One of the first modifications I made to the metro was removing the inner part of its stock double wall exhaust pipe that was stock from the manifold to the catalytic converter (about 13" long). I wasn't really intending to focus on MPGs at the time so I thought it would be good to have a very low restriction exhaust for better power. In hindsight it was a mistake, a 1.0 liter engine just doesn't need a 1-3/4" exhaust pipe (that's the size the remaining stock outer tubing was). Driving the car with the large exhaust I could feel how the sweet spot (when the motor really ran best) was way above the normal operating RPM range so I decided to put some smaller tubing back in place. After a doing some research on several websites I settled on 1" ID tubing which worked out well since I had some 1" EMT (conduit) laying around and a bender to bend it with too. Here are some pics comparing the difference between the old and new pipes.
