I'm looking to put an intercooler in my new to me 1984 300sd. I'm wandering what would be better. An air to air intercooler or a water to air intercooler. This is going to be my daily drive and looking to improve the gas mileage and the power.

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84 300sd 2.47 dif 360k
26 mpg needs lots of love
given to me in pitty

89 civic hatchback auto 140k
33mpgs soon to be low to mid 40's
been in the family since mile 1

85 300d with unknown knock. driven over 30k with knock and still going. sold for $800