I think I misunderstood bribenz, I thought he was talking about an exhaust on the euro MOTOR, not putting a euro exhaust on a US car. Yes, to put the euro exhaust on a US car, you will need euro exhaust manifolds. The driver's side US manifold flange is parallel to the ground (goes behind motor), and it needs to be pointing down so you can run a pipe under the car. The other side on US cars is somewhat like a T, or Y-pipe, where the pipe from the back of the motor from the driver's side hooks into and the passenger side, and then it goes under the car. The euro is true dual, and you need true dual manifolds. I'm purely speculating, but a complete exhaust, no resonators, either no cat or high flow unit, free flow muffler, you'd probably get a good 20HP. If you get a complete factory exhaust from Timevalve with resonators and OE muffler, less. If I can get the pic to work, here is a pic of the manifolds you'll need:
'86 420SE Euro
904 Midnight Blue, Gray Velour
Dad bought it new, now I own it.
"A Mercedes-Benz is like a fine wine, it only gets better with age."