For top performance and efficiency, your car's engine needs to “breathe” well. And, a big part of that is an efficient and unrestricted path to allow exhaust gasses to escape.
Typical factory-made and even some “performance” aftermarket systems use “crush-bent” pipes in the exhaust system. And, crush bending machines such as the Huth are the staple of local muffler shops everywhere. This manufacturing technique bends the pipe efficiently but unfortunately it allows the pipe to “crinkle” in the bend which decreases the inside diameter and causes restriction which robs your engine of both power and economy. Think of bending a drinking straw or even kinking a garden hose: It's exactly the same problem.
The alternative to “crush-bent” is mandrel bent pipes. Both processes use heavy concave dies outside to hold and form the pipe as it is bent. They both use hydraulic pressure to bend the pipe around the dies. But, mandrel bending adds an additional step, a die (the mandrel) that supports the pipe from the inside as it is bent and prevents the crinkling and collapsing. This inside die is mounted on a long shaft and it is articulated, meaning that it moves during the bending process. It has three, four or even five “donuts” that are ever so slightly smaller than the inside diameter of the pipe and are held to the tool by a ball and socket and spring arrangement. These donuts swivel and pivot as the pipe passes over them. White grease is injected through the mandrel to the inside of the pipe during the bending process.
The results are smooth, uniform, constant-diameter bends that greatly reduce restriction and improve both the performance and efficiency of your engine. One of the most immediately noticeable advantages of a mandrel bent aftermarket exhaust system is the deeper and more aggressive rumble and “growl” of your exhaust.
A circle is the strongest shape that you can achieve for tubing. By using a mandrel bender instead of a standard bender you are able to keep the round shape and not get the more egg like shape you would get from another type of bender. Many people claim they are using a mandrel bender when in fact they are not. The easiest way to tell if the roll bar has been made with a mandrel bender is to feel the tubing at the middle of the bend. If it feels egg shaped, it was definitely not mandrel bent.