The only measurable gains I've ever seen on a dyno run when using headers were on race engines at high RPM, using equal-length tubes for every cylinder. I don't know of any street headers that are built that way. For best results, we also had to calculate the length of each individual intake manifold runner and include that in the overall length of the flow from carburetor to exhaust collector. By adjusting the total length of the intake/exhaust column, an improvement in torque for a specific RPM range could be dialed in. Rams' horns will usually provide more torque over a wider RPM range than custom-built headers at any normal street RPM, so headers are just eye candy to impress the gawkers at a car show. They have no practical purpose on a street machine. Block-huggers with their short-radius bends and unequal length tubes are really a giant leap backwards.
Jerry
