What is the purpose of it?
for manual transmission turbo cars, the premium switch activation bumps up timing and allows more boost (at least in 87-88)
Auto cars also have it. Both of the 88 TC's Ive had/have were both autos swapped to 5 speeds. Otherwise BornInAFord is correct
Speaking of which, they should have never even offered an auto with the turbos, but they gotta please everybody (the very stupid and lazy masses) lmao.
Im glad for multiple reasons they did an auto! For one a friend of mine has a condition that limits the strength in his legs, and he has driven a TC because of the auto. On top of that for spirited driving I actually like the 3.73 gears with a 5 speed. Decent fuel mileage and gets up to speed quickly without running out at 60
When in the regular position, timing advance is roughly 3 to 4 degrees less than when in premium across the whole timing map, and the BCS will never open, limiting boost to 10 psi on a standard trans car. The switch actually is wired through a relay (located behind the glovebox) that signals the EEC.