Well performance of a vehicle depends on a lot of factors. But when considering of exhaust system design: exhaust back pressure on engine is more important from fuel economy stand point. Depending on type of engine (gasoline or diesel) exhaust temp plays a big role in higher efficiency of emission reduction after-treatment systems. Type of exhaust system changes along with engine cylinder orientation. I-4s normally use 4-2-1 exhaust system. If the vehicle is equipped with single exhaust.