Sorry, I gotta disagree with Hotrod Lincoln. A PROPERLY split manifold and dual exhausts will free up some horsepower. This is tricky, because the owner/driver usually drives the car alittle harder/faster, which may make the power/mileage gains SEEM to disappear. The above description of how to split a manifold is somewhat sketchy at best. I started doing them when I was about 16 or 17, and I'm still doing them nearly 60 years later.