The FC3S is designed with a very restrictive air intake system.  Even when replacing the stock, panel-type air filter with a freer flowing unit (i.e. K&N drop-in), only a very small amount of power can be freed.  The restriction is not with the air filter itself but with the stock air box.

To eliminate the restrictive, stock air intake design, you can switch over to a cone-type air filter.  For 1986-1988 FC3S you need a cone filter adapter mounted to the front of the stock air flow meter; we, at FC3S Pro, offer our own airflow meter adapter that is cast aluminum unit that has a 3" O.D. inlet to fit all universal cone filters that is typically sized with a 3" I.D. outlet.   For 1989-1991 FC3S, no adapter is needed - the sliding-cone air flow meter can directly use any cone-type air-filter can clamp down to it's 2.5" OD inlet.

One word of warning about using the cone-style air filter:  caution must be exercised to keep hot air produced by engine heat from entering the intake to a minimum.  The air filter ends up sitting in a very hot engine bay.  The FC3s does not lend itself well to a "cold-air induction" system well - it is not easy to plumb a big enough opening into the front of the engine bay without cutting heavily into the car itself.

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