The stock FC3S oil cooler is good unit for a stock OEM oil
cooler.  It is one of the largest OEM oil coolers ever
made for a mass production automobile.  There is very
little gains trying to upgrade the stock oil cooler.
Mazda Competition does offer an upgrade unit that is a drop-in
Even with extensive mods to your FC3S, the stock oil cooler
is sufficient enough to handle most oil cooling requirements.
The oil cooler should be exposed to constantly flowing air
and the floorpan kept in place.  This keeps air from
bypassing the oil cooler if the floorpan was eliminated.
The only other reason to remove the stock oil cooler is if you
need to relocate your oil cooling system away from the front nose
due to competing requirements from a larger radiator and/or
front-mount intercooler.  Front bumper space becomes a premium
when trying to fit these heat exchangers, especially with the
stock front bumper.  An aftermarket front bumper can help
solve this problem with larger ducts for your heat exchangers.
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