutting or removal of bumper supports was allowed, however the support could be replaced with an equal strength item. Now with airbags it is a different story. Air bag sensors are fitted to the very front of the vehicle. A FMIC could possibly affect the operation of the SRS therefore it raises a lot of questions. 'Could possibly' are the words often thrown around. As a car owner you have to prove that it will not.
With any modification today you are guilty until you prove your innocence.

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