[roflwaffle]

Heat exchangers are a PITA IMO, like Coyote X said restrictions are the way to go. Old domestics had a spring/flap on the exhaust for warm up, old skool internal EGR I guess. The flap would stay closed'ish and not all the hot gas would exit the combustion chamber until the manifold warmed up enough/the spring rate dropped enough and the flap popped open.

Edit- What about one of those cable actuated exhaust cutouts?